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PROGRESS IN ENDOCRINOLOGY AND

Metabolic Basis for Disorders of Degradation

Irving H. Fox

Purine nucleotide degradation refers to a regulated series of reactions by which purine and are degraded to uric in . Two major types of disorders occur in this pathway. A block of degradation occurs with syndromes involving immune deficiency. myopathy or renal calculi. Increased degradation of occurs with syndromes characterized by and , renal calculi, anemia or acute . Management of disorders of purine nucleotide degradation is dependent upon modifying the specific molecular pathology underlying each disease state.

N EXPLOSION of information about disorders dation of purine nucleotides. Activation of nucleotide A of purine nucleotide degradation in humans has degradation to and occurs in occurred during the past 8 yr. The discovery of seven ascites tumor cells following incubation with 2-deoxy- new abnormalities associated with specific glucose or glucose.‘4 There is rapid utilization of ATP clinical syndromes and the intensive research to during the of these compounds. The elucidate the underlying molecular pathology have sudden diminution of intracellular ATP concentra- provided the basis for the major advances. New tions and the restthing elevation of AMP and IMP concepts have related tissue ATP levels and their levels appear to be triggering factors for the activation depletion by hypoxic or metabolic mechanisms to of nucleotide degradation. common clinical abnormalities. Experimental evidence in humans indicates that Disorders of purine nucleotide degradation now altered regulation of the pathway may accelerate the encompass a range of previously unsuspected disease degradation of purine nucleotides. A model for activa- associations. Immunodeficiency, myopathy, renal cal- tion of purine nucleotide degradation in humans is culi, hyperuricemia and gout, anemia, central nervous provided by the rapid infusion of . In less than system dysfunction and tissue hypoxia occur with 60 min after intravenous fructose there is an increase abnormalities of this biochemical pathway. In this of the serum urate concentration and an elevation of review, diseases of purine nucleotide degradation will urinary , oxypurine (hypoxanthine and be described. Two major types of disorders occur; ) and inosine (Fig. 2).5-9 In human blocks of purine nucleotide degradation and increased there is a depletion of total nucleotides, activity of this pathway. The metabolic basis underly- predominantly ATP, and inorganic within ing altered regulation of purine nucleotide degradation 30 min of a fructose infusion.” These observations to uric acid in these diseases will be described as it is suggest that the rapid phosphorylation of infused fruc- understood at the present time focusing on the more tose by ATP leads to hepatic ATP degradation to the recent advances. putine compounds measured in blood and . Vigorous muscular in man causes a rise of the REGULATION OF PURINE NUCLEOTIDE serum urate level and an elevation of plasma and DEGRADATION urinary oxypurines.“--I3 This may be related to an Purine monophosphate derivatives are increase of uric acid synthesis from the degradation of degraded to uric acid in humans by a final common muscle ATP during exercise, since there is a diminu- pathway (Fig. 1). Complex regulation of this pathway tion of vastus lateralis muscle ATP concentrations.‘3 is evident from experiments which increase the degra- These observations are compatible with an activation of purine nucleotide degradation, initiated by a sudden From the Human Purine Research Center, Departments of decrease of intracellular ATP and inorganic phosphate Internal Medicine and RioLogicar Chemistry, Clinical Research concentrations during exercise. Center, University of Michigan, Ann Arbor, Michigan. Receivedfor publication October 28, 1980. This work supported in part by USPHS grants AM 19674 and SMOI RR42. and grants from the Michigan Heart Association and The dephosphorylation of nucleoside 5’-monophos- the Warner Lambert Company. phates (Fig. 1, reaction 1) is the first committed and Address reprint requests to Dr. I. H. Fox, University Hospital, irreversible reaction of purine nucleotide degradation. Clinical Research Center, W-4642 Main Hospital, Ann Arbor, Michigan 48109. It has been proposed to be the major site of regulation 0 I981 by Grune & Stratton, Inc. of the pathway.14 Four distinctive subcellular types of 0026-o495/8i/3006~l7%02.00/0 5’ have been recognized to hydrolyze

616 Metabolism, Vol. 30, No. 6 (Junel. 198 1 PIJRINE NUCLEOTIDE DEGRADATION 617

dAMP CAMP - 11

6 1 I Adenine

I- 2 - dGuanosine d lnosine dAdenosine I lnosine * 8 3 10 S-Adenosylhomocysteine I

S-Adenosylmethionine

4

Uric Acid

Fig. 1. Pathway of purine nucleotide degradation. Purine nucleoside monophosphate derivatives are degraded to uric acid in humans by a final common pathway. Nucleotides are dephosphorylated to form the nucleoside derivatives. Specific 5’-phosphomonoesterase (E.C.3.1.3.5) and non-specific jE.C.3.1.3.2) hydrolyze AMP, IMP, GMP or their deoxynucleoside monophosphates to the nucleoside or deoxynucleoside derivatives (reaction 1 j. AMP is deaminated to IMP by AMP deaminase (E.C.3.5.4.6, reaction 12). Inosine, deoxyinosine, deoxygusnosine, and guanosine are converted to purine bases and -l-phosphate or -l-phosphate by purine nucleoside phosphorylase (E.C.2.4.2.1, reaction 3). Adenosino and deoxyedenosine are deaminated to inosine and deoxyinosine by fE.C.3.5.4.0, reaction 21. Guanine is deaminated to xanthine by lE.C.3.5.4.3, reaction 5). Hypoxanthine and xanthine are oxidized to uric acid by jE.C.l.2.3.2, reaction 4) located primarily in the liver and jejunem of . These pathways are interrupted by reactions which allow the resynthasis of nucleotides. Nucleoside kineses catalyze the conversion of adenosine to AMP, to dAMP. deoxyinosine to dlMP and deoxyguenosine to dGMP (reaction 6). Hypoxanthine-guenine phosphoribosyltransferase (E.C.2.4.2.8, reaction 7) converts hypoxanthine or guanine to IMP or GMP. Adenine phosphoribosyltransferase (E.C.2.4.2.7, reaction 11) catalyzes the formation of AMP from adenine. The precursor substrates of the final common pathway of purine nucleotide degradation are formed from the of dietary nucleoprotein, the degradation of nucleic to nucleoside monophosphates and the formation of nucleotides by de novo purine synthesis and the purine salvage pathways. In addition, S-adenoeylhomocysteine is degraded to edenosine and homocysteine by S-adenosyfhomocysteine jE.C.3.3.1 .l, reaction Sj in an essential step of S-adenosylmethionine mediated methylation reactions (reaction 9). S-adenosylmethionine may also be decarboxylated to enter polyamine metabolism (reaction 10). Degradation of deoxyribonucleotides follows a similar pattern as indicated. However, dAMP may be converted to dlMP at only 6% of the V,, of AMP (2321 (reaction 121. while dlMP may be converted to dGMP or dAMP at virtually the same V,, as IMP (233-236). dGMP may be converted back to dlMP at about 1% of the V,, of GMP (237). The dephosphorylation of deoxynucleotides. the of deoxyedenosine and the phosphorolysis of deoxyinosine or are all similar to the reactions of the ribosyl derivatives (Reviewed in Reference 14). GMP, guanosine 5’-monophosphate: dGMP, deoxyguanosine 5’-monophosphate; IMP. inosine 5’-monophosphate; dlMP. deoxyinosine I’-monophosphate; AMP, adenosine 5’- monophosphate: dAMP, deoxyadenosine 5’-monophosphate.

purine nucleoside 5’-monophosphates.” These include membrane under usual circumstances. A vectorial in plasma membrane, microsomes, lysosomal transport system for the adenosine component of AMP membrane, and . The plasma membrane into myocardial cells has been suggested during 5’-nucleotidase has been the focus of many studies perfusion with AMP.24 although its function is not clear. The enzyme faces Plasma membrane 5’-nucleotidase may be an the outer surface of the plasma membrane.‘6-22 important site for the regulation of purine nucleotide Substrate specificity is localized to 5’-monophosphate degradation, since activation of this pathway occurs compounds. The enzyme is inhibited by nucleoside with a depletion of ATP, an inhibitor of this enzyme, diphosphate and triphosphate derivatives and concan- and an accumulation of IMP and AMP, substrates for avalin A.“.2’.“.?’ Extracellular nucleoside 5’-mono- this enzyme. However, recent observations indicate are degraded to their nucleoside deriva- that plasma membrane 5’-nucleotidase may not tives in preparation for uptake, since the negatively influence intracellular nucleotide degradation.2s A 5’- charged nucleotides are not transported across the nucleotidase that occurs in the supernatant fraction of 618 IRVING H. FOX

Fructose - I -P

Ribose - I-P Xanthine

-1 Uric Acid

Fig. 2. Mechanism of fructose-induced purine nucleotide degradation. Fructose triggers the rapid breakdown of purine nucleotides to uric acid in the liver. The phosphorylation of fructose to fructose-l-phosphate causes ATP to be degraded to ADP. Fructose-l-P tends to accumulate and thus traps inorganic phosphate. ADP is converted back to ATP by the mitochondrial electron transport system or , which use inorganic phosphate, or by adenylate . The latter reaction also forms AMP. The net result is a diminution of intracellular ATP and inorganic phosphate and buildup of AMP. The elevated AMP concentrations also lead to increased IMP concentration. Dephosphorylation of 5’-nucleotidase is triggered. If AMP and IMP concentrations are high enough, then non-specific can be activated. Once dephosphorylation is activated, there is a cascade of nucleotide degradation through the catabolic pathways leading to increased synthesis of uric acid and accounting for hyperuricemia and the elevated urinary excretion of inosine. hypoxanthine, xanthine and uric acid. Inhibition is indicated by dotted lines. Vertical arrows beside ATP, Pi end AMP show changes caused by-the fructose infusion (From Fox, Reference 14). chicken liver26*27and rat liver28-30 may catalyze cyto- concentrations is the major factor increasing the activ- plasmic nucleoside 5’-monophosphate dephosphoryla- ity of AMP deaminase,33 while altered ATP, GTP and tion. This enzyme preferentially hydrolyzes IMP and GDP levels do not change the activity.3”36 Therefore, GMP, is activated by ATP and ADP and is inhibited it remains unclear whether AMP deamination is limit- by inorganic phosphate. Although it is highly likely ing for purine nucleotide degradation. The compli- that important regulation of purine nucleotide degra- cated regulation of this enzyme, its important location dation occurs at the level of dephosphorylation, the in the pathway, and the fact that adenine nucleotides nature of the regulation at this reaction remains to be constitute the majority of free cellular nucleotides clearly delineated. make this a serious consideration. AMP deaminase has an additional important role as AMP Deamination a component of the purine nucleotide cycle in skeletal The deamination of AMP (Fig. 1, reaction 12) has muscle.37 This reaction sequence causes the release of been proposed as the rate-limiting reaction in adenine during muscle contraction and the resynthe- nucleotide degradation instead of 5’-nucleotidase.3’ sis of AMP from IMP. The sequence may be impor- Inorganic phosphate and GTP at physiological tant to energy metabolism and ATP synthesis during concentrations inhibit AMP deaminase by 95%3” and muscle contraction. are decreased in concentration during fructose infu- Reutilization sion. This is proposed to activate the enzyme and lead to adenine nucleotide degradation3* However, in The major regulatory mechanism of purine nucleo- contrast to this proposal, under physiological condi- tide degradation after dephosphorylation may be tions or in intact cells a rise in AMP, ADP and H+ reutilization pathways. Guanine and hypoxanthine are PURINE NUCLEOTIDE DEGRADATION 619 resynthesized to their respective nucleotides by hypo- to initiate degradation. The measurement of intracel- xanthine-guanine phosphoribosyltransferase (Fig. 1, lular nucleotides and the quantitation of hypoxan- reaction 7). This enzyme is regulated by the availabil- thine, inosine, and adenosine allow an assessment of ity of intracellular phosphoribosylpyrophosphate and the activity of this pathway. A refinement of this product inhibition. The importance of this reaction is technique is to prelabel the adenine nucleotide pool by illustrated by its deficiency, which leads to excessive incubation with radioactively labeled adenine.‘,* The synthesis of uric acid in part, from an inability to stimulation of the breakdown of ATP under these reutilize the substrate purine bases.38 conditions allows a clear documentation of the path- Adenosine, deoxyadenosine, deoxyinosine and ways of ATP degradation through the purine catabolic deoxyguanosine can be phosphorylated to their respec- pathways. tive nucleoside 5’-monophosphates by kinase enzymes (Fig. 1, reactions 6). Adenosine phosphorylation may In Vivo Measurements be important in maintaining normal intracellular ATP Plasma and urinary . Measurements of concentrations. In erythrocytes with increased adeno- plasma and urinary purines in man by standard spec- sine deaminase activity there is a decrease in ATP trophotometric methods and more recently by high concentrations and hemolytic anemia.39 The removal pressure liquid chromatography allow an assessment of adenosine by its deamination in this disease suggests of disorders of the nucleotide degradation pathway. that erythrocytes require adenosine phosphorylation to An increase in the serum urate concentration and in synthesize normal quantities of ATP. Adenosine the urine uric acid excretion indicates increased activ- kinase is regulated by adenosine, ATP and Mg ity of the purine nucleotide degradation pathway, concentrations, and is inhibited by ADP, AMP, deoxy- since it demonstrates the increased synthesis of uric adenosine and S-adenosylhomocysteine.40 Therefore, acid. On the other hand, a diminution of the serum alterations in kinase activity may occur when adeno- urate concentration with very low urine uric acid sine or deoxyadenosine accumulates. Attention has excretion may indicate a block in uric acid synthesis. been focused upon these reactions in adenosine deami- An accumulation of purine catabolic intermediates nase deficiency, where both adenosine and deoxyaden- may localize such a block. For example, the excessive osine accumulate in affected patients. Although there excretion of hypoxanthine and xanthine and hypouri- is evidence to suggest that adenosine and deoxy- cemia may indicate a block at xanthine oxidase. adenosine are phosphorylated by the same enzyme, Excretion of inosine, deoxyinosine, guanosine, and some experiments demonstrate the existence of a deoxyguanosine and may indicate a distinct enzyme for deoxyadenosine phosphoryla- block at purine nucleoside phosphorylase (Fig. 1). tion4’ 4s possibly associated with kinase Thus, simple measurements of plasma and urinary activity.45 Observations with highly purified adenosine purines may provide a clue to an underlying disorder kinase suggest that and deoxyadeno- of purine nucleotide degradation. Other research sine kinase are distinct enzymes, with the former also methods are summarized below. having only a small amount of deoxyadenosine phos- Fructose induced purine . A probe of phorylating activity.44 the intactness and activity of the purine catabolic pathway may be carried out by the use of a fructose MEASUREMENT OF PURINE infusion.‘.” Fructose administered rapidly intrave- NUCLEOTIDE DEGRADATION nously activates purine nucleotide degradation (Fig. The development of methods for the quantitation of 2). A block in the pathway will be reflected by a purine nucleotide degradation in whole cells has lead change of the normal pattern of urinary purine excre- to refined techniques to measure this pathway in tion following the fructose infusion. Alterations in the humans. purine excretion following fructose infusion have been observed in deficiencies of purine nucleoside phosphor- Measurements ylase, xanthine oxidase and hypoxanthine-guanine The basis for the measurement of purine nucleotide phosphoribosyltransferase.4N8 degradation in cells is to alter the normal regulation of Radioactive labeling of the adenine nucleotide the pathway by causing a breakdown of ATP.14 This pool. A radioactive isotope of adenine is adminis- initiates a cascade of purine nucleotide degradation tered intravenously to patients. This small quantity of with a flow of metabolites through the pathway to adenine does not alter purine pool sizes and is incorpo- adenosine, inosine, and hypoxanthine. Deoxyglucose rated into the adenine nucleotide pool. As the adenine or inhibitors of the mitochondrial electron transport nucleotide pool is turned over every day, there is a flow system and oxidative phosphorylation have been used of radioactively labeled compounds through the purine 620 IRVING H. FOX

nucleotide degradation pathway. The intermediates of Table 1. Disorders of Purine Nucleotide Degradation purine nucleotide degradation are labeled separately Block of degradation from purines formed by the pathway of purine synthe- 5’-Nucleotidase deficiency of lymphocytes sis de novo. Some of these radioactively labeled Adenosine deaminase deficiency Purine nucleoside phosphorylase deficiency purines will be excreted in the urine and can be easily Myoadenylate deaminasa deficiency measured. Block of the pathway or alteration of reutil- Guanine deaminase deficiency ization steps may change the quantity and pattern of Xanthine oxidase deficiency urine radioactive purine excretion. This method has Increased degradation been used to demonstrate decreased hypoxanthine Increased enzyme activity adenosine deaminase overactivity reutilization in Lesch-Nyhan Syndrome.48 xanthine oxidase overactivity Adenine labeling combined with fructose infu- Decreased reutilization sion. The two techniques described above may be hypoxanthine-guanine phosphoribosyltransferase combined for a sensitive probe to assess for alterations deficiency of the purine nucleotide degradation pathway in wine nucleoside phosphorylase deficiency adenine phosphoribosyltransferase deficiency humans. The adenine nucleotide pool may be prela- Increased substrate beled three to four days prior to fructose infusion. Increased de novo purine synthesis Under these conditions the normal response to a fruc- Increased phosphoribosylpyrophosphate tose infusion is a lo-fold elevation of the urinary synthetase radioactivity over the baseline excretion (Edwards NL Hypoxanthine-guanine phosphoribosyltransferase deficiency and Fox IH, unpublished results); the greatest Purine nucleoside phosphorylase deficiency increase of radioactivity is found in inosine and hypo- Increased turnover of nucleic acrd xanthine in subjects with normal enzyme activity.48 Hematological abnormalities Alteration in the amount or pattern of radioactivity Psoriasis excretion following the fructose infusion may indicate Decreased ATP Increased degradation of ATP disorders of the purine nucleotide degradation path- Glucose-6-phosphatase deficiency way. Such an alteration has been discerned in hypo- Hereditary fructose intolerance xanthine-guanine phosphoribosyltransferase deficien- (Possibly) fructose-l, 6diphosphatasa cy,48 but not in patients with lymphocyte plasma deficiency membrane 5’-nucleotidase deficiency.49 Muscular exercise Fructose infusion DISORDERS OF PURINE NUCLEOTIDE DEGRADATION Decreased synthesis of ATP Tissue hypoxia-, respiratory failure. Two major types of abnormalities of human purine impairment of blood flow nucleotide degradation occur; a block in the pathway Hypophosphatemia and an increased activity of the pathway. Table 1 outlines a classification of disorders associated with alteration of purine nucleotide degradation. Blocks of lymphocytes is decreased in certain patients with purine nucleotide degradation may lead to an accumu- congenital agammaglobulinemia, common variable lation of the intermediates or their metabolites in hypogammaglobulinemia, and selective IgA deficien- human body fluids or cells. The accumulation of such cy. 49s~53 The enzyme deficiency is related to a intermediates may be toxic to certain cells and lead to decrease in the number of 5’-nucleotidase positive clinical disorders such as immunodeficiency. An lymphocytes.54,55 The enzyme deficiency is measured elevated rate of purine nucleotide degradation may in peripheral sheep erythrocyte rosette-forming lym- lead to an increase in the end-products of the pathway. phocytes49,5”58 and in peripheral mononuclear cells On the basis of the observations with fructose-induced depleted of monocytes56-59 and represents a block in purine nucleotide degradation which is triggered by the ability of these cells to degrade extracellular massive ATP utilization (Fig. 2), an elevated rate of nucleoside 5’-monophosphates. The diminished activ- purine nucleotide degradation may be expressed by ity of lymphocyte 5’-nucleotidase in these diseases the following metabolic variables: an increase in the appears to be related to decreased T-lymphocyte serum urate concentration and an elevation of urinary enzyme activity and decreased numbers of B-lympho- uric acid, inosine, and oxypurine excretion.50 cytes. The question arises whether 5’-nucleotidase defi- Blocks of Purine Nucleotide Degradation ciency causes immune dysfunction or results from S-Nucleotidase deficiency of lymphocytes. The abnormal lymphocytes. The data suggest that abnor- surface 5’-phosphomonoesterase activity of peripheral mal lymphocytes are the basis for the enzyme deficien- PURINE NUCLEOTIDE DEGRADATION 621 cy. No structural alteration of the deficient enzyme ciency and central nervous system disease. How these has yet been demonstrated.‘* The enzyme deficient biochemical abnormalities lead to immune dysfunc- patients have no evidence for a systemic disorder of tion is discussed below. purine nucleotide degradation and appear to have the Adenosine deaminase is structurally altered in deficiency only on lymphocytescg Erythrocyte and tissues from patients with severe combined immunode- lymphocyte deoxynucleoside triphosphates are normal ficiency disease. *o-85 The properties of the mutant in S-nucleotidase deficiency.60 Finally, earlier stages enzymes provide support for genetic heterogeneity of of lymphocyte maturation may be characterized by the involving the structural coding for lower values of S-nucleotidase, supporting the possi- adenosine deaminase. Structural alterations of adeno- bility that abnormal lymphocyte populations cause sine deaminase also occur in patients with the enzyme 5’-nucleotidase deficiency.58.6’ deficiency and normal immune function.86*87 In these A deficiency of lymphocyte 5’-nucleotidase has also patients white cell adenosine deaminase is less severely been observed in chronic lymphocytic leukemia, acute deficient; this amount is adequate for normal function lymphoblastic leukemia and transiently in acute infec- of the immune system. tious mononucleosis.62-65 Experimental therapies attempt to restore immune Adenosine deaminase de$ciency. In 1972, the function in patients with severe combined immunode- deficiency of adenosine deaminase was found in two ficiency and adenosine deaminase deficiency. The unrelated patients with severe combined immunodefi- results of these trials support an etiological relation- ciency disease.66 The enzyme deficiency accounts for ship between the enzyme deficiency and the immune about half of the patients with this syndrome and disorder. About 50% of the patients with severe autosomal recessive inheritance. In 85%--90% of combined immunodeficiency disease and adenosine adenosine deaminase deficient patients there is severe deaminase deficiency respond to enzyme replacement lymphopenia, failure to thrive, and infections, diar- therapy with packed irradiated erythrocytes.88m9’ The rhea, malabsorption, and candidiasis with atrophic response includes an increase in the absolute lympho- tonsils, adenoids, and thymus in the first few months cyte count and improved responsiveness to the mixed of . In 10%15% of patients the disease may lymphocyte culture and to phytohemagglutinin stimu- become evident later than 3-6 mo and immunoglobu- lation. As well, there is a diminution in the tissue lin retention may occur.67.68 X-rays demonstrate deoxyadenosine triphosphate concentrations, a de- evidence of osteoporosis, small or absent thymus crease in the excretion of adenosine and deoxyadeno- gland, and chondro-osseous dysplasia at costochondral sine and an elevation of erythrocyte S-adenosylhomo- junctions, apophysis of iliac bones and vertebral hydrolase.78.89-9’ Bone marrow transplantation bodies. There is a disorder of decreased platelet aggre- corrects most of the immunological and biochemical gation.69.70 Neurological abnormalities including nys- abnormalities9’m93 and appears to be the treatment of tagmus, head lag, spasticity, athetosis, and develop- choice. A low purine diet, which reduces the urinary mental delay occur in 10%-l 5% of patients.” excretion and the plasma concentrations of adenosine Biochemical abnormalities appear to account for and deoxyadenosine,” and thymosin administration the clinical syndrome. Elevated adenosine and deoxy- may be useful adjuncts to the management of these adenosine concentrations in the urine, plasma and patients.94 erythrocytes result from the adenosine deaminase defi- The molecular basis for the immune dysfunction ciency. Increased concentrations of dATP and dADP has been assessed in cell culture models simulating in erythrocytes, lymphocytes and bone marrow cells adenosine deaminase deficiency. Cytoxicity or immu- and, in some instances, decreased concentration of nosuppression follow the addition of adenosine or ATP in erythrocytes from these patients result from deoxyadenosine to cultured diploid fibroblasts, human the deoxyadenosine accumulation and may have toxic lymphoblasts, lymphosarcoma T-cells, Chinese ham- properties toward the immune system.60*72-76Increased ster ovary cells, and mouse fibroblasts (Reviewed in lymphocyte and platelet cyclic-AMP concentrations Reference 95). These compounds also inhibit mitogen- may also contribute to the dysfunction of these cells.” mediated lymphoblastogenesis and monocyte to A secondary severe deficiency of erythrocyte S-adeno- macrophage transformation. Toxicity from adenosine syl-homocysteine hydrolase has been observed.78,79 or deoxyadenosine is potentiated with inhibition of This may be related to the deoxyadenosine accumula- adenosine deaminase by specific inhibitors, coformy- tion, since deoxyadenosine has been shown to inacti- tin, deoxycoformycin or erythro-9(2-hydroxyl-3- vate S-adenosylhomocysteine hydrolase.78 A possible nonyl) adenine. A block of adenosine deaminase alone, interruption of methylation reactions by this inactiva- or addition of deoxyguanosine may also have similar tion may potentially contribute to the immunodefi- cytotoxicity or immunosuppressive effects.43*74*96-‘” 622 IRVING H. FOX

The biochemical features of cells susceptible to tion in B-lymphoblasts may result from the high deoxynucleoside toxicity have been defined using activity of S-nucleotidase in the cytoplasmz5 or plasma cultured human lymphoblast cell lines. Deoxyadeno- membrane.‘8*‘0’*‘02 The high 5’-nucleotidase activity sine is selectively toxic to cultured T-lymphoblasts may degrade dAMP back to deoxyadenosine, prevent- during adenosine deaminase inhibition.963993’00 Deoxy- ing its conversion to dATP in B-lymphoblasts. A close adenosine mediated cytotoxicity in T-lymphoblasts is correlation exists between plasma membrane 5’- accompanied by increased concentrations of dATP. nucleotidase and the ability to accumulate dATP, T-lymphoblasts have a 20-45fold greater capacity to supporting a relationship between these two variables accumulate deoxyadenosine nucleotides than B- in murine lymphocytes.“’ Since human thymocytes lymphoblasts at deoxyadenosine concentrations of 50 have 5’-nucleotidase activity as low as cultured T- /JM. “‘J’~ The accumulation of deoxyadenosine nucleo- lymphoblasts,58~‘~‘06 human thymocytes may share tides in T-lymphoblasts may result from the small common properties with T-lymphoblasts and may be a quantity of S-nucleotidase activity either on the cell at risk for toxicity when deoxynucleosides accu- plasma membrane58*‘0’3’02 or in the cytoplasm.2s Simi- mulate. larly, the lack of deoxyadenosine nucleotide accumula- How does deoxynucleoside triphosphate accumula-

ADP GDP UDP CDP mm--- -II--- 1 dATP------7 pJ 3 7 3 dADP t t t dGDP t dUDP t dCDP dADP 1 + + t t +dATP t dGTP t dTTP t dCTP dAMP

t dADENOSlNE

# piiq DNA + SYNTHESIS dlNOSINE

A

ADP GDP UDP CDP ----I --I-I RR ----dGTP+ rt I dADP t ddDP + dU’DP t d&P t dGDP + + + I dATP + dGTP t dTTP t dCTP t

\\ // dGI++E ,

DNA PNP + SYNTHESIS GUANINE B

Fig. 3. hypothesis. (FlRI is the enzyme that converts ADP. GDP, UDP, and CDP to dADP, dGDP, dUDP, and dCDP. respectively. IA) In edenosine deaminese (ADA) deficiency, deoxyadenosine cannot be deamineted to deoxyinosine. and is thus phosphoryleted to dAMP, dADP. and dATP. The accumulation of dATP potently inhibits all the reactions of ribonucleotide reductsse. This potent inhibitory effect may decrease the synthesis of dGDP. (IUDP, and dCDP. reduce the substrates available for DNA synthesis and inhibit DNA synthesis. IB) In purine nucleoside phosphorylese (PNP) deficiency. deoxyguanosine, which ordinarily undergoes phosphorolysis to guanine is phosphorylated to dGMP. dGDP, and (IGTP. Deoxyguanosine triphosphate accumulates in purine nucleoside phosphorylase deficiency end is e potent inhibitor of three of the four components of the ribonucleotide reductese reaction. Inhibition of these reactions can lead to a reduction of DNA synthesis. PURINE NUCLEOTIDE DEGRADATION 623 tion cause cell toxicity? A block at ribonucleotide / t S-Adenyylmethwkw reductase is currently the most popular hypothe- sis43.72.73.97,98 (Fig. 3). This reaction leads to the synthe- sis of the triphosphate substrates 4 necessary for DNA formation. It is potently inhibited S-Adenosylhomocysteine by dATP. A block at ribonucleotide reductase from ,“ilCtl”~tlO” the accumulated dATP could explain immune f--_-____-----~ dysfunction in the deficiencies of adenosine deaminase t \ (Fig. 3). Resistance to deoxynucleoside toxicity with mutations of ribonucleotide reductase support the role of this enzyme in S-49 mouse T-lymphoma ce11s.‘07 t Deoxyadenosme The ribonucleotide reductase hypothesis is further strengthened by the reduction of DNA synthesis, but not RNA synthesis, during incubation with 50 PM deoxyadenosine and an adenosine deaminase inhibitor in cultured human cells.‘08,‘W However, the pattern of Fig. 4. Possible altered methyl&ion in adenosine deaminase deficiency. This diagram shows the pathway involved in methyl- inhibition of DNA synthesis in these experiments was transfer reactions catalyzed by S-adenosylmethionine. The prod- not similar to that expected for ribonucleotide reduc- uct of these methyltransfer reactions is S-adenosylhomocysteine tase inhibition.‘08,‘09 Therefore, an additional or alter- (SAHL a potent inhibitor of the S-adenosylmethionine mediated methykransfer reactions. S-adenosylhomocysteine is removed by native mechanism of block of DNA synthesis needs to its degradation to homocysteine and adenosine. The adenosine is be considered. normally deaminated to inosine, thus ensuring the irreversibility of Inhibition of intracellular methylation reactions S-adenosylhomocysteine hydrolase. In adenosine deaminase (ADAI deficiency, there is an accumulation of adenosine end this provides an additional hypothesis to explain immune may reverse the S-adenosylhomocysteine hydrolase reaction and dysfunction. This possibility originates from the secon- cause an accumulation of S-adenosylhomocysteine. As well, in dary decrease of erythrocyte S-adenosylhomocysteine adenosine deaminase deficiency, deoxyadenosine accumulates and this is known to cause suicide inactivation of S-adenysylho- hydrolase in adenosine deaminase deficiency.7x,79 An mocysteine hydrolase. which may lead to an accumulation of accumulation of S-adenosylhomocysteine, which may S-adenosylhomocysteine. The increased concentration of S- result from the hydrolase deficiency, inhibits methyla- adenosylhomocysteine could inhibit methyltransfer reactions and lead to toxic effects. tion reactions and has cytotoxic and immunosuppres- sive activities.78.“0-“4 Although the exact mechanism for toxicity remains unclear, recent studies indicate immune responsiveness and immune cytolysis.“9~‘2’ that enzymatic methylation of phospholipids may be Elevated concentrations of CAMP may be responsible critical for the transduction of mediated for decreased platelet aggregation in adenosine deami- signals through cell membranes.“5 A block of this nase deficiency providing evidence for the operation of reaction may also be toxic to cells of the human this mechanism. Increased levels of CAMP in cells immune system. Deoxyadenosine, which accumulates remain a possible mechanism of immunodeficiency in in adenosine deaminase deficiency, irreversibly inacti- adenosine deaminase deficiency. vates S-adenosylhomocysteine hydro1ase78,“4 and Biochemical mechanisms other than the ones probably accounts for the secondary hydrolase defi- discussed above may account for the association ciency (Fig. 4). However, this proposal remains between purine enzyme defects and immune disorders unproven since elevated concentrations of S-adenosyl- or more than one mechanism may be involved. Direct homocysteine have not been observed in cells from proof for a particular mechanism by its demonstration patients with adenosine deaminase deficiency. The in the tissues of the enzyme deficient patient is not yet selectivity of this mechanism may be directed toward available. B-lymphocytes.“6 Purine nucleoside phosphorylase dejiciency. Pu- The hypothesis that increased levels of CAMP rine nucleoside phosphorylase deficiency was first concentrations account for the immune dysfunction described in 1975 in association with a disturbance of has been proposed based upon elevated CAMP concen- cellular immunity.‘22 This autosomal recessive distur- trations in leukocytes and platelets from patients with bance of T-cell function is characterized by a marked adenosine deaminase deficiency.70,77 Adenosine in- reduction in T-lymphocyte numbers and by a lack of a creases CAMP concentrations in cells derived from the proliferative response of lymphocytes to mitogens and immune system by activating an allogeneic cells. The normal induction of T-cell matu- and stimulating plasma membrane adenylate cy- ration in bone marrow precursors by human thymic clase.“7 ‘*OIncreased concentrations of CAMP inhibit epithelium conditioned medium or thymosin in two 624 IRVING H. FOX brothers with purine nucleoside phosphorylase defi- forming cells without an increase in the lymphocyte ciency suggest normal T-cell generation and intact count. The lymphocytes stimulate with phytohemag- thymic epithelial function.‘23 glutinin. A slight reaction to tests became evident Humoral immune function appears intact or in one patient. Transfusion therapy in these patients enhanced as shown by normal or elevated immuno- increases the serum urate concentration and the urine globulin concentrations, normal numbers of peripheral uric acid excretion and decreases urine nucleoside B-lymphocytes, and normal response to anti- eXcretion.‘*3.‘35”36 Partial and transient responses to gens. The occurrence of monoclonal gammopathy, thymosin administration or thymus epithelial trans- rheumatoid factor, antinuclear antibody, and Coombs- plants are reported.‘l’ Administration of and positive hemolytic anemia indicate excessive antibody hypoxanthine with or without has no production.‘23m’2s Features resembling systemic lupus apparent beneficial effect on the immune function. erythematosus are evident in some of these patients, Since in vitro studies suggest that deoxycytidine may suggesting that the enzyme deficiency may provide a reverse deoxyadenosine or deoxyguanosine induced model for this autoimmune disorder. toxicity,43*96-‘00 it is possible that this therapy in Patients with this disorder have evidence of a block patients may be helpful. of purine nucleotide degradation. There is decreased The molecular pathology by which the enzyme synthesis of uric acid with hypouricemia, hypouricos- deficiency causes the immune dysfunction may be uria and an excessive urinary excretion of inosine, related to deoxyguanosine accumulation. Deoxygua- guanosine, deoxyinosine, and deoxyguanosine.46*‘229 nosine is toxic to T-lymphoblasts and not to B-lympho- ‘*u*’ Fructose infusion indicates a block of purine blasts. 9~98~‘38T-lymphoblasts accumulate dGTP and nucleotide degradation by no change in serum urate B-lymphoblasts do not. Inhibition of ribonucleotide concentration or urinary uric acid and oxypurine reductase by dGTP and subsequent inhibition of DNA excretion and a massive increase in urinary inosine synthesis is the major mechanism by which toxicity to excretion.46 These patients are overproducers of the immune system may occur’o9 (Fig. 3B). The role of purines despite the hypouricemia. The overproduction a decrease in S-adenosylhomocysteine hydrolase in of purines is accompanied by an elevation in the this disease has not been detined.79 concentration of phosphoribosylpyrophosphate in ery- Myoadenylate deaminase deficiency. A new en- throcytes to a level similar to that observed in patients zyme deficiency associated with myopathy has been with a deficiency of hypoxanthine-guanine phosphori- recognized in a series of 5 cases of 250 biopsies.‘39 All bosyltransferase.‘** This is related to a loss of active patients were young males with muscle weakness or reutilization of purine bases in purine nucleoside phos- cramping after exercise, in many instances since child- phorylase deficiency from the lack of formation of hood. Decreased muscle mass, hypotonia and weak- hypoxanthine or guanine, substrates for hypoxanthine- ness are evident on physical exam. There is a mildly guanine phosphoribosyltransferase. Deoxyguanosine elevated creatine phosphokinase and nonspecifically triphosphate concentrations are elevated in erythro- abnormal electromyograms. Although the patients cytes. 60~‘29~‘30This is believed to muft from the accu- release lactate into the venous blood during exercise, mulation of deoxyguanosine and its subsequent phos- there is a failure to release ammonia. While the phorylation. Increased concentrations of dGTP may muscle biopsy is normal, the stain for adenyIate deam- be the toxic factor to the immune system. The degree inase is negative. Homogenized muscle is deficient in of abnormality in uric acid levels, nucleoside excre- adenylate deaminase, while the enzyme is normal in tion, and dGTP concentrations reflect the severity of erythrocytes and neutrophils. the enzyme deficiency.13’ These patients also have an The original observations about this disorder have 80% decrease in erythrocyte S-adenosylhomocysteine been expanded’40’4’ and now are confirmed by other hydrolase activityy9 but the significance of this obser- workers.‘42 Males and females may have this disorder. vation remains unclear. Three of 6 female patients have a poorly understood Structural alterations of the decreased purine associated collagen disease such as systemic lupus nucleoside phosphorylase provides evidence for struc- erythematosus, polymyositis or mixed connective tural gene mutations and genetic heterogeneity in this tissue disease.‘43 Rabbit antiserum to human purified disorder.‘3*‘34 myoadenylate deaminase reacts with human enzyme Enzyme replacement therapy with blood transfu- from muscle, but not from erythrocytes, neutrophils or sions have only partial success in patients with purine platelets.14’ The data suggest a separate genetic origin nucleoside phosphorylase deficiency.‘35-‘37 Two of four for the muscle enzyme and explains the basis for a patients have modest improvement. There is an deficiency of the muscle enzyme alone. increase in the percentage of peripheral E-rosette The frequency (5-6 per 250 muscle biopsies)‘39*‘42 of PURINE NUCLEOTIDE DEGRADATION 625 the enzyme deficiency and the diversity of patients of adenosine by elevated adenosine deaminase activi- involved have suggested that this enzyme deficiency ty. may represent a normal variant or a subclinical state Hepatic xanthine oxidase is increased in gouty rather than an actual disease.‘42 However, recent patients who exhibit an overproduction of uric acid experiments indicate an important possible role of and in one patient with a partial deficiency of hypo- adenylate deaminase in the maintenance of a normal xanthine-guanine phosphoribosyltransferase.‘473’48 adenine nucleotide ~001.‘~~ ATP depletion is acceler- This may be a secondary change related to the induc- ated in the contracting muscle from a patient with this tion of xanthine oxidase.‘45 Administration of RNA, enzyme deficiency. This may be related to the inter- hypoxanthine, ethylaminothiadiazole or fructose to ruption of the purine nucleotide cycle of muscle.37 human subjects causes liver xanthine oxidase to Xanthine oxidase deficiency. is a increase from 2 to 4 times higher than the control rare disorder characterized by low serum urate group.‘47 concentrations and urinary uric acid excretion and Decreased reutilization and increased sub- elevated urinary excretion of hypoxanthine and strate. Seventy-five percent of adenine nucleotide xanthine (Reviewed in Reference 145). This autoso- degraded to hypoxanthine in normal individuals is ma1 recessively inherited disease is associated with a reutilized to IMP.48 This may represent an important gross deficiency of xanthine oxidase in the jejunal homeostatic mechanism for the maintenance of the mucosa or liver. Xanthine stones, hypoxanthine and nucleotide pool, since a loss of this pathway leads to xanthine in muscle, and have been increased purine excretion and elevated IMP forma- observed in this disorder. Fructose infusion in a patient tion by de novo purine synthesis.‘49 The formation of with xanthine oxidase deficiency shows an increase of increased quantities of nucleotides by this mechanism urinary hypoxanthine and no change in the serum or by increased degradation will provide urate level or urinary uric acid excretion. These increased substrate for the purine catabolic pathway changes illustrate the block in purine nucleotide and will result in an accelerated rate of degradation. degradation at xanthine oxidase.47 The ability to reutilize hypoxanthine is lost in Guanine deaminase deficiency. The complete hypoxanthine-guanine phosphoribosyltranferase defi- absence of guanine deaminase activity and of the ciency.48 Patients with this X-linked enzyme defi- inhibitory has been reported in a single ciency overproduce uric acid and develop hyperuri- newborn, full-term boy, who died two days after cemia and .‘49 The inability to reutilize birth.‘46 No specific clinical features were described. It hypoxanthine is an important contributor to purine remains unclear what is the significance of this asso- overproduction in these patients, since oxidation to ciation of guanine deaminase with newborn death. uric acid then remains the only pathway for hypoxan- thine metabolism. Inability to reutilize hypoxanthine and increased activity of purine de novo Increased Degradation from increased intracellular phosphoribosylpyrophos- Increased nucleotide degradation may occur by a phate together account for the overproduction of uric number of different mechanisms which alter the regu- acid (Fig. 5). In the complete enzyme deficiency, lation of the pathway. If the subsequent purine cata- patients have Lesch-Nyhan syndrome, a disorder bolic pathway is not blocked, this may result in an characterized by self-mutilation, choreoathetosis. increased synthesis of uric acid and may be clinically mental retardation, spasticity and uric acid calculi expressed as hyperuricemia, hyperuricosuria and gout. (Reviewed in Reference 149). In the partial enzyme Not all disorders of nucleotide degradation ultimately deficiency, patients have Kelley-Seegmiller syndrome, alter uric acid synthesis. a disorder characterized by gout, recurrent uric acid Increased enzyme activity. A 4570-fold increase calculi and occasional mild central nervous system of erythrocyte adenosine deaminase activity has been disorders (Reviewed in Reference 149). In purine observed in the kindred with hereditary hemolytic nucleoside phosphorylase deficiency (described anemia.39 Patients with this dominantly inherited above), there is a secondary loss of hypoxanthine entity have a mild anemia and a decrease of erythro- reutilization and increased de novo purine synthesis. cyte adenine nucleotide levels to less than 50% of that This results from the inability to form hypoxanthine, a comparable with reticulocyte-rich blood. The de- product of purine nucleoside phosphorylase, and creased erythrocyte adenine nucleotide concentration increased intracellular phosphoribosylpyrophosphate appears to be responsible for the hemolytic anemia. levels, respectively. This may result from diminished reutilization of The complete deficiency of adenine phosphoribosyl- adenosine to AMP as a result of excessive destruction is characterized by a loss of the ability to 626 IRVING H. FOX

GLUTAMINE nucleotide substrate for degradation. This may result in increased synthesis of uric acid. Hematological

+[ 1 malignancies or other hematological disorders leading to bone marrow hyperplasia classically cause hyperu- ricemia on this basis. In psoriasis, the increased turn- over of epithelial tissue may lead to hyperuricemia by means of increased synthesis of uric acid. Decreased ATP. A sudden diminution of ATP t REUTILIZATION concentration and the resultant elevation of AMP and IMP levels appear to activate a cascade of nucleotide t HYPOXANTHINE I breakdown to purine catabolic intermediates and uric t acid. A substantial decrease in the intracellular t XANTHINE concentration of ATP may result in profound morpho- logical and functional changes with persistent ATP 1 4 URIC ACID levels of 20%25% of control values.“’ In the myocar- dium there is a correlation between a diminution of Fig. 5. Mechanism for hyperuricemia in hypoxanthine-guan- ATP content of muscle and impairment of left ventric- ine phosphoribosyltransfeferase(HGPRTI deficiency. The absence of HGPRT leads to a loss in the ability to reutilize hypoxanthine. ular function.‘56 Hypophosphatemia leads to decreased Thus all hypoxanthine formed is oxidized to uric acid. The ATP concentrations and ’57m’59 or even decreased hypoxanthine reutilization leads to a sparing of PP- .‘60 In myocardial the degree ribose-P, the other substrate for HGPRT. PP-ribose-P is a rate- limiting substrate for purine biosynthesis de nova. The resultant of ATP loss is correlated with a lethal .‘6”‘62 elevated intracellular concentration of PP-ribose-P causes an (A) increased degradation of ATP. Increased increase in purine biosynthesis de nova. Thus both decreased degradation of ATP has been described above for the reutilization of hypoxanthine and increased purine biosynthesis de nova lead to overproduction of uric acid in HGPRT deficiency. fructose infusion model, where ATP is consumed in the formation of fructose-l-P, and in muscular exer- cise, where ATP is consumed during contraction of reutilize adenine. There results an excessive excretion and myosin. of adenine and its oxidation product, 2,8-dihydrox- Hyperuricemia, gouty and uric acid calculi adenine.‘50~15’ The increased degradation of these may complicate the clinical course of Glycogen Stor- compounds is small in relation to total purine excre- age Disease Type I.‘63.‘64 The glucose-6-phosphatase tion. However, 2,8-dihydroxyadenine is insoluble and deficiency may activate a mechanism similar to fruc- stones composed of this compound are formed. The tose-induced hyperuricemia.‘h5m’67 In this disorder the major feature of almost all patients with this autoso- triggering mechanism for increased production of uric ma1 recessively inherited disorder is recurrent renal acid may be the insulin counter-regulatory hormonal calculi formed from adenine and its oxidation prod- response to hypoglycemia and the inability to synthe- ucts. Since these stones are similar to uric acid, they size glucose (Fig. 6). Recent studies indicate that may be erroneously identified as such. These patients parenteral causes a rise in the serum urate have a normal serum urate concentration and no level and urinary uric acid excretion in enzyme defi- evidence for gout. In contrast to the complete deficien- cient patients.‘66 This activity of glucagon is accom- cy, the partial deficiency of adenine phosphoribosyl- panied by a reduction in hepatic ATP concentrations transferase has no definite associated clinical and a marked elevation of glucose-6-phosphate, fruc- features.15* tose-6-phosphate and fructose- 1,6-diphosphate.16’ The Increased substrate alone may cause elevated depletion of ATP and the trapping of inorganic phos- purine nucleotide degradation. An error of purine phate in the form of phosphorylated sugar compounds metabolism, increased activity of phosphoribosylpyro- set the stage for activation of purine nucleotide degra- phosphate synthetase, leads to increased intracellular dation and may explain the increased synthesis of uric concentrations of phosphoribosylpyrophosphate and acid (Fig. 6). The elevated rate of purine biosynthesis increased de novo purine synthesis (Reviewed in de novo observed in this disease’63.‘64 may result secon- Reference 153). In this rare X-linked disorder’54 there darily from the depletion of the adenine nucleotide is a massive overproduction of uric acid with gout and pool. Increased lactate formation inhibits the renal uric acid calculi, but no evidence of central nervous excretion of urate and accentuates the hyperuricemia. system dysfunction. A large tumor or hyperplastic Further support for the operation of these mechanisms tissue may have increased turnover of nucleic acid is derived from the therapeutic approach of providing (Reviewed in Reference 153) and generate increased continuous nutrition and preventing hypoglycemia and PURINE NUCLEOTIDE DEGRADATION 627

GLYCOGEN 4 GLUCAGON

+ PHOSPHORYLASE (a) +--- I4 ,

+ GLUCOSE-6-P

4 FRUCTOSE -6-P

Fig. 6. Mechanisms for hyparuricamia in glucose-6-phos- phataaa (GL-6-P’ASE) deficiency. Hypoglycemia may be the trigger for the abnormalities in this enzyme deficiency. Hypo- glycemia cauaa5 glucagon release and this activates glycogan phoaphorylaaa to dagrada glycogan to glucose-6-phosphate. There results an increased intracellular concentration of glucose-(i-phosphate, fructose-bphoaphate and fructose-l .6- dipho5phate. These events lead to a depletion of ATP concan- trationa, an activation of Purina nucleotide degradation and 4 XANTHINE increased synthesis of uric acid. The increased accumulation ) RENAL EXCRETION of phoaphorylatad sugars leads to hyparlacticacidamia and this OF URATE decreases the renal axcratin of urata. 4 URI;ACID

the associated hormonal response. This treatment A lack of any of these components will impair ATP corrects the hyperuricemia, and hyperuricosuria and synthesis. The resultant decrease of ATP concentra- other metabolic abnormalities which accompany this tion may trigger a cascade of purine nucleotide degra- enzyme deficiency.‘65.‘67 dation as described above. Tissue hypoxia and hypo- The hyperuricemia accompanying disorders of fruc- phosphatemia are recognized causes of decreased ATP tose metabolism may result from similar mechanisms. synthesis. The effects of hypophosphatemia were Hypoglycemia complicates fructose- 1,&diphospha- described above.‘s7~i60 tase deficiency. This could contribute to the observed Tissue hypoxia has been a well recognized cause of hyperuricemia by increased production of uric acid cellular adenine nucleotide depletion. The adenine and decreased urate excretion from the hyperlacticaci- nucleotide depletion is accompanied by the appear- demia in a manner similar to glucose-6-phosphatase ance of purine catabolic intermediates, which are deficiency. In hereditary fructose intolerance there is a metabolic markers for tissue ATP degration. In block in the further metabolism of fructose-l-phos- ischemic mammalian myocardium there is a marked phate and this compound may accumulate.‘68 Fruc- decrease in ATP and creatine phosphate levels, with tose-induced hyperuricemia is more pronounced in an increase in organic phosphate, inosine, adenosine, these patients, whose disease state is worsened by IMP, and a small increase in ADP and AMP concen- administration of this sugar in any amount. Thus trations.‘62.‘69.‘70 Inorganic phosphate, inosine, and fructose intake may trigger increased uric acid synthe- hypoxanthine have been measured in myocardial sis from the phosphorylation of fructose, an accumula- venous drainage and found to correlate with adenine tion of fructose- 1-phosphate, subsequent depletion of nucleotide and phosphocreatine depletion over hypoxia hepatic ATP concentrations, and activation of purine in dog heart muscle.“’ In sequential biopsies during nucleotide degradation. Hereditary fructose intoler- experimental in dogs a substan- ance is treated by limiting fructose ingestion.‘@ This tial diminution of ATP content occurs in both avoids the activation of purine nucleotide degradation. infarcted and noninfarcted areas of the heart as (B) Decreased synthesis of ATP. ATP is formed compared to uninjured cardiac muscle.‘72.‘73 Similar by mitochondrial respiratory-chain phosphorylation, mechanisms appear to exist in humans since elevated using ADP, O,, and inorganic phosphate as substrates concentrations of adenosine or hypoxanthine have according to the following overall equation: been detected in coronary sinus blood of patients with angina pectoris induced by atria1 pacing.‘74 “’ 3ADP + 3Pi + l/2 O2 Evidence for depletion of tissue adenine nucleotides + NADH + H’ - 3ATP + 4Hz0 + NAD’ and release of purine nucleotide degradation interme- 628 IRVING H. FOX diates have been observed in other situations including the pathway at xanthine oxidase and preventing the hemorrhagic shock, hypoxia, hypothermia, renal conversion of hypoxanthine to useless metabolic end &hernia, hyperthermic stress, muscle ischemia, and products. The accumulated hypoxanthine could be brain ischemia in animals.“‘-‘93 Elevated blood levels reutilized to form nucleotides and ultimately ATP.‘” of uric acid, or hypoxanthine provide a useful In tissues known not to contain xanthine oxidase, plasma correlation with these changes. Allantoin, an aliopurinol may be active at another site either directly oxidation product of uric acid, is the end-product of or as one of its metabolites2’ In humans, allopurinol is in certain animals. In humans, rapidly oxidized to by xanthine oxidase. hyperuricemia may result from diseases leading to Allopurinol or oxipurinol may be phosphorylated to tissue hypoxia. In acute myocardial infarction in man ribonucleotide derivatives.2’62’6 A modification of there is hyperuricemia with an expansion of the uric purine nucleotide degradation by allopurinol is acid pool and an increased uric acid turnover supported by its stimulation of hypoxanthine or rate. I96197 Hyperuricemia also occurs during human adenine uptake into myocardial cell nucleotides.2’7 circulatory collapse,“’ smoke inhalation,‘99 respiratory Pretreatment with allopurinol usually improves acidosis,*“” and decompensation in chronic bronchi- function and nucleotide levels, but treatment after the tis.*” Increased serum urate concentrations and acute event may not be consistently effective. Sodium urinary uric acid excretion are observed to accompany allopurinol in dosages ranging from 50-100 mg/kg perinatal hypoxia.“* Respiratory distress syndrome or has been shown to reverse the effects of experimental other hypoxic problems are accompanied by more myocardial hypoxia,2’s*2’9 and irreversible hemor- pronounced increases of these variables*‘* and by rhagic shock in dogs treated with hypoxanthine.‘*’ In a elevated plasma,*03 cerebrospinal fluid,*04 urinary205 or similar experiment, allopurinol, adenine, hypoxan- renal tissue206 hypoxanthine concentration. thine and oxaloacetate produced the best survival The prevention of further purine nucleotide degra- (43%).222 Allopurinol has also been found to preserve dation and stimulation of ATP synthesis together with kidneys223.224 and small intestine”’ and to increase the reversal of the precipitating factor may provide an hepatic adenine nucleotide resynthesis after olige- optimal approach for the management of patients with mia,226 further demonstrating a potential role in main- decreased synthesis of ATP. The consequences of taining normal cell integrity during ischemia. hypophosphatemia may be managed by inorganic However, this drug did not modify infarct size when phosphate replacement therapy. In tissue hypoxia, administered 15 minutes after infarction,227 perhaps increased synthesis of ATP may be promoted by because an irreversible depletion of ATP had already reversal of hypoxia and by different combinations of occurred. allopurinol, hypoxanthine, adenine, inosine, Kreb’s The application of these promising experimental cycle intermediates, and glucose, potassium and observations to human disease processes that have insulin. Therapy of myocardial infarction is aimed at proven refractory to current management appears to increasing ATP synthesis and decreasing its utilization be indicated. by improving myocardial perfusion, augmenting ATP production by anerobic glucolysis with glucose- SUMMARY AND CONCLUSIONS insulin-potassium and hypertonic glucose, and reduc- The pathway of purine nucleotide degradation is a ing myocardial ATP consumption by beta blocking regulated series of reactions by which purine ribonu- agents and balloon counter-pulsation.*” Glucose, cleotides are degraded to uric acid in man. Two major insulin and potassium or inhalation of an rich categories of disorders occur. A block of degradation is gas mixture have been found to minimize cardiac associated with syndromes involving immune dysfunc- necrosis and to maintain ATP levels in the infarcting tion, myopathy or renal calculi. Increased degradation myocardium2” or to have an inotropic response in of nucleotides occurs with syndromes characterized by congestive heart failure.*@’ Inosine, a purine nucleo- hyperuricemia, gout, renal calculi, anemia or acute side, improves the performance of ischemic myocar- hypoxia. Some disorders associated with increased dium2” and enhances the preservation of ischemic purine nucleotide degradation are characterized by .*” Inosine increases ATP levels by its degrada- marked decreases in intracellular ATP concentration tion to hypoxanthine and ribose-l-phosphate, com- and increases in the serum urate concentration and pounds which promote ATP synthesis. ATP-Mg elevated uric acid, oxypurine, and inosine excretion. increases ATP concentrations during hepatic isch- Management of disorders of purine nucleotide emia.2’2 degradation is dependent upon an understanding of Allopurinol may be useful in the management of the metabolic mechanisms of the disease state. In disorders of purine nucleotide degradation by blocking blocks of the pathway, symptomatic therapy may PURINE NUCLEOTIDE DEGRADATION 629

reverse the consequence of the block. In other disorder, to inhibit further purine nucleotide degrada- instances, specific biochemical therapy to correct an tion and to stimulate ATP synthesis. enzymatic defect has been attempted, including Application of the promising experimental observa- enzyme replacement therapy. Patients with increased tions about the molecular pathology underlying disor- purine nucleotide degradation with overproduction of ders of purine nucleotide degradation will provide uric acid need protection from the adverse effects of innovative approaches to diagnosis and management excess uric acid production. This is achieved by inhib- of the associated diseases. Already, concepts of iting uric acid synthesis with allopurinol. Patients with enzyme blocks and alteration of immune function has disorders leading to decreased intracellular concentra- lead to clinical trials of deoxycoformycin, a potent tions of ATP require therapy to reverse the underlying inhibitor of adenosine deaminase.22s-23’

REFERENCES

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