REVIEW J Am Soc Nephrol 12: 2181–2189, 2001

The Kidney and Metabolism

ALLON N. FRIEDMAN,*† ANDREW G. BOSTOM,*‡ JACOB SELHUB,* ANDREW S. LEVEY,† and IRWIN H. ROSENBERG* *Vitamin Metabolism and Aging, Jean Mayer United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, Massachusetts; †Division of Nephrology, Tufts University-New England Medical Center, Boston, Massachusetts; and ‡Division of General Internal Medicine, Memorial Hospital of Rhode Island, Pawtucket, Rhode Island.

Abstract. Homocysteine (Hcy) is an intermediate of methio- renal Hcy handling and should be considered when measuring nine metabolism that, at elevated levels, is an independent risk Hcy plasma flux and renal clearance. The underlying cause of factor for vascular disease and atherothrombosis. Patients with hyperhomocysteinemia in renal disease is not entirely under- renal disease, who exhibit unusually high rates of cardiovas- stood but seems to involve reduced clearance of plasma Hcy. cular morbidity and death, tend to be hyperhomocysteinemic, This reduction may be attributable to defective renal clearance particularly as renal function declines. This observation and the and/or extrarenal clearance and metabolism, the latter possibly inverse relationship between Hcy levels and GFR implicate the resulting from retained uremic inhibitory substances. Although kidney as an important participant in Hcy handling. The nor- the currently available evidence is not conclusive, it seems mal kidney plays a major role in plasma clearance more likely that a reduction in renal Hcy clearance and me- and metabolism. The existence in the kidney of specific Hcy tabolism is the cause of the hyperhomocysteinemic state. Ef- uptake mechanisms and Hcy-metabolizing enzymes suggests forts to resolve this important issue will advance the search for that this role extends to Hcy. Dietary protein intake may affect effective Hcy-lowering therapies in patients with renal disease.

Homocysteine (Hcy) is an amino acid metabolite that has been introduces the issues of Hcy plasma flux, protein binding, and implicated, in retrospective and prospective studies, as a potential general metabolism, and reviews amino acid/Hcy metabolism in atherogenic agent and risk factor for cardiovascular disease normal kidneys, including the effects of dietary intake on this (CVD) (1–3). Hyperhomocysteinemia, the state of elevated process. It then reviews the association between Hcy and the plasma Hcy levels, is very common among patients with chronic GFR, amino acid/Hcy metabolism in diseased kidneys, and in- renal insufficiency (defined as the range of kidney function below sights gleaned from Hcy-lowering trials. It concludes with the normal but above that requiring renal replacement therapy) and hypothesis that the hyperhomocysteinemia of renal disease is occurs almost uniformly in the end-stage renal disease (ESRD) primarily attributable to reduced renal clearance and intrarenal population (4). This is of particular importance for the latter group metabolism. of patients, in which CVD is the major cause of death. In fact, these patients may have up to a 30 times higher risk of CVD- related death than the general population (5). Interestingly, tradi- Plasma Hcy Levels, Protein Binding, and Flux tional cardiac risk factors may not be able to account for this high Average fasting plasma total Hcy levels for healthy human mortality rate (6). Despite the abundance of data on Hcy metab- subjects in the current era of flour and grain folic acid fortifi- olism, the pathogenesis of hyperhomocysteinemia in renal disease cation range between 6 and 12 ␮M (Table 1) (7), with “mod- remains unclear. A better understanding of this deranged state erate” hyperhomocysteinemia occurring when levels are be- would advance current knowledge of renal physiologic processes, tween 12 and 30 ␮M, “intermediate” hyperhomocysteinemia as well as efforts to find an effective therapy. Although there is occurring when levels are between 31 and 100 ␮M, and “se- abundant evidence suggesting that the kidney plays a prominent vere” hyperhomocysteinemia occurring when levels are greater role in Hcy metabolism, there is considerable controversy sur- than 100 ␮M (8). In normal subjects, approximately 75% of rounding the extent and mechanisms of this role. This article total plasma Hcy is bound via a disulfide bond, to protein, primarily albumin, [bound Hcy (bHcy)], while the remaining 25% exists in a free unbound form [free Hcy (fHcy)] (Figure 1) Received January 24, 2001. Accepted March 24, 2001. (9,10). fHcy is composed almost entirely of oxidized, disul- Correspondence to Dr. Allon N. Friedman, Vitamin Metabolism and Aging, fide-linked heterodimers (Hcy-) or homodimers (Hcy- Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts Hcy, or homocystine), with perhaps 1 to 2% existing in a University, 711 Washington Street, Room 829, Boston, MA 02111. Phone: 617-556-3007; Fax: 617-556-3166; E-mail: [email protected] reduced sulfhydryl state (11). Unfortunately, fHcy is inherently 1046-6673/1210-2181 unstable, and accurate levels may be difficult to measure. Of Journal of the American Society of Nephrology importance, only the fHcy fraction is thought to be freely Copyright © 2001 by the American Society of Nephrology filtered at the glomerulus. The fHcy/bHcy ratio varies among 2182 Journal of the American Society of Nephrology J Am Soc Nephrol 12: 2181–2189, 2001

Table 1. Comparison of ranges of total Hcy levels in different populationsa

Geometric Means 10th to 90th Percentile Group (␮M) Total Hcy Levels (␮M)

End-stage renal disease 24 12 to 39 Renal transplantationb 15 9to25 Chronic renal insufficiencyb 15 9to25 Normal renal function (population-based controls)c 9 6 to 12

a Hcy, homocysteine. Modified from reference 7, with permission. b Renal transplant recipients receiving standard immunosuppressive therapy and patients with chronic renal insufficiency receiving no immunosuppressive drugs, with equivalent renal function. c In the current era of folic acid-fortified cereal grain flour. species. For example, in contrast to humans, approximately 65 Hyperhomocysteinemia is a condition in which the regula- to 75% of Hcy in rats is in the free form (12). tion of intracellular Hcy levels is disrupted and Hcy export to Hcy production occurs in all cells as a consequence of the the plasma compartment is accelerated and/or normal Hcy normal methylation process (Figure 2). The Hcy volume of dis- plasma clearance is decreased. Human kinetic studies suggest tribution in healthy subjects was observed to be approximately 0.4 that hyperhomocysteinemia in those with vitamin B12 or folate L/kg, similar to that in subjects with severe renal insufficiency deficiency is caused by enhanced tissue export of Hcy (19). In (13). Intracellular Hcy levels rise with enhanced intracellular Hcy contrast, hyperhomocysteinemia in renal disease is related to production and/or inhibition of intracellular metabolism. To main- reduced plasma Hcy clearance (13). The underlying cause of tain low intracellular levels of this putatively cytotoxic substance, this reduction is unknown but involves a defect in renal and/or Hcy that is not metabolized within the cell is exported to the extrarenal clearance. As noted below, the kidney likely plays plasma compartment (14,15). Calculations based on steady-state an important role in Hcy clearance and metabolism. kinetics in healthy adult humans estimate that 1.2 mmol of Hcy, or approximately 5 to 10% of the total daily cellular production, General Metabolism is delivered daily to the plasma compartment (16,17). Because Hcy is an endogenous sulfur-containing amino acid inter- Hcy is constantly produced and exported by cells, it must also be mediate of the essential amino acid and is not constantly cleared for plasma levels to remain within 10% of obtained from the diet. An overview of the metabolic pathway baseline values, as they do in healthy human subjects (18). Plasma is presented in Figure 2. Methionine enters the one-carbon Hcy levels are not known to be actively regulated. metabolic cycle either through the dietary consumption of

Figure 1. Homocysteine (Hcy) and the major related disulfides in normal human plasma. Reprinted from reference 9, with permission. J Am Soc Nephrol 12: 2181–2189, 2001 The Kidney and Hcy Metabolism 2183

Figure 2. Hcy metabolism. Enzyme reactions that are regulated by S-adenosylmethionine (SAM) and 5-methyltetrahydrofolate (MethylTHF) are indicated by large arrows. Open arrows indicate activation, and closed arrows indicate inhibition. Enzymes are as follows: 1, 5,10- methylenetetrahydrofolate reductase; 2, methionine synthase; 3, S-adenosylmethionine synthase; 4, S-adenosylhomocysteine hydrolase; 5, ␤-synthase; 6, betaine/Hcy methyltransferase; 7, N-methyltransferase; 8, hydroxymethyltransferase; 9, cystathio- nase. THF, tetrahydrofolate; FAD, flavin adenine dinucleotide; PLP, pyridoxal-5-phosphate. Modified from reference 4, with permission. methionine-containing protein or through endogenous protein Folate or B vitamin deficiencies and inborn errors of me- breakdown. It is then converted intracellularly to S-adenosyl- tabolism are well recognized causes of hyperhomocysteinemia methionine, which functions as a universal methyl donor for a (14). Folate and vitamin B12 deficiencies cause fasting Hcy variety of important acceptors, including nucleic acids, neuro- levels to increase by impairing Hcy remethylation. Vitamin B6 transmitters, hormones, and phospholipids. S-Adenosylhomo- deficiency, in contrast, primarily affects transsulfuration and cysteine, a byproduct of these reactions, is hydrolyzed to form tends to induce hyperhomocysteinemia after a methionine load Hcy and adenosine. This reaction actually favors the produc- (14). Most healthy people without inborn errors of metabolism tion of S-adenosylhomocysteine, although the normally rapid can be successfully treated for hyperhomocysteinemia with egress or metabolism of intracellular Hcy and adenosine allows modest vitamin supplementation, such as daily doses of 0.4 mg this reaction to continue forward (20). Hcy then follows one of of folic acid (21). Despite the greatly increased prevalence of the following two metabolic pathways: (1) remethylation to hyperhomocysteinemia among patients with renal disease, methionine by methionine synthase using vitamin B12 (cobal- these patients are not disproportionately affected by vitamin amin) as a cofactor and 5-methyltetrahydrofolate as a substrate; deficiencies (22) or inborn metabolic defects (23). or, alternatively, by betaine/Hcy methyltransferase in the pres- ence of betaine (in human subjects, the latter reaction is mainly Normal Kidneys and Hcy Handling confined to the liver and kidney); (2) transsulfuration to cys- Amino Acid Metabolism tathionine by cystathionine ␤-synthase, in an irreversible vita- It is well established that the kidney plays a major role in min B6 (pyridoxal-5-phosphate)-dependent reaction; cystathi- amino acid metabolism. Unbound amino acids are freely fil- onine is then degraded by cystathionase to ␣-ketobutyrate, tered at the glomerulus, where the filtered load reflects the ammonium, and cysteine. plasma amino acid concentration and GFR. Approximately 450 2184 Journal of the American Society of Nephrology J Am Soc Nephrol 12: 2181–2189, 2001 mmol of amino acids are filtered daily (24). More than 99% of tion and tubular reabsorption. In this case, renal Hcy-metabo- amino acids are then reabsorbed along the proximal renal lizing enzymes would be upregulated. Conversely, when the tubule, with only approximately 5 mmol being ultimately ex- GFR decreases, intrarenal Hcy metabolism also would fall. The creted in the urine (24,25). An alternative mechanism of cel- effect of these alterations ensures that changes in Hcy filtration lular uptake is via the tubule cell basolateral surface. This do not affect renal Hcy export to plasma. In summary, as with occurs mainly in distal tubule cells, possibly to provide meta- other amino acids, the healthy kidney has the capability to bolic substrates to cells in which luminal amino acid delivery filter, reabsorb, and metabolize Hcy, which it may routinely do is decreased (24). Hcy may potentially be taken up in this way in large amounts. In addition to the filtered load, Hcy uptake as well, because cystine, an amino acid that shares an uptake may also occur on the basolateral tubule cell surface. mechanism with homocystine, is known to be taken up in this Arteriovenous (A-V) studies, which represent an accepted manner (24). These tubular metabolic processes can be quite method of measuring the capacity of an organ to extract a complex, with tubule cells producing and exporting certain particular substance from blood, have been performed in nor- amino acids while simultaneously taking up and degrading mal rat and human kidneys. Hcy extraction is determined by others (24–26). simultaneously measuring Hcy concentrations in the arterial system and in the renal vein and calculating the difference, Hcy Metabolism while taking into account renal plasma flow and Hcy lost in the The kidney seems to be just as capable of filtering and urine. A-V studies in rats have consistently demonstrated that metabolizing Hcy as it does other amino acids. Hcy has a the kidney is capable of clearing Hcy in large quantities. A molecular mass of 135 D (27), which is well within the study in six healthy postprandial rats demonstrated a positive filtration range of normal glomeruli. Assuming plasma fHcy renal A-V difference of approximately 20% of arterial levels concentration of 3 ␮M (10) and a normal GFR of 125 ml/min, (28). This value would be equivalent to approximately 1 mmol the daily amount of filtered Hcy would be approximately 0.5 of Hcy in human subjects, or 80% of the load delivered daily mmol. As with other amino acids, there is abundant evidence to plasma. When an acute hyperhomocysteinemic state was that filtered Hcy is avidly reabsorbed and only minimally precipitated by Hcy infusion, rat kidneys manifested apprecia- excreted (6 ␮mol/d, or 1%) in the urine (10,11,28,29). As ble reserve and increased Hcy extraction by fourfold (12). described below, this may be an underestimation of the true However, differences in the fHcy/bHcy ratio among species filtered load of Hcy. make extrapolation to human subjects problematic. The only Tubular uptake mechanisms specific for Hcy have been published A-V study in humans observed that 20 fasting sub- identified. Kinetic studies of minced rat renal cortical tissue jects with normal renal function exhibited, on average, no identified low-Km/high-affinity and high-Km/low-affinity ho- significant renal extraction of Hcy (39). However, differences mocystine uptake systems, the former shared with cystine and in experimental conditions between the studies make direct the dibasic amino acids , , and (30). comparisons difficult. Additionally, the results of that study do This finding is supported by studies in which rats and human not exclude renal extraction of up to 30 to 40% of the presumed subjects exhibited dramatically increased urinary homocystine daily Hcy production. The renal A-V difference may also be excretion after intravenous boluses of arginine and lysine (31) altered in the postprandial state. This is supported by renal A-V or ␣-aminoisobutyric acid (32), an inhibitor of lysine, arginine, experiments in dogs that demonstrated alterations in renal and ornithine tubular reabsorption. A host of studies have amino acid handling after amino acid intake (40) and data on confirmed that animal and human kidneys contain the neces- the effects of protein intake on renal Hcy filtration, as de- sary Hcy-metabolizing enzymes. Rat kidney extracts contain scribed below. transsulfuration (cystathionine ␤-synthase and cystathionase) and remethylation (methionine synthase) enzymes in signifi- Effects of Dietary Intake on Hcy Filtration cant amounts (Figure 2) (33). In human subjects, appreciable Experimental data suggest that food intake affects Hcy pro- levels of these enzymes are found primarily in liver and kidney tein binding and, consequently, Hcy glomerular filtration. A tissue. Compared with liver, the kidney contains more betaine/ carefully performed study in healthy human subjects fed a Hcy methyltransferase (300%) and less cystathionase and me- protein-rich meal demonstrated a significant postprandial in- thionine synthase (30 and 50%, respectively) (14,34). Mea- crease in plasma Hcy levels, which peaked at 8 h (18). Of surements of the enzyme cystathionine ␤-synthase in two particular importance, fHcy levels increased more rapidly and tissue samples identified small amounts, compared with liver. to a greater extent than did bHcy levels, peaking 2 to 4 h after However, cystathionine ␤-synthase gene expression has been the meal by 34 Ϯ 20%, or 0.6 Ϯ 0.3 ␮M (mean Ϯ SD), documented in the kidney (35,36). Although both enzymatic compared with premeal levels. The fHcy/bHcy ratio also in- pathways can theoretically be used, in vitro and in vivo rat creased, by a mean of 49 Ϯ 16%. The authors observed experiments demonstrated that Hcy was metabolized primarily synchronous fluctuations in the free/bound ratios of aminothi- via transsulfuration (12,37). Some hypothesize that the kidney ols (Hcy, cysteine, and cysteineglycine). They hypothesized may compensate for changes in glomerular filtration by up- or that a change in the free/bound levels of one of the aminothiols downregulating these biochemical pathways, thereby keeping triggers a cascade of thiol-disulfide exchange reactions, lead- constant the amount of Hcy returned to plasma (38). Under this ing to altered ratios for the others. These intriguing results are model, an increase in GFR would lead to increased Hcy filtra- supported by previous studies in healthy and homocystinuric J Am Soc Nephrol 12: 2181–2189, 2001 The Kidney and Hcy Metabolism 2185 subjects (41–43). Studies that did not demonstrate an increase methyl group donations from S-adenosylmethionine and is in postprandial fHcy or total Hcy levels were limited by dietary therefore closely linked to Hcy production (Figure 2) (17). It is protein load size or inadequate follow-up periods or lacked not clear, then, whether inversely predicts Hcy levels fractionated Hcy measurements (44–46). because it is a marker of GFR or simply because it is linked to These results may help explain how diet-related increases in Hcy production via a common biochemical pathway. Studies Hcy levels are normalized. Dietary methionine-containing pro- using highly accurate GFR measurements (with iohexol and tein loads increase plasma levels of Hcy and cysteine. Disul- 51Cr-ethylenediaminetetraacetate) were performed in healthy fide exchange reactions and competition for protein binding and diabetic adults, a large proportion of whom exhibited cause a disproportionate increase in fHcy levels, allowing normal serum creatinine levels (Յ1.3 to 1.5 mg/dl) (38,54,55). excess Hcy in the free form to be filtered by the kidney. The Those studies confirmed the strong inverse relationship be- increase in fHcy levels may also accelerate extrarenal uptake. tween Hcy levels and renal function, even in the range of Therefore, a combination of increased renal Hcy filtration and minimally reduced to supranormal GFR (60 to 165 ml/min). possibly upregulation of intrarenal degradative pathways They also demonstrated that creatinine lost its predictive value would lead to increased plasma clearance. after controlling for GFR, suggesting that the predictive Recent data on the effects of N-acetylcysteine (NAC), a strength of creatinine is associated with its role as a GFR mucolytic agent and treatment for acetaminophen overdose, on marker. Other, more sensitive, markers of GFR, such as cys- aminothiol protein binding and renal excretion support this tatin C, confirm the relationship between GFR and Hcy by hypothesis. NAC has a free sulfhydryl group, which preferen- independently predicting fasting Hcy levels in renal transplant tially interacts with cysteine and other endogenous sulfhydryl- recipients and patients with coronary artery disease with nor- containing substances, including Hcy, displacing them from mal plasma creatinine levels (56,57). In conclusion, a strik- their protein binding sites and forming mixed disulfides (e.g., ingly consistent inverse relationship exists between Hcy levels NAC-Hcy and NAC-cysteine) (47). Human subjects treated and renal function. It has been observed using multiple GFR with NAC orally or intravenously demonstrate almost imme- markers over a broad range of renal function, from severe renal diate, dramatic, and dose-dependent reductions in total plasma insufficiency to a level of GFR far above the uremic range. Hcy and cysteine levels (48–50). NAC administration also This relationship is powerful indirect evidence that elevated increases the fHcy/bHcy ratio and urinary Hcy and cysteine Hcy levels in renal disease are intimately linked to kidney excretion (49,50). In summary, these results suggest that dis- function. ruption of Hcy protein binding, by either diet or drugs, can transiently increase fHcy levels, leading to increased glomer- Plasma Protein Binding ular filtration and renal metabolism. Absolute fHcy, bHcy, and total Hcy levels are all increased If this hypothesis is confirmed, it will be necessary to in renal disease, although the proportion of fHcy remains reassess the calculated amount of Hcy delivered to the plasma constant or decreases (58,59). Therefore, uremic subjects, who compartment (1.2 mmol). This value was derived from Hcy are often hypoalbuminemic, tend to exhibit a greater propor- loading studies that assumed that Hcy was cleared from plasma tion of protein-bound Hcy. It is possible that retained uremic at a constant rate. However, dietary protein loads may acutely plasma proteins play a role in binding Hcy. Additional studies accelerate renal Hcy clearance, making it difficult to extrapo- are needed to confirm the proportion of fHcy, the effects of late this result to a nonfasting setting. For similar reasons, the retained uremic solutes on Hcy binding, and the potential amount of Hcy filtered daily at the glomeruli will also need to toxicities of reduced Hcy, fHcy, and bHcy in the uremic be recalculated. Not only can dietary protein intake acutely milieu. increase fHcy levels, but protein itself can cause temporary elevations in GFR, further increasing fHcy clearance. The Amino Acid Metabolism mechanism of the latter process is not entirely understood but Amino acid handling and elimination are profoundly altered is probably neurohormonal and/or hemodynamic in nature as a result of uremia-induced malnutrition, retained toxins, (51). deranged hormonal levels, increased urinary amino acid excre- tion, and, of particular relevance here, altered capacity of the Diseased Kidneys and Hcy Handling kidney to clear, degrade, and synthesize certain amino acids Association between Hcy Levels and GFR (25,26,60). Reductions in plasma levels of essential amino Data on the relationship between Hcy levels and GFR are acids, such as , , , , and ly- consistent with the presumption that normal kidneys play a sine, and increases in levels of cystine, , methylhisti- prominent role in plasma Hcy handling. Hcy levels increase as dine, glycine, hydroxyproline, and total nonessential amino renal function declines and progresses to ESRD, with the vast acids are commonly observed in renal disease (25). Although majority (Ͼ85%) of dialysis patients ultimately experiencing normal urinary amino acid excretion is minimal, it increases mild-to-moderate hyperhomocysteinemia (5). GFR values es- with progressive renal insufficiency. This may be because of timated from serum creatinine or calculated creatinine clear- higher plasma concentrations of certain amino acids, leading to ance is consistently and inversely correlated with plasma Hcy increases in the filtered load and subsequent filtrate flow rate in levels (52,53). However, the precursor molecule for creatinine the few remaining functioning nephrons (24). This latter pro- is . Creatine accounts for approximately 90% of all cess tends to decrease the time of contact between amino acids 2186 Journal of the American Society of Nephrology J Am Soc Nephrol 12: 2181–2189, 2001 and renal tubular cells, impairing the ability of these cells to levels of Hcy-metabolizing enzymes were no different than take up and metabolize filtered amino acids (24,25). Increased those in a pair-fed group (65). By finding no significant renal tubular amino acid secretion may also theoretically contribute Hcy extraction in fasting adults, the human A-V study can be to the higher excretion rate. interpreted as indirectly supporting this hypothesis as well.

Renal and Extrarenal Hcy Metabolism Hcy-Lowering Trials The fact that hyperhomocysteinemia in kidney disease is the In contrast to healthy patients, those with renal disease and result of defective clearance of Hcy from plasma rather than hyperhomocysteinemia are partially refractory to all available increased delivery to plasma was demonstrated in a kinetic treatments, including vitamin B6, vitamin B12, folic acid and its study of eight subjects with severe renal insufficiency who reduced forms, betaine, and serine (4). This is a consistent were given oral and intravenous Hcy loads (13). Plasma Hcy finding in patients with renal disease. Moreover, resistance to clearance in these subjects was reduced by 70%, compared treatment increases as renal function declines. Hyperhomocys- with control subjects and subjects with isolated folate or vita- teinemia in chronic renal insufficiency can often be normalized min B12 deficiencies. This result can be ascribed to a decrease with supraphysiologic folic acid treatment, such as doses of 2 in intrarenal Hcy clearance resulting from a reduction in func- to 5 mg/d (66,67), whereas the majority of patients with ESRD tioning renal mass and/or a decrease in extrarenal clearance, fail to normalize with doses as high as 60 mg (68). This perhaps attributable to retained uremic solutes that inhibit phenomenon was reflected in one study that compared the metabolism. Unfortunately, studies of Hcy handling in the responsiveness of renal transplant recipients and dialysis pa- renal disease population are sparse. As with amino acids in tients, with equivalent levels of hyperhomocysteinemia, to general, urinary Hcy excretion increases progressively with supraphysiologic doses of folic acid and B vitamins. Despite renal disease. Subjects with a calculated creatinine clearance of being treated with lower vitamin doses, the renal transplant 19 ml/min and urinary Hcy levels of 1.4 ␮mol/h were esti- recipients exhibited much larger reductions in Hcy levels, with mated to excrete 15% of filtered Hcy via the urine (13). a significantly greater percentage of patients experiencing nor- Whether renal disease affects the specific metabolic pathways malization of levels (50 versus 5%) (22). One important issue is not known, but a stable isotope study with four hyperhomo- confounding the interpretation of these Hcy-lowering studies is cysteinemic subjects undergoing hemodialysis demonstrated that many of them include relatively vitamin-deficient subjects. decreased Hcy remethylation with a trend toward lower trans- In fact, when compliance with a vitamin supplement (e.g., sulfuration, compared with control subjects (61). Nephrocap [Fleming & Co., Fenton, MO] or Nephrovite [R&D The paucity of data on Hcy extraction and metabolism by Laboratories, Marina Del Rey, CA]) was enforced in patients diseased kidneys makes it impossible to definitively identify with ESRD, ensuring that all subjects were vitamin-replete, the the source of the clearance defect. There is good evidence that effect of even massive doses of vitamin supplementation be- normal kidneys play a major role in amino acid and Hcy came negligible (69). How can this striking resistance to treat- clearance and metabolism. The existence of Hcy-metabolizing ment be explained? If normal kidneys play a primary role in enzymes and uptake systems in renal tubular cells has been clearing and metabolizing Hcy, then the atrophic, nonfunction- confirmed, and Hcy extraction studies in animal kidneys doc- ing, renal parenchyma in ESRD would not significantly re- umented significant Hcy uptake (12,28,30,36). Without impli- spond to vitamin supplementation, no matter how high the cating other complex processes, it seems reasonable to expect dose. Alternatively, uremic solutes may effectively inhibit ex- that the loss of metabolically active kidney tissue normally trarenal Hcy metabolism. The fact that a minority of patients involved in Hcy handling would decrease Hcy clearance and with ESRD do respond, albeit in an attenuated manner, to increase plasma levels. The inverse relationship between Hcy supraphysiologic vitamin supplementation suggests some level levels and GFR, which is consistent throughout a nonuremic of residual metabolic renal function and/or enhanced extrarenal range of kidney function, supports the premise that it is re- Hcy metabolism. It may also suggest a vitamin-deficient state duced renal function, not the accumulation of uremic toxins, at baseline. that causes Hcy levels to increase. There has been some concern that common genetic poly- An alternative theory involves currently unidentified uremic morphisms affecting Hcy metabolism, in particular the meth- substances that inhibit normal extrarenal Hcy metabolism. The ylenetetrahydrofolate thermolabile mutation (C677T), have liver is considered the most likely target organ, given its major significant effects in patients with renal disease, including regulatory role in protein metabolism, its high levels of Hcy- ESRD. However, this defect has been demonstrated to cause metabolizing enzymes, and its capacity to export massive fasting hyperhomocysteinemia only in the setting of low folate amounts of Hcy in vitro (15). Although an in vitro study levels, regardless of renal function (70). Consequently, this demonstrated that folic acid transport was inhibited in the mutation has a negligible effect among dialysis patients who uremic milieu (62), two subsequent, well designed trials using are folate replete (71,72). the reduced folates L-folinic acid and L-5-methyltetrahydrofo- Routine hemodialysis treatments acutely decrease Hcy lev- late to decrease Hcy levels in patients with ESRD found no els by approximately 30 to 40%, but levels quickly increase to evidence of impaired folate absorption, transport, or conjuga- their elevated pretreatment values (27,73). There are ongoing tion (63,64). Another study demonstrated abnormal hepatic attempts to manipulate the dialysis modality to improve Hcy sulfur amino acid metabolism in uremic rat livers, although clearance. High-flux dialysis membranes, which have the ca- J Am Soc Nephrol 12: 2181–2189, 2001 The Kidney and Hcy Metabolism 2187 pacity to clear large plasma molecules, including potential hyperhomocysteinemia as an adverse cardiovascular risk factor uremic inhibitory substances, have no significant long-term in end-stage renal disease. Circulation 97: 138–141, 1998 Hcy-lowering effect (27). Of interest, increasing the frequency 4. Bostom AG, Culleton BF: Hyperhomocysteinemia in chronic of dialysis may lead to significantly lower fasting Hcy levels renal disease. J Am Soc Nephrol 10: 891–900, 1999 (74). One potential explanation for this is that Hcy diffuses 5. Foley RN, Parfrey PS, Sarnak MJ: Clinical epidemiology of cardiovascular disease in chronic renal disease. Am J Kidney Dis directly into the dialysate, and more frequent dialysis allows a 32[Suppl 3]: S112–S119, 1998 lower steady-state level to be reached. Alternatively, uremic 6. Sarnak MJ, Levey AS: Cardiovascular disease and chronic renal inhibitory factors may be removed, facilitating normal Hcy disease: A new paradigm. Am J Kidney Dis 35[Suppl 4]: S117– metabolism. S131, 2000 7. Bostom AG, Gohh RY, Morrissey P: Hyperhomocysteinemia in Conclusion chronic renal transplant recipients. Graft 3: 195-204, 2000 Hcy has been demonstrated to be an independent CVD risk 8. Kang SS, Wong PWK, Malinow MR: Hyperhomocyst(e)inemia as a risk factor for occlusive vascular disease. Annu Rev Nutr 12: factor. Patients with kidney disease, for whom exceptionally 279–297, 1992 high rates of cardiovascular morbidity and death are observed, 9. Mudd SH, Finkelstein JD, Refsum H, Ueland PM, Malinow MR, exhibit disproportionately elevated plasma with Hcy levels. Lentz SR, Jacobsen DW, Brattstrom L, Wilcken B, Wilcken DE, Hcy levels increase as renal function declines, patients become Blom HJ, Stabler SP, Allen RH, Selhub J, Rosenberg IH: Ho- increasingly refractory to the usual Hcy-lowering therapies. mocysteine and its disulfide derivatives: A suggested consensus The role of the kidney in plasma Hcy handling is an area of terminology. Arterioscler Thromb Vasc Biol 20: 1704–1706, ongoing research. Current data suggest that the healthy kidney 2000 plays a major role in Hcy clearance and metabolism, as it does 10. Refsum H, Helland S, Ueland PM: Radioenzymic determination with other amino acids. Renal Hcy filtration and clearance may of homocysteine in plasma and urine. Clin Chem 31: 624–628, be affected by dietary protein intake and should be measured 1985 under nonfasting conditions. Confirmation of this effect would 11. Ueland PM: Homocysteine species as components of plasma redox thiol status. 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