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J Clin Pathol: first published as 10.1136/jcp.s1-2.1.7 on 1 January 1969. Downloaded from J. clin. Path., 22, suppl. (Ass. clin. Path.), 2, 7-12 Disorders of fructose metabolism E. R. FROESCH From the Metabolic Unit, Department of Medicine, University ofZurich, Switzerland Fructose is and has always been an important THE MAJOR ROUTE OF FRUCTOSE METABOLISM nutriment for man. Table I shows the fructose con- tent of some fruit and vegetables. As may be seen Three organs share the 'specific' route of fructose all of those listecI contain glucose, fructose, and metabolism by which more than 70% is utilized: the saccharose. Of these three sugars fructose is the liver, kidney, and the mucosa of the small bowel sweetest, saccharo,se comes next, and glucose last. (Fig. 1). Fructose is phosphorylated to fructose-1- The daily fructose intake lies between 50 and 100 g. phosphate by the enzyme fructokinase, which also It has risen enormc)usly in the last 20 years due to the phosphorylates tagatose and sorbose (Leuthardt and ever increasing c,onsumption of sucrose which Testa, 1950; Hers, 1952; Kuyper, 1959). Fructose-i- contains one molescule of glucose and one molecule phosphate is then split to phosphodihydroxyacetone of fructose. and the non-phosphorylated triose glyceraldehyde. The enzyme which accomplishes this step, fructose-1- TABLE I phosphate aldolase, is present only in liver, kidney, GLUCOSE, FRUCTOSE, AND SACCHAROSE CONTENT OF SOME and small bowel (Leuthardt, Testa, and Wolf, 1953; FRUTIT AND VEGETABLES1 Hens and Kusaka, 1953). Fructose-l-phosphate copyright. Source Percentage of Weight aldolase has been investigated in detail by Penhoet Glucose Fructose Saccharose and others who managed, among other remarkable biochemical to Apple 1-7 5.0 3-1 achievements, produce hybrids Pear 2-5 5 0 1-5 among subunits of fructose-i-phosphate and Peach 1-5 1-6 6-6 fructose-1, 6-diphosphate aldolases (Penhoet, Lemon juice Blackberry 32 29 0.2 Rajkumar, and Rutter, 1966; Christen, Goschke, Melon 1-2 09 4-4 Leuthardt, and Schmid, 1965). Whereas dihydroxy- http://jcp.bmj.com/ Tomato 1-6 1-2 0-4 acetone phosphate now enters the glycolytic path- Carrot 2-1 1.0 3-1 Vegetable marrow 1 1 1-2 0-4 way, glyceraldehyde is still a product of the 'specific' Sweet potato 0-4 03 401 fructose metabolism in these tissues. Glyceraldehyde Potato Garden pea ?-1 ?1 0167 may be metabolized by three pathways in liver tissue: 'From Hardinge, Swarnei:r, and Crooks (1965). (1) direct phosphorylation by the enzyme triose-kinase to glyceraldehyde phosphate (Hers and Kusaka, The role of fructtose in the pathogenesis of the so- 1953; Heinz and Lamprecht, 1961); (2) oxidation on September 26, 2021 by guest. Protected called diseases of civilization has not been estab- to glycerol and subsequent phosphorylation by the lished. Some claim that fructose is well tolerated by enzyme glycerokinase (Wolf and Leuthardt, 1953); diabetics and of cc)nsiderable help in their manage- and (3) oxidation to glyceric acid and subsequent ment (Mehnert, rMlahrhofer, and Forster, 1964), phosphorylation to phosphoglycerate by the enzyme whereas others tend to believe that the excessive in- glyceric acid kinase (Lamprecht and Heinz, 1958; take of sucrose is aLt the root of a few major diseases Holzer and Holldorf, 1957). of modem life suchi as obesity, diabetes, and athero- According to recent investigations on enzyme matosis (Yudkin, I.964; Yudkin and Roddy, 1964). activities in human liver by Heinz, Lamprecht, and Since the excessive iintake of sucrose is almost always Kirsch (1968), it appears likely that most of the accompanied by:intake of extra fat and extra glyceraldehyde formed during fructose metabolism calories in general, I do not share this latter view but is metabolized by triose-kinase. rather believe thatt the imbalance between calorie The activities of fructokinase and of these other intake and calorie output must be responsible for enzymes of fructose metabolism do not appear to be the 'explosion' of diabetes, obesity, and athero- influenced to any great extent by insulin or insulin matosis. lack (Adelman, Spolter, and Weinhouse, 1966). The 7 J Clin Pathol: first published as 10.1136/jcp.s1-2.1.7 on 1 January 1969. Downloaded from 8 E. R. Froesch Glycogen Glucose-l-phosphate'4 ? 4it Glucose-6-phosphate ) Glucose 6 i - Fructose Fructose-6-phosphate I t Fructose-1, 6-diphosphate Fructose-i-phosphate i 2 f' GIcrldhd+ FIG. 1. Pathways and enzymes I I I offructose metabolism. D-Glyceraldehyde + Dihydroxyacetone + Glyceraldehyde phosphate 3-phosphate 5 4 3 t t J I I I vl 'I,e+ 0 D-Glyceric acid --3-7 L ivcerate-_Z-Dhosnhaxte I I a-Glycerophosphate Enzymes according to number: 1 = fructokinase 2 = fructose-i-phosphate aldolase 3 = triose-kinase 4 = glyceraldehyde dehydrogenase 5 = alcohol dehydrogenase 6 = hexokinase biological half life of fructose is of the order of MINOR ROUTES OF FRUCTOSE METABOLISM magnitude of 18 minutes in normal subjects as well The ubiquitous enzyme hexokinase phosphorylates as in diabetics (Smith, Ettinger, and Seligson, 1953). fructose to fructose-6-phosphate, an intermediate of It is not influenced by insulin. After oral fructose copyright. glycolysis. However, hexokinase has a much higher ingestion the venous blood fructose levels do not exceed 25 mg/100 ml due to the rapid hepatic affinity for glucose than for fructose. Therefore, extraction of fructose. fructose is only phosphorylated when no free glucose is present in the cell. This is true for erythrocytes and leucocytes which can oxidize fructose as well as ABSORPTION OF FRUCTOSE FROM THE GUT glucose, but do so only in the absence of the latter hexose (Froesch, Wolf, Baitsch, Prader, and Labhart, Fructose is absorbed from the gut at a rate of 1963). In these cells the speed of glucose transport http://jcp.bmj.com/ approximately 43 % of that of glucose (Cori, 1925). exceeds the rate of glucose phosphorylation so that The process of fructose absorption is ill defined free glucose is always present inside the cell. The and was long thought to be one of passive diffusion. situation is different in muscle. In the absence of Fructose absorption is not inhibited by phlorizin insulin and below a blood glucose concentration of and, unlike galactose, it does not compete with 50 mg/100 ml no free glucose is present in the cyto- glucose for a common carrier. Metabolic poisons plasm and the fructose entering the cell may be phos- such as dinitrophenol appear to inhibit the con- phorylated by hexokinase to fructose-6-phosphate on September 26, 2021 by guest. Protected version of fructose to glucose, but not the absorption (Froesch, 1965a; Morgan, Henderson, Regen, and of fructose itself (Crane, 1960). The absorption of Park, 1961). Adipose tissue appears to play a major sorbitol is slower still than that of fructose and an role in fructose metabolism, particularly in patients active component in the absorption of fructose from in whom the specific pathway of fructose is deficient. the gut cannot be entirely ruled out. Glucose transport appears to limit the rate of Fructose is absorbed from the gut in three forms: glucose metabolism under almost all circumstances as fructose, as glucose, and as lactate. The percentage in adipose tissue, so that no free glucose is ever of these three varies according to the experimental present (Froesch and Ginsberg, 1962; Crofford and animal and the quantity of fructose used. Conversion Renold, 1965). Fructose on the other hand is of fructose to glucose is by no means an essential transported in a rapid insulin-independent manner, part of its absorption. Thus patients with essential particularly when present in the medium in high fructosuria, who are unable to phosphorylate concentration; moreover in this tissue fructose fructose and to convert it to glucose in the mucosa of phosphorylation by hexokinase is not inhibited by the small bowel, are perfectly able to absorb large free glucose (Froesch, 1965a; Froesch and Ginsberg, quantities of fructose rapidly without any diarrhoea. 1962; Ginsberg, 1965). J Clin Pathol: first published as 10.1136/jcp.s1-2.1.7 on 1 January 1969. Downloaded from Disorders offructose metabolism 9 Fructose is not only metabolized, but is also Fructose (80g.PO.) produced in specialized tissues such as the accessory reproductive glands of the male. Hers has detected two enzymes catalyzing the reduction of glucose to 6 Inorganic Phosphorus sorbitol and fructose, ie, aldose reductase and sorbitol dehydrogenase (Hers, 1960). Fructose 41 a . I A a appears to be the major source of energy for the sperm. The same enzyme system is also present in F100r the brain and nervous system (Gabbay and Glucose O'Sullivan, 1968). The interesting hypothesis has 60 - been put forward that during hyperglycaemia sorbitol and fructose might be formed in excessive 20 Fructose amounts in brain and nerve tissue. Due to the Ita, A a .I I. relative impermeability of the blood-brain barrier 0 60 120 180 240 for fructose and sorbitol, these latter substances Minutes in might be the cause of a relative hyperosmolarity FIG. 2. Oral fructose tolerance test with 80 g offructose the brain during rapid lowering of the blood sugar in a patient with essential fructosuria. Glucose was meas- and might be responsible for the cerebral oedema ured by the difference between total reducing substances which is still one of the major causes of death during andfructose by the resorcinol method. The slight apparent the treatment of diabetic acidosis (Clements, fall of blood glucose may be explained by inadequate Prockop, and Winegrad, 1968). methods. Note the absence of untoward symptoms and the constant serum level of inorganic phosphorus. (We are ESSENTIAL FRUCTOSURIA indebted to Dr StLinitz for permission to reproduce these results.) Essential fructosuria is a rare inborn error of metabolism which should be considered as a harm- was made by Chambers and Pratt in England copyright.
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