Different rates of glucose and fructose metabolism in rat liver tissue in vitro

Different Rates of Glucose and Fructose Metabolism in Rat Liver Tissue In Vitro By J. N. PEREIRAAND NORMAN0. JANGAARD The rate of conversion of uniformly labeled fructose and glucose into various intermediates of carbohydrate and lipid metabolism was studied in rat liver slices. Fructose was converted into every compound measured more rapidly than was glucose. Tbe ratio of the rate of fructose metabolism to the rate of glucose metabolism (F/G) ranged from a low of three

F

for lactic acid, pyruvic acid, carbon dioxide and fatty acids, to a high of 19 for glyceride-glycerol. These results suggest that fructose-induced hyperlipemia may result, at least in part, from the relatively efficient conversion of fructose to cyglycerophosphate in liver, (Metabolism 20: No. 4, April, 392-400, 1971)

EEDING

OF FRUCTOSE OR SUCROSE has been reported to produce hyperlipemia in mar+” and laboratory animals.10-15 This carbohydrateinduced hyperlipemia has been implicated in the etiology of occlusive atherosclerotic disease.14s6 The fructose portion of sucrose appears to be responsible for the hyperlipemic effect. 6~10-1R The basis for the differential effects of fructose and glucose has been examined by other investigators and attributed, variously, to the greater conversion of fructose to acetyl SCoA16 or gIyceride-glycerol,6.‘1,12.14 due to a more rapid rate of uptake from the gut or the bloodstream,12,15 the lesser stimulation of lipoprotein lipase,l” a lesser effect of fructose on insulin release,l? a decreased rate of removal of triglycerides from the plasma pool14 and a greater induction of glycolytic enzymes. 13*17These studies were undertaken to determine the relative rates of formation of various metabolites of fructose or glucose. Rat liver slices were utilized to examine hepatic enzymatic differences and minimize the role played by extrahepatic mechanisms involving triglyceride removal, stimulation of lipoprotein lipase and insulin secretion.

MATERIALSANDMETHODS Fructose and glucose, randomly labeled with carbon-14, were obtained from New England Nuclear Corp. or Calbiochem. Male nonfasted CD rats, obtained from Charles River Breeding Laboratories, were used except where indicated in the text. They were maintained on Ralston-Purina Laboratory Chow and were exposed to fluorescent light on a 12 hr-12 hr light-dark cycle. The rats were killed by stunning and exsanguination. The livers were removed, rinsed in Krebs-Ringer bicarbonate buffer. blotted dry, placed in Krebs-Ringer bicarbonate buffer and minced into pieces of approximately 10 mg each. The incubation flasks were as described earlier. ** Each experimental flask contained 200 mg of minced liver in a total volume of 2 ml. The buffer was Krebs-Ringer bicarbonate, pH

From the Medicul Research Laboratories, Chus. Pfizer & Co., Inc., Groton, Conn. 06340 Received for publication October 26, 1970. J. N. PEREIRA, PH.D.: Pharmacology Department, Medical Research Laborutories, Groton, Corm. NORMAN 0. JANGAARD, PH.D.: Biochemistry Department, Biological Sciences Research Center, Shell Development Co.. Modesto, Calif. 392

METABOLISM, VOL. 20, No.

4 (APRIL),

1971

GLUCOSE

AND FRUCTOSE

393

METABOLISM

36.000 1

-0

4

a

Hoxose

Concontrohon.

12

16

mM

Fig. l.-Effect of substrate concentration (0- 15.6 mM) on oxidation of uniformly labeled glucose and fructose by 100 mg of liver obtained from nonfasted rats. Vertical bars indicate f 1 SD; n=S. Specific activity of hexoses, 0.07 PCilmg.

7.4,containing 1 per cent albumin and aerated with 95 per cent Os-5 per cent CO,. The 3-hr incubation was carried out at 37% Carbon dioxide production was measured as described previously.ls Liver glycogen was recovered by the method described by Stadie and his associates;‘9 lactic acid was measured by the method of Barker and Surnmersonss and pyruvic acid was measured by the Friedemann and HaugerP procedure. The lipid extraction was initiated by adding 2 ml of 60 per cent KOH to the incubation mixture. After shaking for 5 min the solution was neutralized with concentrated HCI. The solution was placed into a 50 ml centrifuge tube containing 5 g of zeolite. The zeolite mixture was extracted with 20 ml of chloroform and the extract filtered through nonfat filter paper. The chloroform extract was taken to dryness and saponified with alcoholic KOH at 1OO“C for 1 hr. Cholesterol was removed by extraction with diethyl ether. After acidification with HCl, the triglyceride fatty acids were recovered by reextracting with diethyl ether. Glyceride-glycerol was recovered by the methods described by Cahill et al.** and Reeves.= RESULTS

The oxidation of uniformly labeled glucose and fructose by 100 mg of minced rat liver is shown in Fig. 1. The incubation period for this experiment was 2 hr. The rate of fructose oxidation exceeded that of glucose oxidation at all concentrations tested. Fructose was oxidized three to four times more rapidly than glucose during this period. The highest hexose concentrations used in this experiment, 15.6 mM (300 mg% ), were used in the remainder of these experiments. A comparison of the percent incorporation of fructose and glucose into carbon dioxide, triglycerides, fatty acids and glyceride-glycerol is shown in Table 1. The numbers represent the mean + SD of 8 determinations. The oxidation of the hexoses to carbon dioxide was the predominant process occurring during this period. The ratio of fructose/glucose oxidized was 3.5, in good agreement with the experiment in Fig. 1. The hexoses were less effectively incorporated into triglycerides, but in each case fructose was the favored substrate. The fructose/ glucose ratios were 3.1 for fatty acids, 11.0 for triglycerides and 18.9 for glyceride-glycerol.

394

PEREIRA

AND

JANGAARD

395

GLUCOSE AND FRUCTOSE METABOLISM

Table 2.-Effects

Substrate

Fructose

Lactate, n-A/liter

of Fructose and Glucose on Medium Lactate and Pyruvate Levels * F/G

Pyruvate, mM/liter

4.2

0.51 k 0.04

2.12 k 0.52

3.1

3.2 Glucose

Lactate/ Pyruvate

F/G

4.0

0.16 k 0.02

0.66 I!z 0.10

* Minced rat liver tissue (200 mg) obtained from fasted rats. Means f SD; n = 5.

The production of pyruvic acid and lactic acid from the two hexoses was determined and the results are shown in Table 2. The data indicate that fructose is converted to lactate and pyruvate more rapidly than is glucose. A factor of 3 describes the relative rate. It is interesting to note that the lactate/pyruvate ratios were the same with both substrates, 4.2 with fructose and 4.0 with glucose. The per cent incorporation of fructose and glucose into liver glycogen is shown in Table 3. Livers from both fed and fasted rats were used. In the fasted state, fructose is incorporated into glycogen 7 times more rapidly than glucose. Both sugars were incorporated more rapidly in the fed state than in the fasted state. The pathway of fructose incorporation appears to be preferentially stimulated in the fed state as indicated by the increase in the fructose/glucose ratio from 7 to 15. The ratio of per cent incorporation for fructose was fed/fasted 4.2; for glucose this ratio was 1.9. DISCUSSION

These data demonstrate that fructose is more rapidly converted to many intermediates of carbohydrate and lipid metabolism than is glucose. It has long been known that the routes of glucose and fructose metabolism are different. In 1913, Tiigel and coworkers24 reported that fructose was metabolized more rapidly than glucose and had a higher respiratory quotient. This finding was substantiated by Higgins in 1916.22 The enzymatic basis for this difference has been partially elucidated and is schematically represented in Fig. 2. 26-32The initial step in fructose metabolism in liver is a phosphorylation of the one position yielding fructose-l-phosphate. This reaction is catalyzed by a specific enzyme, ATP:d-fructose 1-phosphotransferase (E.C.2.7.1.3, ketohexokinase) which is not the same enzyme that catalyzes the phosphorylation of glucose. Cleavage of fructose-l-phosphate by an aldolase, presumably ketose-l-phosphate aldehyde-lyase (E.C.4.1.2.7, ketose-l-phosphate aldolase) , yields glyceraldehyde and dihydroxyacetone-phosphate. The glyceraldehyde is then phosphorylated in a reaction catalyzed by ATP:d-glyceraldeTable 3.-Per Substrate

Fructose

Cent Incorporation of Uniformly Labeled Fructose or Glucose into Liver Glycogen * Fed

F/G

Fasted

0.13 -r- 0.05

Fed/Fasted

4.2 6.6

14.9 Glucose

F/G

0.46 + 0.06

1.94 4 0.28

0.07 k 0.02

* Liver minces derived from fasted or fed rats. Numbers

1.9 indicated

means

+ SD; n - 5.

PEREIRA

396

AND

JANGAARD

GLYCOGENk, “DPG

1 GLUCOSE~I-P>

GL”CO&dCiSE-6-P -11

> FRUCTOSE-6~Pt---FRUCTOSE

FRlfCTO,,l, *

6-D

P

FRUCTOSE-I-P ’

OXALACETATE

Fig. 2.-Summary

of major routes of glucose and fructose metabolism.

Superscript

numerals indicate F/G ratios for each of the metabolites measured. hyde-3-phosphotransferase (E.C.2.7.1.28, triokinase) . The glyceraldehyde-3phosphate and dihydroxyacetone phosphate produced are thought to enter the same metabolic pool as the triose phosphates produced from glucose via the initial steps of glycolysis. There is some evidence for alternate modes of fructose metabolism,28 primarily a direct formation of fructose-6-phosphate. The enzymes involved in fructose metabolism in rat liver are subject to dietary and hormonal regulation.30 The observation that fructose is converted to all metabolites measured at a faster rate than glucose suggests that the initial phosphorylation rate of fructose exceeds that for glucose. Adelman and coworkers30 demonstrated that the activities of ketohexokinase, ketose-l-phosphate aldolase and triokinase were high enough to account for the high rate of fructose metabolism in liver. Zakim and associater? subsequently demonstrated that the activity of ketohexokinase in the livers of rats fed control chow diets exceeded the combined activities of ATP : dhexose 6-phosphotransferase (E.C.2.7.1.1, hexokinase) and ATP:d-glucose 6-phosphotransferase (E.C.2.7.1.2, glucokinase) by a factor of 11. The smallest differential between fructose and glucose occurred in pyruvic acid, lactic acid, carbon dioxide and fatty acids. Fructose was metabolized to these compounds in rat liver slices three times faster than was glucose. This compares to relative ratios of 2% and 3l” seen in other experiments with COproduction in liver slices and ratios of 233 and 416 for fatty acids. Gale and Crawford34 have reported that the in vivo rate of incorporation of fructose into lipids in guinea pigs was four to six times that for glucose. Intravenous administration of fructose and glucose to humans was reported to be followed by an increase in blood pyruvic acid concentrations. 35 Fructose was at least three times

GLUCOSE

AND FRUCTOSE

METABOLISM

397

as effective in this regard as was glucose. These in vivo effects would seem to be at least partially attributable to the differential utilization of fructose and glucose by the liver. The ketohexokinase-catalyzed phosphorylation of fructose to give fructose-lphosphate, followed by cleavage of this hexose to trioses appears to be the major route of fructose metabolism in liver,2Q31 although the direct phosphorylation of fructose to give fructose-6-phosphate may also occur to a limited extent.27 Considering the number of steps required to convert fructose to glycogen and the fact that the regulatory enzyme, d-fructose-l ,6-diphosphate l-phosphohydrolase (E.C.3.1.3.11, hexosediphosphate), catalyzes one of these steps, it is somewhat surprising that fructose is converted to glycogen 7 to 15 times more rapidly than is glucose. Livers from fed rats were more effective in incorporating both glucose and fructose into glycogen than were livers from fasted rats. The ratio of rates for fructose:glucose incorporation were 15 for fed rats and 7 for fasted rats. Wyshak and ChaikofP3 also found fructose to be more rapidly converted to glycogen than was glucose, but found a smaller difference in rates and found that the rate of glucose incorporation was decreased more by fasting than was fructose incorporation. Spiro and Hastings,36 on the other hand, found almost no difference in the rates of fructose and glucose incorporation into glycogen by rat liver slices from normal rats. However, those investigators incubated the two hexoses together and their results are not strictly comparable to the present findings. They did find that fructose was converted to glycogen six times faster than was glucose in liver slices from diabetic rats. The more rapid rate of fructose incorporation is consistent with Cori’s observation37 that fructose and glucose are on a par as glycogen formers in vivo, even though fructose is more slowly absorbed from the intestinal tract. This finding was made in rats receiving oral doses of the two hexoses. The greatest differential in rates was seen in the production of glycerideglycerol, which is presumably derived from a-glycerophosphate. Fructose was converted to a-glycerophosphate 19 times faster than was glucose. Muntz and Vanko3” reported that a-glycerophosphate had the highest specific activity of any of the compounds measured following intraportal administration of fructose14C to rats. Bar-On and Stein14 reported that more fructoseJ4C than g1ucoseJ-C incorporated into the glycerol moiety of triglycerides following intragastric administration of the two compounds to rats. Zakim and Herman”* have studied the effect of intravenous administration of fructose and glucose on hepatic a-glycerophosphate concentrations in the rat. They found that fructose increased a-glycerophosphate concentrations more than glucose did. The peak fructose response occurred 5 min after injection while the glucose response developed more slowly. The results reported in this paper support the view that the rapid rate of conversion of fructose to a-glycerophosphate may play a role in fructose-induced hypertriglyceridemia.~~ll~1~J4 Th is view is also supported by the finding that glycerol feeding produces hypertriglyceridemia3Q and the observation that fructose stimulates acetate incorporation into fatty acids more than glucose does.40 Exton and Park41 found a ninefold increase in a-glycerophosphate concentrations following a 1 hr perfusion of rat livers with 20 mM fructose. Zakim and Herman38

398

PEREIRA

AND

JANGAARD

have recently indicated that the effects of fructose on glyceride metabolism may not be related to the effect of the sugar on hepatic a-glycerophosphate concentrations. This conclusion was reached primarily because of the transient effects of fructose on a-glycerophosphate concentrations observed in their in vivo experiments. However, their data do not exclude an effect of fructose on the turnover rate of a-glycerophosphate. In this regard, it would appear that incorporation of the hexoses into glyceride-glycerol is a superior measure of a-glycerophosphate production. It should be emphasized that a-glycerophosphate production in a liver slice system does not appear to be transient. The work of Chernick and Scow4” demonstrates that triglyceride and phospholipid synthesis from fructose as well as fructose oxidation is linear for at least 4 hours. ln our view, the available evidence suggests that the rapid hepatic conversion of fructose to a-glycerophosphate plays a role in fructose-induced hypertriglyceridemia. This viewpoint is further supported by the finding that single doses of p-chlorophenoxyisobutyric acid (CPIB, clofibrate), a hypolipemic agent. induces the synthesis of the mitochondrial L-glycerol-3-phosphate :oxidoreductase f E.C. 1.1.99.5, GPD) , reduces hepatic levels of a-glycerophosphate and diminishes triglyceride synthesis and secretion.4” This mechanism of fructose-induced hyperlipemia may be augmented by other factors. The observations that fructose is converted to acetyl SCoA faster than is glucose,l’i that fructose produces a lesser effect on insulin release than does glucose,12 that fructose is a less effective stimulator of lipoprotein lipasel-’ and that fructose decreases the rate of triglyceride removal from the blood stream,14 would all be consistent with its effect on plasma triglyceride concentrations and augment its effect on the rate of a-glycerophosphate synthesis. The observations reported in this paper demonstrate that the differential effects of fructose and glucose are apparent over a short time scale in vitro. Nikkila and Ojalall have demonstrated a rapid development of hypertriglyceridemia in rats given fructose. These findings suggest that enzyme induction13*17 might amplify the fructose effect but is not its primary cause. Any effect due to more rapid uptake of fructose by the liver*“rl” would be counterbalanced by the slower absorption of fructose from the gut?? Although the findings reported here indicate that fructose is metabolized by liver tissue more rapidly than is glucose, it is not possible to completely rule out an alternate explanation, that is, that differential isotope dilution in the intracellular pools of glucose and fructose is involved. However, that alternate explanation appears to be very unlikely in view of the findings of Cahill et al.,“” Thielmann et al.4j and Heinz and JunghgneP6 who reported that liver cells are freely permeable to glucose and fructose, the intracellular concentrations of glucose and fructose are determined by the extracellular concentrations and the intracellular and extracellular pools of glucose and fructose equilibrate rapidly. These findings indicate that the extracellular and intracellular compartments of glucose and fructose behave as a single pool. This conclusion, coupled with the fact that the extracellular compartment is large in relation to the intracellular compartment, supports the view that the greater rate of fructose metabolism is due to the more rapid rate of the early steps of fructose metabolism and not to

GLUCOSE

AND FRUCTOSE

399

METABOLISM

differential isotope dilution in the intracellular compartments of glucose and fructose. ACKNOWLEDGMENTS We wish to express our gratitude excellent technical assistance.

to Mr. J. A. Joseph and Mr. D. P. MacDonald

for their

REFERENCES 1. Yudkin, J.: Dietary fat and dietary sugar in relation to ischaemic heart disease and diabetes. Lancet 2:4, 1964. 2. -: Why blame sugar? Chem. Industr. 2: 1464, 1967. 3. - and Morland, J.: Sugar intake and myocardial infarction. Amer. J. Clin. Nutr. 20: 503, 1967. 4. and Roddy, J.: Levels of dietary sucrose in patients with occlusive atherosclerotic disease. Lancet 2:6, 1964. 5. Akinyanju, P. A., Qureshi, R. I-J., Salter, A. J., and Yudkin, J.: Effect of an “atherogenic” diet containing starch or sucrose on the blood lipids of young men. Nature 218:975, 1969. 6. Nikkila, E. A., and Pelkonen, R.: Enhancement of alimentary hyperglyceridemia by fructose and glycerol in man. Proc. Sot. Exp. Biol. Med. 123:91, 1966. 7. Rifkind, B. M., Lawson, D. H., and Gale, M.: Effect of short-term sucrose restriction on serum-lipid levels. Lancet 2: 1379, 1966. 8. MacDonald, I., Braithwaite, D. M.: The influence of dietary carbohydrates on the lipid pattern in serum and in adipose tissue. Clin. Sci. 27:23, 1964. 9. Kuo, P. T., and Bassett, D. R.: Dietary sugar in the production of hyperglyceridemia. Ann. Intern. Med. 62:1199, 1965. 10. Nikkila, E. A., and Ojala, K.: Induction of hyperglyceridemia by fructose in the rat. Life Sci. 4:937, 1965. 11. - and -: Acute effects of fructose and glucose on the concentration and removal rate of plasma triglyceride. Life Sci. 5:89, 1966. 12. Nikkila, E. A.: Modulation of triglyceride kinetics by fructose versus glucose in the rat. Stand. J. Clin. Lab. Invest. 18: 76, 1966. 13. Bailey, E., Taylor, C. B., and Bartley, W.: Effect of dietary carbohydrate on hepatic lipogenesis in the rat. Nature 217: 471, 1968. 14. Bar-On, H., and Stein, Y.: Effect of

glucose and fructose administration on lipid metabolism in the rat. J. Nutr. 9495, 1968. 15. Cohen, A. M., and Teitelbaum, A.: Effect of glucose, fructose, sucrose, and starch on lipogenesis in rats. Life Sci. 7:23, 1968. 16. Zakim, D., Pardini, R. S., Herman, R. H., and Sauberlich, H. E.: Mechanism for the differential effects of high carbohydrate diets on lipogenesis in rat liver. Biochim. Biophys. Acta 144:242, 1967. 17. Yudkin, J., and Krauss, R.: Dietary starch, dietary sucrose and hepatic pyruvate kinase in rat. Nature 215:75, 1967. 18. Pereira, J. N., Jangaard, N. O., and Pinson, E. R.: Some metabolic effects of phenformin in rat adipose tissue. Diabetes 16:869, 1967. 19. Stadie, W. C., Haugaard, N., and Marsh, J. B.: Combination of insulin with muscle of the hypophysectomized rat. J. Biol. Chem. 188:167, 1951. 20. Barker, S. B., and Summerson, W. H.: The calorimetric determination of lactic acid in biological material. J. Biol. Chem. 138:535, 1941. 21. Friedemann, T. E. and Haugen, G. E.: Pyruvic acid. II. The determination of keto acids in blood and urine. J. Biol. Chem. 147:415, 1943. 22. Cahill, G. F., Leboeuf, B., and Renold, A. E.: Studies on rat adipose tissue in vitro. III. Synthesis of glycogen and glyceride-glycerol. J. Biol. Chem. 234:2540, 1959. 23. Reeves, R. E.: The estimation of primary carbinol groups in carbohydrates. J. Amer. Chem. Sot. 63:1476, 1941. 24. Togel, O., Brezina, E., and Durig, A.: The carbohydrate-sparing action of alcohol. Biochem. Z. 1:296, 1913. 25. Higgins, H. L.: The rapidity with which alcohol and some sugars may serve as nutriment. Amer. J. Physiol. 41:258, 1916. 26. Cori, G. T., Ochoa, S., Slein, M. W., and Cori, C. F.: The metabolism of fructose in liver. Isolation of fructose-l-phosphate

400 and inorganic pyrophosphate. Biochim. Biophys. Acta 7:304, 1951. 27. Parks, R. E., Jr., Ben-Gershom, E., and Lardy, H. A.: Liver fructokinase. J. Biol. Chem. 227:231, 1957. 28. DiPietro, D. L.: Some aspects of D-fructose metabolism in rat liver. J. Biol. Chem. 239:4051, 1964. 29. Spolter, P. D., Adelman, R. C., DiPietro, D. L., and Weinhouse, S.: Fructose metabolism and kinetic properties of rabbit liver aldolase. Advances Enzym. Regulat. 3:79, 1965. 30. Adelman, R. C., Spolter, P. D., and Weinhouse, S.: Dietary and hormonal regulation of enzymes of fructose metabolism in rat liver. J. Biol. Chem. 2415467, 1966. 3 1. Heinz, F., and Lamprecht, W.: Enzyme des fructosestoffwechsels. Aktivatgt und verteilung in der leber der ratte. Hoppe Seyler Z. Physiol. Chem. 348:855, 1967. 32. Muntz, J. A., and Vanko, M.: The metabolism of intraportally injected fructose in rat liver in vivo. J. Biol. Chem. 237: 3582, 1962. 33. Wyshak. G. H., and Chaikoff, I. L.: Metabolic defects in the liver of fasted rats as shown by utilization of 14C-labeled glucose and fructose. J. Biol. Chem. 200: 851, 1955. 34. Gale, M. M., and Crawford, M. A.: Different rate of incorporation of glucose and fructose into plasma and liver lipids in guinea pig. Metabolism 18:1021, 1969. 35. Miller, M., Drucker, W. R., Owens, J. E.. Craig, J. W., and Woodward, H., Jr.: Metabolism of intravenous fructose and glucose in normal and diabetic subjects. J. Clin. Invest. 31:115, 1952. 36. Spiro, R. G., and Hastings, A. B.: Studies on carbohydrate metabolism in rat liver slices. XI. Effect of prolonged insulin administration to the alloxan-diabetic animal. J. Biol. Chem. 230:751, 1958. 37. Cori, C. F.: The fate of sugar in the animal body. III. The rate of glycogen for-

PEREIRA

AND JANGAARD

mation in the liver of normal and insulinized rats during the absorption of glucose, fructose, and galactose. J. Biol. Chem. 70: 577. 1926. 38. Zakim. D., and Herman, R. H.: The effect of intravenous fructose and glucose on the hepatic a-glycerophosphate concentration in the rat. Biochim. Biophys. Acta 165:374, 1968. 39. Nikkila, E. A. and Ojala, K.: Hyperglyceridemia induced by glycerol feeding. Life Sci. 3:1021, 1964. 40. Christophe, J., and Mayer, J.: Influence of diet on utilization of glucose and incorporation of acetate-l-l% into liver fatty acids and cholesterol in rats. Amer. J. Physiol. 197:55-59, 1959. 41. Exton. J. H., and Park, C. R.: Control of gluconeogenesis in the perfused liver of normal an1 adrenalectomized rats. J. Biol. Chem. 24O:PC955, 1965. 42. Chernick, S. S., and Scow, R. 0.: Synthesis in vitro of glyceride-glycerol by the liver of normal and pancreatectomized rats. J. Biol. Chem. 239:2416, 1964. 43. Pereira. J. N.. and Holland, G. F.: Studies of the mechanism of action of p-chlorophenoxyisobutyrate (CPIB). Presented at the Second International Symposium on Atherosclerosis. Chicago, November 2-5, 1969. 44. Cahill. G. F.. Ashmore. J.. Earle, A. S. Zottu, S.: Glucose penetration into liver. Amer. J. Physiol. 192:491, 1958. 45. Thielmann, K., Blume, E.. Kraul, M. and Frunder, H.: Der Stoffwechsel geschadigter Gewebe, XIV. Glucose, Fructose und Glucose-6-phosphat in Leber und Muskel normaler und alloxandiabetischer Mause nach Glucose-, Fructose- und Glucase-6-phosphat-Belastung. Hoppe Seyler Z. Physiol. Chem. 322:241, 1960. 46. Heinz, F. and Junghlnel, J.: Metabolitmuster in Rattenleber nach Fructoseapplikation. Hoppe Seyler Z. Physiol. Chem. 350:859, 1969.