ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05313.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract:Sucrose in synthetized in the green leaves of plants. With increasing economical status the sucrose from sugar cane and beets, like fat, supplies an increasing fraction of our food. Sucrose is easily metabolized and utilized. Too high consumption is, however, not desirable from nutritional point of view, since this highly refined product contains calories but no essential nutrients.The general concept is that animals (and humans) do not synthesize sucrose. A single report of a sucrose‐synthesizing patient needs confirmation from other sources before it can be accepted.The intestinal absorption of sucrose can only occur if the hydrolyzing enzyme, invertase (sucrase), is present in the mucosal cells. Acid hydrolysis in the stomach has been suggested, but does not occur. The intestinal invertase is an a‐glucosidase.In the human intestine invertase, as well as the other a‐glucosidases, is developed very early in fetal life — much earlier than lactase. In the animals studied so far, in contrast, intestinal invertase and other a‐glucosidases are weak until the weaning period, when lactase disappears and the a‐glucosidase develops. The reason for these species differences is yet unexplained.Human populations with low sucrose consumption have approximately equally high intestinal invertase activity as those in countries with high sucrose consumption. In Greenland Eskimos the adults have for a very long period of time (probably thousands of years) consumed nearly no carbohydrates at all. The average intestinal activity of invertase and the other a‐glucosidases in the Greenland Eskimos is nevertheless nearly the same as ours. Specific enzyme defects, which among us occur only as very rare cases of «inborn errors of metabolism« are, however, rather frequent in the Gr

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05314.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.The metabolism of fructose by the small intestine can be analyzed in terms of the following scheme: 1) hydrolysis of fructose containing saccharides especially sucrose; 2) movement of fructose into the intestinal cell; 3) transformation of fructose into glycolytic metabolic intermediates; 4) formation of fructose from glucose via sorbitol; 5) adaptive regulation of fructose metabolizing enzymes; 6) adaptive responses of other enzymes to fructose. The hydrolysis of sucrose is dependent upon the brush border enzyme sucrase which shows an adaptive response to sucrose diets. The entrance of fructose into the small intestine and the intermediary metabolism of fructose is reviewed. Fructose metabolizing enzymes, fructokinase and fructose‐1‐phosphate aldolase, are regulated by the presence or absence of fructose, folic acid and drugs. Fructose causes adaptive changes in small intestine glycolytic enzymes and decreases the gluconeogenic enzyme fructose‐1,6‐diphosphatase. Actinomycin D inhibits the adaptive effect of fructose on glycolytic enzymes which suggests that fructose acts via the protein synthetic me

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05315.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.For liver a metabolic pathway for fructose has been described in which the ketosugar is phosphorylated by ketohexokinase to fructose‐1‐phosphate, which is then converted by liver aldolase to D‐glyceraldehyde and dihydroxyacetone phosphate, an intermediate of the glycolytic pathway. D‐glyceraldehyde could be oxidized to glycerate. By phosphorylation glycerate becomes an intermediate of the Embden‐Meyerhof pathway, like D‐glyceraldehyde if phosphorylated directly by triokinase. The reduction of D‐glyceraldehyde forming glycerol, which could be phosphorylated to L‐glycerol 3‐phosphate is in variance with isotope studies with fructose‐6‐14C.This special metabolic pathway is limited to warmblooded animals and man, because only in the liver of these species ketohexokinase could be detected.An adaption of enzymes was found in rats, which have had a high fructose diet over three weeks.The activity of ketohexokinase estimated under optimal conditions and 37°C agrees well with fructose extraction rates found in liver perfusion for rat and in in vivo experiments for human liver. The fast catabolism of fructose in human liver, in contrast to glucose, is due to higher enzyme levels of ketohexokinase, in contrast to hexokinase and glucokinase. By this high phosphorylation capacity and the low activity of aldolase together with the action of metabolic inhibitors on this enzyme and the equilibrium far at the side of fructose‐1‐phosphate, fructose‐1‐phosphate will accumulate if high fructose concentrations were offered. But even under these conditions, the levels of metabolites following fructose‐1‐phosphate were enlarged.Enzyme regulations based on the high fructose‐1‐phosp

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05316.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.The rate limiting step of fructose metabolism in adipose tissue is fructose transport into the cell. The step is accelerated by insulin only in the absence of glucose, so that insulin has no effect on fructose transport under physiologic conditions. The fructose carrier has a relatively high apparent Kmfor fructose transport so that significant quantities are transported only at relatively high concentrations of fructose in the blood.Fructose is the phosphorylated to fructose‐6‐phosphate by the enzyme hexokinase. Glucose does not compete because it is not present intracellularly in sufficiently high concentrations. This statement is correct only for adipose tissue.Fructose uptake by adipose tissue of diabetic animals is reduced as is glucose uptake, but to a lesser extent.In vivo, most of the C14‐incorporation of fructose‐C14into triglycerides or C14O2by adipose tissue occurs after hepatic conversion of C14‐fructose to C14‐glucose. Insulin is required for glucose transport into adi

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05317.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.Physiological factors which may regulate the catalytic activity of three key enzymes of hepatic fructose metabolism, fructokinase, aldolase and triokinase, are reviewed. Possible relationships between these potential control mechanisms and hepatic fructose metabolismper seare discussed at three distinct levels: 1) characterizationin vitroof purified enzyme preparations; 2) variation of enzyme activityin vivounder various dietary and hormonal conditions; and 3) variation of enzyme activityin vivofollowing development of liver tumors of different growth rate and degree of cellular differentiation.

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05318.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.In normal subjects infusion of fructose (1 g/kg/hr) for 4 hrs resulted in an increase in the glycogen content of them. quadriceps femorisof 3.3 g/kg wet muscle (muscle samples obtained by needle biopsy). This was equal to the amount of glycogen formed after a glucose infusion of the same magnitude. In muscle depleted of glycogen by exercise, infusion of glucose resulted in twice as much glycogen formed, as did a fructose infusion. Formation of liver glycogen was much higher after fructose than after glucose infusion (liver samples obtained by Menghini biopsy). Studies by hepatic vein catheterization indicated that glucose formation by the liver was insufficient to account for the synthesis of muscle glycogen, which presumably occurred directly from fructose taken up by the muscle.The occurrence of high lactic acid concentrations in blood resulting from infusion of fructose could be partly abolished by simultaneous infusion of amino acids, thus lessening the risk of lactic acidosis.

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05319.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.The clinical and metabolic aspects of hereditary fructose intolerance (HFI) are reviewed and some new observations on children with HFI are reported. The acute rise in plasma FFA level which was earlier shown to follow an acute exposure to fructose in these patients, was found to be associated with an increase in epinephrine excretion and a decrease in plasma TG level. Abolition of the fructose induced hypoglycemia by an infusion of glucose after injection of fructose prevented the rise in plasma FFA, but did not modify the increase in plasma lactate or urate concentration. Maintaining normal or high plasma inorganic phosphate levels after fructose injection by infusion of phosphate buffer did not alter the fructose‐induced changes in blood glucose or lactate, or in plasma FFA or urate concentration. The hyperuricemia and hyperuricosuria after fructose were larger in HFI patients than in normal children. Administration of fructose in the small doses of 0.33‐1.3 g/kg daily for 3–5 days brought about a gradual fall in plasma cholesterol level, and a rise in the excretion of urate and 17‐ketogenic steroids, but no increase in prot

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05320.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.Two hereditary alterations of fructose metabolizing enzymes are known. We have shown that in essential fructosuria, there is in fact a deficiency of fructokinase activity. We have shown that in hereditary fructose intolerance (HFI) some abnormal properties of aldolase in liver are related to aldolases A (muscle type) and C (brain type) which are normally synthesized by embryo, and which persist without change. In livers with HFI, we have found a protein immunologically related to aldolase B (liver adult type), the enzymatic activity of which is about 3 per cent of the normal value. Its Michaelis constant for fructose‐1‐phosphate is greatly increased.We conclude that, in hereditary fructose intolerance, there is a mutation of the structural g

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05321.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY

摘要:

Abstract.The changes in the metabolite content in freeze‐clamped livers of fed rats occurring on perfusion with 10 mM D‐fructose have been examined under aerobic and anaerobic conditions. During aerobic perfusion the main effects of fructose were an accumulation of fructose 1‐phosphate, as already known, up to 8.7 μmol/g of liver within 10 min, a loss of total adenine nucleotides (up to 35 % after 40 min) with a decrease in the ATP content to 23 % within 10 min, a seven‐fold rise in the concentration of IMP to 1.1 μmol/g and an eight‐fold rise of a‐glycerophosphate to 1.1 μmol/g. There was a transient decrease in Pifrom 4.2 to 1.7 μmol/g. Within 40 min the Picontent recovered to the normal value. The content of lactate increased to 4.3 μmol/g at 80 min; pyruvate also increased and the [lactate]/[pyruvate]ratio remained within physiological limits. The concentration of free fructose within the liver remained much below that in the perfusion medium, indicating that the rate of penetration of fructose into the tissue was lower than the rate of utilisation. The fission of fructose 1‐phosphate by liver aldolase is inhibited by several phosphorylated intermediates, especially by IMP. This inhibition is competive with a Kiof 0.1 mM. The maximal rates of the enzyme synthesising and splitting fructose 1‐phosphate are about equal. The accumulation of fructose 1‐phosphate on fructose loading is due to the inhibition of the fission of fructose 1‐phosphate by the IMP arising from the degradation of the adenine nucleotides. When the conditions were anaerobic fructose was readily converted to lactate and the fructose 1‐phosphate content of the liver after 40 min rose to 5.5 μmol/g. The total adenine nucleotide content decreased to 1.74 μmol/g. The contents of a‐glycerophosphate and lactate were 4.3 μmol/g and 5.6 μmol/g respectively, the [la

ISSN:0001-6101    

DOI:10.1111/j.0954-6820.1972.tb05322.x

出版商:Blackwell Publishing Ltd    

年代:1972

数据来源: WILEY