D-Fructose in Liver, Intestine, and Kidney

Liver is the principal site of D-fructose metabolism. D-Fructose is transported to the liver from the small intestine by way of the portal blood-vessel. Experiments with perfused pig and rat livers revealed that the rate of elimination of D-fructose from blood is a function of the sugar concentration,26,27 and follows Michaelis-Menten kinetics.27,28 Carrier-mediated, liver-membrane transport of D-fructose has a high29 Km and Vmax, in comparison to the intracellular phosphorylation constants of D-fructose in both pigeon and rat livers.27,28 For example, the calculated rat-liver transport for D-fructose has a Km of 67 mM and a Vmax of 30 /u,mole.min. g-1, in contrast to the lower, calculated fruc-tokinase Km of 1.0 mM and Vmax of 10.3 pmole. min r1. g 1 with D-fruc-tose and Km of 0.54 mM with adenosine 5 -triphosphate (ATP). In perfused pig-liver,28 the transport Km for D-fructose is only ten times that of intracellular phosphorylation by fructokinase. Hence, D-fructose-transport values suggested that, at physiological D-fructose concentrations, membrane transport limits the rate of uptake, thereby protecting the liver from severe depletion of adenine nucleotide.28,29 

Liver metabolizes up to 85% of orally administered D-fructose. Fructokinase, aldolase, and triose kinase are important control enzymes for hepatic D-fructose metabolism.62 The activities of these enzymes are subject to dietary composition and hormonal control. Total fructokinase, aldolase, and triose kinase decrease to about half or less of their normal activity on fasting for 48 to 72 hours, and are restored to normal in 24 hours upon D-fructose administration. Longterm feeding of D-fructose results in the maintenance of a considerably higher level of all three enzymes, an effect also seen with rats maintained on a high-fat or high-protein diet.62 

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