Fructokinase and

Table 1-12-4. Comparison of Fructokinase and Fructose 1-Pho hate Aldolase Deficiencies...

Enzymes required for fructose to enter intermediary metabolism, Fructose is first phosphorylated to fructose 1-phosphate by fructokinase, and then cleaved by aldolase B to dihydroxyacetone phosphate and glyceraldehyde. These enzymes are found in the liver, kidney, and small intestinal mucosa. 

A. Fructokinase and galactokinase phosphorylate their substrates at carbon 1. 

The metabolism of fructose occurs principally in the liver and to a lesser extent in the small intestinal mucosa and proximal epithelium of the renal tnbnle, becanse these tissues have both fructokinase and aldolase B. Aldolase exists as several isoforms aldolases A, B, C, and fetal aldolase. Although all of these aldolase isoforms can cleave fructose 1,6-bisphosphate, the intermediate of glycolysis, only aldolase B can also cleave fructose 1-phosphate. Aldolase A, present in muscle and most other tissues, and aldolase C, present in brain, have almost no ability to cleave fructose 1-phosphate. Fetal aldolase, present in the liver before birth, is similar to aldolase C. 

Ca and K + Ca " " activates a number of enzyme systems pancreatic lipase is activated by Ca in the presence of bile and insoluble triglycerides, and bacterial 2-oxyglutarate dehydrogenase is also activated by Ca. The apoenzyme of calf intestinal alkaline phosphatase is activated by Ca. K" " activates phosphate fructokinase and pyruvate kinase, respectively. 

Glucose is not the only hexose used for glycolysis— fructose, mannose, and galactose can also enter the glycolytic cycle after phosphorylation. Like glucose, fructose can be used only after phosphorylation in one of three ways [33] (1) phosphorylation to fructose-6-phosphate by hexokinase, (2) phosphorylation to fructose-6-phosphate by a specific fructokinase, and (3) phosphorylation to fructose-1-phosphate by fructokinase (Fig. 1-7). It is well established that the glu-cokinase of liver and muscle can also phosphorylate fructose. Fructose can enter muscle metabolism only in the form of fructose-6-phosphate. This is strikingly different from liver metabolism in which fructose is converted to fructose-1-phosphate by a specific fructokinase. 

Renz A., Merlo L., and Stitt M. 1993. Partial purification from potato tubers of three fructokinases and three hexokinases which show differing organ and developmental specificity. Planta 190 156-165. 

Fructose phosphates are intermediates in the breakdown of glucose according to the Embden-Meyerhof pathway, but free fructose is broken down in a different fashion. At first it is phosphorylated by a fructokinase and ATP to produce/ructose... 

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