Comparison between d-[3-3H]- and d-[5-3H]glucose and fructose utilization in pancreatic islets from control and hereditarily diabetic rats

The metabolism of d-glucose and/or d-fructose was investigated in pancreatic islets from control rats and hereditarily diabetic GK rats. In the case of both d-glucose and d-fructose metabolism, a preferential alteration of oxidative events was observed in islets from GK rats. The generation of 3HOH from d-[5-3H]glucose (or d-[5-3H]fructose) exceeded that from d-[3-3H]glucose (or d-[3-3H]fructose) in both control and GK rats. This difference, which is possibly attributable to a partial escape from glycolysis of tritiated dihydroxyacetone phosphate, was accentuated whenever the rate of glycolysis was decreased, e.g., in the absence of extracellular Ca2+ or presence of exogenous d-glyceraldehyde. d-Mannoheptulose, which inhibited d-glucose metabolism, exerted only limited effects upon d-fructose metabolism. In the presence of both hexoses, the paired ratio between d-[U-14C]fructose oxidation and d-[3-3H]fructose or d-[5-3H]fructose utilization was considerably increased, this being probably attributable, in part at least, to a preferential stimulation by the aldohexose of mitochondrial oxidative events. Moreover, this coincided with the fact that d-mannoheptulose now severely inhibited the catabolism of d-[5-3H]fructose and d-[U-14C]fructose. The latter situation is consistent with both the knowledge that d-glucose augments d-fructose phosphorylation by glucokinase and the findings that d-mannoheptulose, which fails to affect d-fructose phosphorylation by fructokinase, inhibits the phosphorylation of d-fructose by glucokinase.