Generation of 3HOH from D-[6-3H]glucose by erythrocytes: role of pyruvate alanine interconversion.

Human and rat erythrocytes were found to generate 3HOH from D-[6(N)-3H]glucose. The rate of 3HOH production represented 7-10% of the glycolytic flux. The generation of 3HOH appeared attributable, in part at least, to the detritiation of [3-3H]pyruvate during the interconversion of the 2-keto acid and L-alanine in the reaction catalyzed by glutamate-pyruvate transaminase. Indeed, purified pig heart glutamate-pyruvate transaminase, as well as homogenates prepared from rat erythrocytes or pancreatic islets, catalyzed the generation of 3HOH from L-[3-3H]alanine. When the production of tritiated pyruvate from L-[3-3H]alanine was coupled to the conversion of the 2-keto acid to L-lactate, the production of 3HOH accounted for one-third of the reaction velocity, the latter failing to display isotopic discrimination. In these experiments, the production of 3HOH was abolished by amino-oxyacetate. Likewise, in intact rat erythrocytes, aminooxyacetate inhibited the generation of 3HOH and tritiated L-alanine from D-[6-3H]glucose (or D-[1-3H]glucose), as well as the generation of 3HOH from L-[3-3H]alanine. In pancreatic islets, however, aminooxyacetate failed to affect significantly the generation of 3HOH from D-[6-3H]glucose. These findings indicate that the generation of 3HOH from D-[6-3H]glucose is mainly attributable to an intermolecular tritium transfer in transaminase reaction, at least in cells devoid of mitochondria.