Metabolism of tritiated d-glucose in rat erythrocytes

Abstract

The metabolism of d-[U- 14C]glucose, d-[1- 14C]glucose, d-[6- 14C]glucose, d-[1- 3H]glucose, d-[2- 3H]glucose, d-[3- 3H]glucose, d-[3,4- 3H]glucose, d-[5- 3H]glucose, and d-[6- 3H]glucose was examined in rat erythrocytes. There was a fair agreement between the rate of 3HOH production from either d-[3- 3H]glucose and d-[5- 3H]glucose, the decrease in the 2,3-diphosphoglycerate pool, its fractional turnover rate, the production of 14C-labeled lactate from d-[U- 14C]glucose, and the total lactate output. The generation of both 3HOH and tritiated acidic metabolites from d-[3,4- 3H]glucose indicated uncomplete detritiation of the C 4 during interconversion of fructose-1,6-bisphosphate and triose phosphates. Erythrocytes unexpectedly generated 3HOH from d-[6- 3H]glucose, a phenomenon possibly attributable to the detritiation of [3- 3H]pyruvate in the reaction catalyzed by glutamate pyruvate transaminase. The production of 3HOH from d-[2- 3H]glucose was lower than that from d-[5- 3H]glucose, suggesting enzyme-to-enzyme tunneling of glycolytic intermediates in the hexokinase/phosphoglucoisomerase/phosphofructokinase sequence. The production of 3HOH from d-[1- 3H]glucose largely exceeded that of 14CO 2 from d-[1- 14C]glucose, a situation tentatively ascribed to the generation of 3HOH in the phosphomannoisomerase reaction. It is further speculated that the adjustment in specific radioactivity of d-[1- 3H]glucose-6-phosphate cannot simultaneously match the vastly different degrees of isotopic discrimination in velocity at the levels of the reactions catalyzed by either glucose-6-phosphate dehydrogenase or phosphoglucoisomerase. The interpretation of the present findings thus raises a number of questions, which are proposed as a scope for further investigations.