Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates: a model for the interconversion of d-[2-3H]glucose 6-phosphate and d-[1-3H]fructose 6-phosphate

Abstract Based on experimental data, a model is proposed for the interconversion of either unlabelled hexose phosphates or d -[2- 3 H]glucose 6-phosphate and d -[1- 3 H]fructose 6-phosphate in the reaction catalyzed by phosphoglucoisomerase. This model takes into account the known differences in maximal velocity and affinity for each substrate, the intramolecular transfer of tritium between C 1 and C 2 , and the isotopic discrimination between unlabelled and tritiated esters. This model revelas that, in a close system characterized by the progressive detritiation of hexose phosphates, the concentration ratio of d -glucose 6-phosphate is much higher with the tritiated than unlabelled esters, a paradoxical increase in the specific radioactivity of d -glucose 6-phosphate above its initial value being even observed during the initial period of exposure of d -[2- 3 H]glucose 6-phosphate to phosphoglucoisomerase. The extension of this model to an open system may be essential for the correct interpretation of radioactive data collected in intact cells exposed to d -[2- 3 H]glucose.