Interconversion of D-fructose 1,6-bisphosphate and triose phosphates in human erythrocytes

Aldolase and triose phosphate isomerase both display strict specificity towards the enantiomers of [1- 3 H]glycerone 3-phosphate. The enantiomer generated from d -[1- 3 H]glyceraldehyde 3-phosphate produces 3 HOH in the aldolase reaction, whilst the other enantiomer generated from d -[3- 3 H]fructose 1,6-bisphosphate is solely detritiated in the reaction catalyzed by triose phosphate isomerase. Advantage was taken of such a specificity to assess, in human erythrocytes exposed to either d -[3- 3 H]glucose or d -[3,4- 3 H]glucose, the extent of d -glyceraldehyde 3-phosphate sequential conversion to glycerone 3-phosphate and d -fructose 1,6-bisphosphate, relative to net glycolytic flux. At 37°C and in the presence of 5.6 mM d -glucose, only 55% of the metabolites of d -[4- 3 H]glucose underwent detritiation in the reactions catalyzed by triose phosphate isomerase and aldolase. Such a percentage was further decreased at low temperature (8°C) or lower concentrations of d -glucose (0.2 and 1.0 mM). However, when the erythrocytes were exposed to menadione, the increase in 3 HOH production from either d -[3- 3 H]glucose or d -[3,4- 3 H]glucose indicated that the majority of the 3 H atoms initially located on the C 4 of d -glucose were recovered as 3 HOH upon circulation through the pentose phosphate pathway. These findings suggest that, under physiological conditions, a large fraction of d -glyceraldehyde 3-phosphate generated from exogenous d -glucose may undergo enzyme-to-enzyme channelling in the glycolytic pathway.