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-H-3]glycerone 3-phosphate. The enantiomer generated from D-[1-H-3]glyceraldehyde 3-phosphate produces (HOH)-H-3 in the aldolase reaction, whilst the other enantiomer generated from D-[3-H-3]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-H-3]glucose or D-[3,4-H-3]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-degrees-C and in the presence of 5.6 MM D-glucose, only 55% of the metabolites of D-[4-H-3]glucose underwent detritiation in the reactions catalyzed by triose phosphate isomerase and aldolase. Such a percentage was further decreased at low temperature (8-degrees-C) or lower concentrations of D-glucose (0.2 and 1.0 mM). However, when the erythrocytes were exposed to menadione, the increase in (HOH)-H-3 production from either D-[3-H-3]glucose or D-[3,4-H-3]glucose indicated that the majority of the H-3 atoms initially located on the C4 Of D-glucose were recovered as (HOH)-H-3 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.