Variable expression of isotopic discrimination in metabolic flows.

Isotopic discrimination in reaction velocity may affect to a variable extent the estimation of metabolic flow when a metabolic intermediate is catabolized by two pathways with different degrees of discrimination. This was explored in erythrocytes exposed to 14C- or 3H-labelled D-glucose in the absence or presence of menadione. In the absence of menadione, when the pentose phosphate pathway accounted for only 5% of the D-glucose 6-phosphate turnover, the oxidation of C1-protonated or C1-deuterated D-[U-14C]glucose and D-[1-14C]glucose, mixed with the homologous non-radioactive D-[1-1H]glucose or D-[1-2H]glucose, indicated that, relative to the phosphorylation of the hexose, C1-deuterated D-glucose was less efficiently converted to 14CO2 than C1-protonated D-glucose. Moreover, in the absence of menadione, non-deuterated D-[U-14C]glucose and D-[1-14C]glucose were more efficiently oxidized in cells exposed to D-[1-2H]glucose rather than D-[1-1H]glucose. In the presence of menadione, which increased more than ten-fold the flow rate through the pentose phosphate pathway, the phenomenon of isotopic discrimination was either revealed or masked. These data indicate that the phenomenon of isotopic discrimination may indeed affect to a variable extent the estimation of a given metabolic flow.