Methaemoglobin Reduction: Studies Using Galactose as Substrate

The effectiveness of galactose as a substrate for methaemoglobin reduction in human red cells has been investigated. The rate of methaemoglobin reduction has been found to depend on the concentration of galactose up to levels of 0.5 M. Methylene blue, which markedly accelerated methaemoglobin reduction when glucose was a substrate, only minimally accelerated methaemoglobin reduction with a galactose substrate. Pre‐incubation with high concentrations of galactose did not inhibit methaemoglobin reduction in the presence of glucose and methylene blue. No inhibition of G‐6‐PD activity was observed with very high concentrations of Gal‐1‐P, glucose‐1‐P, or galactose, even at low concentrations of G‐6‐P. Several possibilities are presented to explain the lack of stimulation of methaemoglobin reduction by methylene blue when galactose is the substrate: 1) inhibition of methylene blue‐stimulated methaemoglobin reduction by galactose or its metabolites; 2) inability of the red cell to metabolize very low concentrations of glucose‐6‐phosphate by way of the hexose monophosphate pathway; and 3) that β‐glucose‐6‐phosphate is not an important product of galactose metabolism. In the latter case, which appears the most likely, a previously undescribed pathway of carbohydrate metabolism must exist in the human erythrocyte. It is suggested that this pathway may represent the direct conversion of α G‐6‐P to fructose‐6‐phosphate, by‐passing the hexose monophosphate pathway, which requires β G‐6‐P as substrate.