Ability of Cytosolic Malate Dehydrogenase and Lactate Dehydrogenase to Increase the Ratio of NADPH to NADH Oxidation by Cytosolic Glycerol-3-phosphate Dehydrogenase

At the normal pH of the cytosol (7.0 to 7.1) and in the presence of physiological (1.0 mM) levels of free Mg2+, theVmaxof the NADPH oxidation is only slightly lower than theVmaxof NADH oxidation in the cytosolic glycerol-3-phosphate dehydrogenase (E.C. 1.1.1.8) reaction. Under these conditions physiological (30 μM) levels of cytosolic malate dehydrogenase (E.C. 1.1.1.37) inhibited oxidation of 20 μM NADH but had no effect on oxidation of 20 μM NADPH by glycerol-3-phosphate dehydrogenase. Consequently malate dehydrogenase increased the ratio of NADPH to NADH oxidation of glycerol-3-phosphate dehydrogenase. On the basis of the measuredKDof complexes between malate dehydrogenase and these reduced pyridine nucleotides, and theirKmin the glycerol-3-phosphate dehydrogenase reactions, it could be concluded that malate dehydrogenase would have markedly inhibited NADPH oxidation and inhibited NADH oxidation considerably more than observed if its only effect were to decrease the level of free NADH or NADPH. This indicates that due to the opposite chiral specificity of the two enzymes with respect to reduced pyridine nucleotides, complexes between malate dehydrogenase and NADH or NADPH can function as substrates for glycerol-3-phosphate dehydrogenase, but the complex with NADH is less active than free NADH, while the complex with NADPH is as active as free NADPH. Mg2+enhanced the interactions between malate dehydrogenase and glycerol-3-phosphate dehydrogenase described above. Lactate dehydrogenase (E.C. 1.1.1.27) had effects similar to those of malate dehydrogenase only in the presence of Mg2+. In the absence of Mg2+, there was no evidence of interaction between lactate dehydrogenase and glycerol-3-phosphate dehydrogenase.