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 physiol ogical (1.0 mM) levels of free Mg2+, the V-max of the NADPH oxidation is on ly slightly lower than the V-max of NADH oxidation in the cytosolic glycero l-3-phosphate dehydrogenase (E.C. 1.1.1.8) reaction. Under these conditions physiological (30 mu M) levels of cytosolic malate dehydrogenase (E.C. 1.1 .1.37) inhibited oxidation of 20 mu M NADH but had no effect on oxidation o f 20 mu M 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 measured K-D of complexes bet ween malate dehydrogenase and these reduced pyridine nucleotides, and their K-m in the glycerol-3-phosphate dehydrogenase reactions, it could be concl uded that malate dehydrogenase would have markedly inhibited NADPH oxidatio n and inhibited NADH oxidation considerably more than observed if its only effect were to decrease the level of free NADH or NADPH. This indicates tha t due to the opposite chiral specificity of the two enzymes with respect to reduced pyridine nucleotides, complexes between malate dehydrogenase and N ADH or NADPH can function as substrates for glycerol-3-phosphate dehydrogen ase, but the complex with NADH is less active than free NADH, while the com plex with NADPH is as active as free NADPH. Mg2+ enhanced the interactions between malate dehydrogenase and glycerol-3-phosphate dehydrogenase describ ed above. Lactate dehydrogenase (E.C. 1.1.1.27) had effects similar to thos e of malate dehydrogenase only in the presence of Mg2+, In the absence of M g2+, there was no evidence of interaction between lactate dehydrogenase and glycerol-3-phosphate dehydrogenase. (C) 1999 Academic Press.