Ability of cytosolic malate dehydrogenase and lactate dehydrogenase to increase the ratio of NADPH to NADH oxidation by cytosolic glycerol-3-phosphate dehydrogenase
La. Fahien et al., Ability of cytosolic malate dehydrogenase and lactate dehydrogenase to increase the ratio of NADPH to NADH oxidation by cytosolic glycerol-3-phosphate dehydrogenase, ARCH BIOCH, 364(2), 1999, pp. 185-194
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.