A study of the control of NADP(+)-dependent isocitrate dehydrogenase activity during gonadotropin-induced development of the rat ovary.

The concentration of cytoplasmic NADP(+)-dependent isocitrate dehydrogenase increased 20.2-fold during gonadotropin-induced development of the immature rat ovary. Measurement was by protein (Western) blotting using polyclonal antibodies raised against purified enzyme from the porcine corpus luteum. The increase in enzyme concentration during development correlated well with the 18.5-fold increase observed for the specific activity of the enzyme in the cytosolic fraction. An immunochemical similarity was demonstrated between the cytoplasmic enzyme from the ovary, testes, placenta, skeletal muscle, brain, liver, kidney, mammary and adrenal gland. However the mitochondrial NADP(+)-dependent isocitrate dehydrogenase from these tissues was found to be immunochemically distinct from the cytoplasmic enzyme. The concentration of the substrate D(+/-)-threo-isocitrate in the ovaries was measured by fluorometry and found to increase 3.1-fold during hormone-induced development. The intracellular concentration of substrate was estimated to be of the same order of magnitude as the enzyme concentration. We conclude that the increase in cytoplasmic NADP(+)-dependent isocitrate dehydrogenase activity observed during the gonadotropin-stimulated development of the rat ovary is due to increased concentration of enzyme rather than to an activation of the enzyme. The activity of the enzyme in vivo appears to be regulated by the availability of the substrate D(+/-)-threo-isocitrate.