[Participation of thyroxine in the regulation of the rate of isocitrate dehydrogenase reactions in the brain and liver].

A study was made of the effect of thyroxin on the activity of NAD- and NADP-dependent isocitratedehydrogenases in the brain and the liver of chick embryos. It was found that in both of the organs thyroxin accelerated isocitrate oxidation in the mitochondria through the NAD-dependent route and elevated the velocity of the NADP-isocitratedehydrogenase reaction in the citoplasm. At the same time the activity of the mitochondrial NADP-dependent isocitratedehydrogenase increased after the administration of the hormone to the embryos only in the mitochondria of the brain cells; this was apparently associated with the intensive biosynthesis of specific lipids in the developing brain. There was noted a more marked change in the activity of the enzymes in the 18-day embryos, in comparison with the 12-day ones.