[Change in the tricarboxylic acid cycle dehydrogenase activity under the action of thyroxine in the brain of animals of varying age].

The effect of thyroxin on the activities of tricarboxylic acid cycle enzymes and NADP-malate dehydrogenase in the rat brain was studied. Experiments were conducted on 20- and 40-day old and on adult rats. Injection of L-thyroxin (50 microgram/100 g body weight) to thyroidectomized rats induced some changes in the activities of citrate-syntase, dehydrogeases of TAC and malic enzymes in the mitochondrial and cytoplasmic fractions of the brain. The most significant effect of the hormone was found in the brain of 20-day old rats there was an increase of the enzyme activities by 40--60%. The effect of the hormone on the enzymes activities could be prevented by simultaneous administration of actinomycin D (10 microgram intracerebrally). This fact indicates that the main mechanism in increasing the enzymic activity lies, probably, in stimulation of the enzyme biosynthesis by the hormone.