Abstract

Administration of a single small dose of triiodothymonine (T₃) greatly increased liver mitochondrial L-α-glycerophosphate dehydrogenase activity of thyroidectomized rats.

The induction of liver mitochondrial L-α-glycerophoshate dehydrogenase by T₃ could be blocked by simultaneous administration of puromycin; in addition, time incorporation of L-leucine-¹⁴C into total liver proteins and into proteins of all subcellular fractions was greatly inhibited. Furthermore, puromycin blocked further induction when it was administered after the administration of T₃. L-Ethionine prevented the induction of enzyme formation, and the inhibition could be partially reversed by L-methionine. "Pulse labeling" was used to study the incorporation of L-leucine-¹⁴C into liver proteins, and the data indicate that an increased rate of protein synthesis precedes the maximal increase in liver mitochondrial L-α-glycerophosphate dehydrogenase produced by T₃-administration. These observations suggest that T₃-induction of L-α-glycerophosphate dehydrogenase in the thyroidectomized rat results from acceleration of enzyme synthesis. Similar observations were reported earlier with the euthyroid rat.

Administration of actinomycin D along with the T₃ abolished the increase of L-α-glycerophosphate dehydrogenase in liver mitochondria. The induction was also partially inhibited by 5-fluorouracil. These results indicate that the induction process depends on the formation of an adequate amount of renewable messenger-RNA molecules.