Synthesis of Carbamyl Phosphate Synthetase in Liver Slices from Thyroxine-treated Tadpoles.

Metamorphosis of tadpoles into frogs is accompanied by the induction of the enzymes of the urea cycle in the liver (I), which leads to the transition from ammonotelism in tadpoles to ureotelism in the frog. Paik and Cohen (2) observed that thyroxine, which is known to promote anatomical metamorphosis (3, 4), induces a marked increase in the level of carbamyl phosphate synthetase activity in the liver of tadpoles before gross morphological changes are observed. It was established by Metzenberg, Marshall, Paik, and Cohen (5) in experiments in viva that the thyroxine-induced increase in carbamyl phcsphate synthetase activity represents net synthesis de novo of the enzyme. Evidence for this was based on n-leucine-‘4C incorporation into carbamyl phosphate synthetase which was precipitated by means of a specific antibody. The present paper deals with the active incorporation of radioactive leucine into carbamyl phosphate synthetase in liver slices from thyroxine-treated tadpoles. The evidence from this study indicates that carbamyl phosphate synthetase can be synthesized de nova in liver slices of thyroxine-treated tadpoles.