Studies on carbamoylphosphate synthetase in livers of rats fed a protein free diet supplemented with methionine and threonine and injected with N-acetylglutamic acid.

Supplementation of methionine and threonine to a protein free diet reduces urinary nitrogen excretion in rats, and it has been assumed that N-acetylglutamic acid may have a regulatory role in urea synthesis. The present paper reports the effect of supplementation of methionine and threonine on the levels of N-acetylglutamic acid and of carbamoylphosphate synthetase (EC 2.7.2.5) in liver. In spite of a significant decrease in urinary urea when methionine and threonine were supplemented to the protein free diet, the activity of carbamoylphosphate synthetase was not reduced. On the other hand, following injection of glutamic acid, N-acetylglutamic acid or N-acetylglutamine, the excretion of urinary urea of rats fed the protein free diet supplemented with methionine and threonine was significantly reduced whereas the level of carbamoylphosphate synthetase was the same in each group. Furthermore, the concentrations of N-acetylglutamic acid in liver of rats fed the protein free diet or the protein free diet supplemented with methionine and threonine were the same. Therefore, these results indicated that, when methionine and threonine were supplemented to the protein free diet, the changes in the levels of N-acetylglutamic acid and of carbamoylphosphate synthetase in liver does not contribute to the reduced body weight loss and urinary nitrogen excretion.