Variation with age of the enzymes of the urea cycle and aspartate transcarbamylase in rat liver.

Abstract

The activities of aspartate transcarbamylase and of five enzymes involved in the urea cycle were determined in the liver of rats from the embryonic state to adulthood. Aspartate transcarbamylase activity is very high in the embryo and at birth. It remains high until the rat reaches a body weight of 50 g, after which there is a rapid decrease which levels off to a plateau at adulthood. The enzymatic activities of carbamyl phosphate synthetase, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinase, and arginase are very low at the embryonic stage. The activity of these enzymes increases gradually with age until a plateau is reached, except for argininosuccinase which also increases in young animals but decreases in adult life. Of these enzymes, argininosuccinate synthetase always has the lowest activity and seems to be the limiting factor in the synthesis of urea. These results indicate that the biosynthesis of pyrimidines and urea vary inversely at different ages that correspond to different metabolic activities of the animals. Thus, an inverse relation is established between the two pathways from carbamyl phosphate, leading to protein biosynthesis (formation of RNA from orotic acid) and to protein catabolism (formation of urea).