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).

Enzymes of Arginine and Urea Systhesis

“…The results of a recent study on rats by Raiha and Suihkonen (205) confirm those of earlier investigators (206)(207)(208). The pattern of change (Fig.…”

Regulation of Ureagenesis in Neonatal Rat Liver: Influence of Cycloheximide in vivo and in vitro

“…It is known from previous studies that, 24 h after birth, urea cycle enzymes present a marked rise in enzyme activities [4,7,8,12,16,21,22]. It was then interesting to specify the nature of the physiological mechanism which causes this postnatal increase.…”

“…), located in the cytosolic fraction. The developmental pat tern of these enzyme activities in perinatal liver in vivo has been previously studied [4,7,8,12,15,21,22], These activities regularly increased during the late fetal period and pre sented a marked rise 24 h after birth. Argininosuccinate synthetase activity appeared to be the rate-limiting step of the urea cycle [11][12][13][14]21].…”

Gluconeogenesis in the Newborn Rat: the Substrates and their Quantitative Significance

“…The rate-limiting enzyme in urea biosynthesis is argininosuccinate synthetase (17) and this enzyme appears to be the most re sponsive and sensitive index of protein catabolism (52,188). The neonatal devel opment of the urea cycle enzymes has been described (26,61,84,98,129,154) and the patterns for arginase and argininosuccinate synthetase are shown in figures 5b, c. The neonatal developmental profiles of the two enzymes correlate well with each other and with that of urea secretion and also show a transient elevation of activity at about 5 days postpartum before the major increase at weaning.…”