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

Abstract

In order to establish whether the postnatal rise in five urea cycle enzyme activities is associated with protein synthesis, the effects of cycloheximide on urea cycle enzyme activities, citrulline and urea productions in vivo and in vitro were studied. A single injection of cycloheximide (10 μg) 9 h after birth significantly prevented the rise in enzyme activities and citrulline concentration which normally occurred after birth, while the urea level remained unchanged. These data suggest that the postnatal rise in urea cycle enzyme activities might be associated with a protein synthesis. The rate of citrulline synthesis by liver slices was reduced (about 50%) in cycloheximide-treated neonatal liver, while the rate of urea production was not significantly decreased. The results obtained in vivo are in good agreement with in vitro experiments: citrullinogenesis was only affected by cycloheximide treatment. When cycloheximide 10 mM was added to the incubation medium, the ability of control neonatal liver slices to produce citrulline and urea was reduced to 42 and 11%, respectively. Since argininosuccinic acid addition in the medium did not produce a rise in urea synthesis, it is concluded that argininosuccinase is most responsive after incubation in cycloheximide and therefore becomes the rate-limiting step of the urea synthesis.

Changes in Argininosuccinate Lyase Gene Expression in the Rat Liver during Development

“…The ontogenesis of rat liver ASL was first examined by Raiha and Suihkonen [12], Miller and Chu [35] and Gautier et al [13], In this work we extended this data, showing that ASL activity could be measured as soon as day 15.5 of gestation and that its rise occurred concomitantly with the accumulation of the enzyme protein. This parallel increase provides direct evidence that the emergence of the catalytic activity repre sents de novo synthesis and supports our pre vious conclusions concerning the require ment for protein synthesis for ASL induction in the cycloheximide-treated newborns [36]. Such a constant ratio of enzyme activity and amount of protein during development had been observed for other rat liver enzymes [37], but the quantitative measurements of liver ASL made here were the first direct determinations of the amount of this enzyme during development.…”