Studies on urea cycle enzymes in rat liver during acute uraemia.

Activities of urea cycle enzymes were measured in the liver of starved rats 12 and 48 h after bilateral nephrectomy. Control experiments (sham-operated, starved rats) revealed that the activities of only two enzymes of the cycle are altered in the uraemic state: argininosuccinic acid synthetase (EC 6.3.4.5.), which is considered to be rate limiting for urea production and carbamyl phosphate synthetase (EC 2.7.2.5.). Alterations in ornithine concentration of the liver, a possible cause of an increased urea production rate, could not be detected previously (21). Our present results do not support the concept that a decrease of the activity of ornithine-delta-amino transferase (EC 2.6.1.13), leading to an increase in the ornithine content of the liver is responsible for the accelerated urea production rate in the liver of acute uraemic rats.