The influence of ornithine cycle enzyme activity on the plasma urea concentration in sheep

Abstract

The influence of plane of nutrition and protein content of the diet on the in vitro hepatic activity of the rate-limiting enzyme of the ornithine cycle, arginine synthetase, and the relationship between arginine synthetase activity and plasma urea concentration were studied in lambs and mature sheep. Arginine synthetase activity was positively related to the protein content of the diet, and was reduced when food and thus protein intake was restricted. These findings indicated that in sheep arginine synthetase activity was influenced by the daily protein intake. The plasma urea concentration was positively related to arginine synthetase activity irrespective of the base used to express enzyme activity. There were differences between experiments in the regression equations describing the relationships between plasma urea concentration and arginine synthetase activity, which suggest that the relationships may not have broad application. The implications of these findings are discussed in relation to the control of plasma urea concentration in sheep.