Urea cycle enzyme activity in lambs fed different nitrogen levels at low dietary intakes.

Eleven Southdown male lambs averaging 19.8 kg were randomly allotted to two groups and fed diets containing 7.7% (low-N) or 15.8% (high-N) crude protein. All of the supplemental nitrogen in the high-N diet was supplied as urea. Intake of the low-N and high-N diets averaged 372.3 g and 340.5 g/day, respectively. Findings at the end of the thirty-day trial were: (1) mean body weights unchanged for the two groups; (2) plasma urea nitrogen three-fold higher in the high-N (19.07 mg/100 ml) than the low-N (6.57) animals; (3) similar hepatic activity levels of three urea cycle enzymes (ornithine transcarbamylase, argininosuccinase, arginase) in the two groups, and (4) similar liver weights and liver protein concentration. The absence of adaptive change in enzyme levels suggests the hypothesis that addition of non-protein nitrogen to maintenance diets may cause ammonia intoxication by exceeding the liver's reserve capacity for urea synthesis.