Effect of a maternal fast on the urea cycle enzymes of the ovine fetus.

Activities of five urea cycle enzymes were measured in maternal and fetal sheep liver during the normal fed state and following 5 days of fasting. Six ewes and 10 fetuses were studied in both the fed and fasted periods at 132 days gestation (term: 147 days) for liver protein and enzyme levels. Results indicated that protein content increased during fasting in both the maternal and fetal liver. Fetal liver weight was decreased during fasting from 108 +/- 8.5 to 71 +/- 8.2 g (mean +/- SD) (p less than 0.001). Fed state fetal enzyme activities per gram liver were 50-125% of maternal values. After fasting, four of the five fetal enzymes increased approximately twofold to fivefold (per gram tissue) (ornithine transcarbamylase did not change). Only one enzyme (argininosuccinase) increased significantly in maternal liver. Total liver activities gave similar results. These data indicate that the in vivo studies that demonstrate a doubling in fetal urea production in the fasted sheep in later gestation are associated with parallel increases in the fetal hepatic activities of several enzymes that are responsible for urea synthesis.