Urea biosynthesis in normal human fetuses

Normal human fetuses at different gestation periods were collected on ice after hysterotomy and the enzymes of the urea cycle were measured in the liver. The activity of all enzymes increased with increasing gestational age towards the adult value, however, in no case did the values reach the normal adult level. The bladder fluid of these fetuses contained urea and ammonia nitrogen at concentrations which were akin to the concentrations found in fetal blood. The ornithine transcarbamylase activity was the lowest when compared to the adult values and appeared to be the rate-limiting enzyme in the cycle, along with argininosuccinic acid synthetase activity, which was also very low. The activity of arginase was found to be the highest in the cycle. The very low ornithine transcarbamylase and argininosuccinic acid synthetase activities and the comparatively higher arginase activity migh lead to the channeling of ornithine into alternate metabolic pathways.