Development of Urea‐synthesizing Enzymes in Human Liver

Summary Liver slices from human foetuses of a gestational age between 16 and 20 weeks produce urea when incubated with optimal amounts of substrate. The activities of the enzymes which synthesize urea (carbamyl phosphate synthetase, ornithine transcarbamylase, arginine synthetase system, argininosuccinase and arginase) have been measured in human liver during development. No rapid postparturition increase in activity was discernible in any of the enzymes studied, although in general the adult levels were somewhat higher than those found at birth. Arginine synthetase has the lowest relative activity of all the five enzymes involved in urea synthesis. The apparent Km for citrulline is similar in magnitude for both foetal and adult liver arginine synthetase.