Urea cycle enzymes in human liver: ontogenesis and interaction with the synthesis of pyrimidines and polyamines.

All the five enzymes of urea synthesis and the formation of urea in vitro can already be demonstrated in human liver as early as the 9th week of fetal development. At this stage the activity of carbamoyl phosphate synthetase is the highest, whereas that of ornithine carbamoyltransferase is the lowest as compared to those in the adult. The kinetic parameters of the urea cycle enzymes are the same in fetal liver as in adult liver, except that the Km values of ornithine carbamoyltransferase for L-ornithine are 3.5 mM and 0.42 mM in the fetus and in adult liver, respectively. Urea formation in vivo seems to begin in the second half of fetal life, and a gradual increase can be detected in the activity of the enzymes of urea synthesis. The activity of ornithine decarboxylase, the glutamine-dependent carbamoyl phosphate synthetase and aspartate carbamoyltransferase, however, changes in the opposite direction. The concentration of carbamoyl phosphate and aspartate remains constant, but that of ornithine gradually decreases during ontogenesis. The ornithine, carbamoylphosphate and aspartate pools are probably utilized in the polyamine, pyrimidine and urea syntheses at varying rates.