Karsai T, Elödi P
Mol Cell Biochem. 1982 Mar 19;43(2):105-10. doi: 10.1007/BF00423098.
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.
早在胎儿发育的第9周，人体肝脏中就能检测到尿素合成的所有五种酶以及体外尿素的形成。在此阶段，氨甲酰磷酸合成酶的活性最高，而与成人相比，鸟氨酸氨甲酰转移酶的活性最低。除了胎儿肝脏和成人肝脏中鸟氨酸氨甲酰转移酶对L-鸟氨酸的Km值分别为3.5 mM和0.42 mM外，胎儿肝脏中尿素循环酶的动力学参数与成人肝脏相同。体内尿素的形成似乎始于胎儿期的后半段，并且可以检测到尿素合成酶的活性逐渐增加。然而，鸟氨酸脱羧酶、谷氨酰胺依赖性氨甲酰磷酸合成酶和天冬氨酸氨甲酰转移酶的活性则呈相反方向变化。氨甲酰磷酸和天冬氨酸的浓度保持恒定，但鸟氨酸的浓度在个体发育过程中逐渐降低。鸟氨酸、氨甲酰磷酸和天冬氨酸池可能以不同的速率用于多胺、嘧啶和尿素的合成。
