Shambaugh G E
Am J Clin Nutr. 1977 Dec;30(12):2083-7. doi: 10.1093/ajcn/30.12.2083.
The urea cycle consist of five enzymatically controlled steps that are catalyzed by carbamyl phosphate synthetase, ornithine transcarbamylase, argininosuccinate synthetase, argininosuccinase, and arginase, respectively. The complete cycle is present in physiological meaningful levels in the liver of terrestrial vertebrates, and in man represents the sole mechanism for ammonia disposal. The formation of carbamyl phosphate and the synthesis of argininosuccinate are potential limiting steps in urea biosynthesis but substrate and not enzymes levels are rate-limiting under physiological conditions. In the adult, urea cycle enzymes change as a unit, and are largely influenced by dietary protein content. The urea cycle is closely linked to the citric acid cycle deriving one of its nitrogens through transamination of oxalacetate to form asparate and returns fumarate to that cycle. The biosynthesis of urea demands the expenditure of energy but less than 20% of the energy derived from metabolism of gluconeogenic amino acids is required for ureogenesis. Embryological development of the urea cycle in the tadpole and in mammalian fetal liver therefore permits use of amino acids as new sources of energy to meet oxidative demands for continuing growth.
尿素循环由五个酶促控制步骤组成，分别由氨甲酰磷酸合成酶、鸟氨酸转氨甲酰酶、精氨琥珀酸合成酶、精氨琥珀酸酶和精氨酸酶催化。完整的循环在陆生脊椎动物的肝脏中以具有生理意义的水平存在，在人类中是氨处理的唯一机制。氨甲酰磷酸的形成和精氨琥珀酸的合成是尿素生物合成中的潜在限速步骤，但在生理条件下，底物水平而非酶水平是限速因素。在成年人中，尿素循环酶作为一个整体发生变化，并且在很大程度上受饮食蛋白质含量的影响。尿素循环与柠檬酸循环紧密相连，通过将草酰乙酸转氨形成天冬氨酸获得其中一个氮，并将延胡索酸返回该循环。尿素的生物合成需要消耗能量，但尿素生成所需的能量不到糖异生氨基酸代谢产生能量的20%。因此，蝌蚪和哺乳动物胎儿肝脏中尿素循环的胚胎发育允许将氨基酸用作新的能量来源，以满足持续生长的氧化需求。
