Wendler P A, Blanding J H, Tremblay G C
Arch Biochem Biophys. 1983 Jul 1;224(1):36-48. doi: 10.1016/0003-9861(83)90188-1.
Two enzymes catalyze the synthesis of carbamylphosphate (CP) in the liver. One is intramitochondrial and utilizes ammonia to make CP for ureagenesis; the second is cytoplasmic and utilizes glutamine to produce CP for pyrimidine biosynthesis. The extent to which the metabolic independence of the two pathways is abridged by the use of a common precursor was examined with measurements of the incorporation of [14C]NaHCO3 into orotic acid, uridine nucleotides, and urea in isolated hepatocytes. Pyrimidine synthesis was markedly stimulated by physiological concentrations of ammonia, and the stimulation was antagonized by ornithine. At intracellular concentrations of ornithine and levels of ammonia found in the portal circulation, some 90% of pyrimidine synthesis was ammonia-dependent. When the glutamine-dependent activity was released from feedback inhibition with galactosamine, the ammonia-dependent incorporation still accounted for 2/3 of pyrimidine synthesis. These results do not support the widely held view that the cytoplasmic enzyme is the sole source of CP for pyrimidine biosynthesis in the liver. They suggest instead that the bulk of the CP incorporated into hepatic pyrimidines is of mitochondrial origin. However, an experiment with intact animals failed to provide decisive evidence on this interpretation. Pyrimidine biosynthesis was sharply inhibited by the addition of uridine, but ureagenesis was unaffected. When physiological levels of ammonia were provided, the sensitivity of pyrimidine biosynthesis to uridine was lost. Although inhibition of the ammonia-dependent enzyme by pyrimidines has been observed with cell-free preparations, it was not evident in the intact cell. Thus, to the extent that the CP consumed in pyrimidine biosynthesis is of mitochondrial origin, feedback control of the orotate pathway appears to be thwarted.
肝脏中有两种酶催化氨甲酰磷酸（CP）的合成。一种存在于线粒体内，利用氨生成CP用于尿素生成；另一种存在于细胞质中，利用谷氨酰胺生成CP用于嘧啶生物合成。通过测量[14C]NaHCO3掺入分离的肝细胞中的乳清酸、尿苷核苷酸和尿素的情况，研究了使用共同前体对这两条途径代谢独立性的影响程度。生理浓度的氨显著刺激嘧啶合成，而鸟氨酸可拮抗这种刺激。在门静脉循环中发现的细胞内鸟氨酸浓度和氨水平下，约90%的嘧啶合成依赖于氨。当用半乳糖胺解除谷氨酰胺依赖性活性的反馈抑制时，氨依赖性掺入仍占嘧啶合成的2/3。这些结果不支持广泛持有的观点，即细胞质酶是肝脏中嘧啶生物合成CP的唯一来源。相反，它们表明掺入肝脏嘧啶中的大部分CP源自线粒体。然而，对完整动物进行的一项实验未能为这一解释提供决定性证据。添加尿苷可显著抑制嘧啶生物合成，但尿素生成不受影响。当提供生理水平的氨时，嘧啶生物合成对尿苷的敏感性丧失。尽管在无细胞制剂中已观察到嘧啶对氨依赖性酶的抑制作用，但在完整细胞中并不明显。因此，就嘧啶生物合成中消耗的CP源自线粒体而言，乳清酸途径似乎受到了反馈控制的阻碍。
