It is now apparent that many of the subtleties of cellular metabolism are intrinsically associated with cell structure and that their physiological study requires techniques that respect the integrity of cells and organs. We have used 15N-substrates to examine urea synthesis in the intact perfused rat liver. This work permits us to determine the extent to which different amino acids donate nitrogen atoms to the two nitrogens of urea. It is apparent that alanine and the amino group of glutamine provide nitrogen for urea synthesis primarily via cytoplasmic aspartate, whereas mitochondrial ammonia is the preferred route of entry for nitrogen from pre-formed ammonia or from the amide nitrogen of glutamine. Most importantly, this methodology permits us to explore for the occurrence of metabolic channels in such a highly organised, physiological system. Our studies indicate that a metabolic channel does not exist between glutaminase and carbamoylphosphate synthetase 1.

中文翻译：

---

细胞代谢的窗口：（15）N标记底物的肝代谢。

现在很明显，细胞代谢的许多细微差别与细胞结构内在相关，并且它们的生理学研究需要尊重细胞和器官完整性的技术。我们已使用15N底物检查完整灌注大鼠肝脏中的尿素合成。这项工作使我们能够确定不同氨基酸将氮原子捐赠给尿素的两个氮的程度。显然，丙氨酸和谷氨酰胺的氨基主要通过细胞质的天冬氨酸为尿素合成提供氮，而线粒体氨是从预先形成的氨或谷氨酰胺的酰胺氮进入氮的优选途径。最重要的是，这种方法使我们能够探索在如此高度组织化的生理系统中代谢通道的发生。