Nissim Itzhak, Horyn Oksana, Luhovyy Bohdan, Lazarow Adam, Daikhin Yevgeny, Nissim Ilana, Yudkoff Marc
Division of Child Development and Rehabilitation Medicine, Department of Pediatrics, Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, PA 19104, USA.
Biochem J. 2003 Nov 15;376(Pt 1):179-88. doi: 10.1042/BJ20030997.
The present study was designed to determine: (i) the role of the reductive amination of alpha-ketoglutarate via the glutamate dehydrogenase reaction in furnishing mitochondrial glutamate and its transamination into aspartate; (ii) the relative incorporation of perfusate 15NH4Cl, [2-15N]glutamine or [5-15N]glutamine into carbamoyl phosphate and aspartate-N and, thereby, [15N]urea isotopomers; and (iii) the extent to which perfusate [15N]aspartate is taken up by the liver and incorporated into [15N]urea. We used a liver-perfusion system containing a physiological mixture of amino acids and ammonia similar to concentrations in vivo, with 15N label only in glutamine, ammonia or aspartate. The results demonstrate that in perfusions with a physiological mixture of amino acids, approx. 45 and 30% of total urea-N output was derived from perfusate ammonia and glutamine-N respectively. Approximately two-thirds of the ammonia utilized for carbamoyl phosphate synthesis was derived from perfusate ammonia and one-third from glutamine. Perfusate [2-15N]glutamine, [5-15N]glutamine or [15N]aspartate provided 24, 10 and 10% respectively of the hepatic aspartate-N pool, whereas perfusate 15NH4Cl provided approx. 37% of aspartate-N utilized for urea synthesis, secondary to the net formation of [15N]glutamate via the glutamate dehydrogenase reaction. The results suggest that the mitochondrial glutamate formed via the reductive amination of alpha-ketoglutarate may have a key role in ammonia detoxification by the following processes: (i) furnishing aspartate-N for ureagenesis; (ii) serving as a scavenger for excess ammonia; and (iii) improving the availability of the mitochondrial [glutamate] for synthesis of N -acetylglutamate. In addition, the current findings suggest that the formation of aspartate via the mitochondrial aspartate aminotransferase reaction may play an important role in the synthesis of cytosolic argininosuccinate.
（i）通过谷氨酸脱氢酶反应使α-酮戊二酸进行还原胺化在提供线粒体谷氨酸及其转氨生成天冬氨酸中的作用；（ii）灌注液中15NH4Cl、[2-15N]谷氨酰胺或[5-15N]谷氨酰胺相对掺入氨基甲酰磷酸和天冬氨酸-N中，从而掺入[15N]尿素异构体中的情况；以及（iii）灌注液中的[15N]天冬氨酸被肝脏摄取并掺入[15N]尿素中的程度。我们使用了一种肝脏灌注系统，其中含有与体内浓度相似的氨基酸和氨的生理混合物，仅在谷氨酰胺、氨或天冬氨酸中含有15N标记。结果表明，在使用氨基酸生理混合物进行灌注时，约45%和30%的总尿素-N输出分别来自灌注液中的氨和谷氨酰胺-N。用于氨基甲酰磷酸合成的氨中，约三分之二来自灌注液中的氨，三分之一来自谷氨酰胺。灌注液中的[2-15N]谷氨酰胺、[5-15N]谷氨酰胺或[15N]天冬氨酸分别提供了肝脏天冬氨酸-N库的24%、10%和10%，而灌注液中的15NH4Cl通过谷氨酸脱氢酶反应净生成[15N]谷氨酸，提供了约37%用于尿素合成的天冬氨酸-N。结果表明，通过α-酮戊二酸还原胺化形成的线粒体谷氨酸可能在以下氨解毒过程中起关键作用：（i）为尿素生成提供天冬氨酸-N；（ii）作为过量氨的清除剂；以及（iii）提高线粒体[谷氨酸]用于合成N-乙酰谷氨酸的可用性。此外，目前的研究结果表明，通过线粒体天冬氨酸转氨酶反应形成天冬氨酸可能在胞质精氨琥珀酸合成中起重要作用。
