Children's Nutrition Research Center, Department of Pediatrics, United States Department of Agriculture/Agricultural Research Service, 1100 Bates Street, Houston, TX 77030, USA.
Am J Physiol Endocrinol Metab. 2010 Jul;299(1):E69-79. doi: 10.1152/ajpendo.00080.2010. Epub 2010 Apr 20.
Although glutamine is considered the main precursor for citrulline synthesis, the current literature does not differentiate between the contribution of glutamine carbon skeleton vs. nonspecific nitrogen (i.e., ammonia) and carbon derived from glutamine oxidation. To elucidate the role of glutamine and nonspecific nitrogen in the synthesis of citrulline, l-[2-(15)N]- and l-[5-(15)N]glutamine and (15)N-ammonium acetate were infused intragastrically in mice. The amino group of glutamine labeled the three nitrogen groups of citrulline almost equally. The amido group and ammonium acetate labeled the ureido and amino groups of citrulline, but not the delta-nitrogen. D(5)-glutamine also infused in this arm of the study, which traces the carbon skeleton of glutamine, was utilized poorly, accounting for only 0.2-0.4% of the circulating citrulline. Dietary glutamine nitrogen (both N groups) incorporation was 25-fold higher than the incorporation of its carbon skeleton into citrulline. To investigate the relative contributions of the carbon skeleton and nonspecific carbon of glutamine, arginine, and proline to citrulline synthesis, U-(13)C(n) tracers of these amino acids were infused intragastrically. Dietary arginine was the main precursor for citrulline synthesis, accounting for approximately 40% of the circulating citrulline. Proline contribution was minor (3.4%), and glutamine was negligible (0.4%). However, the glutamine tracer resulted in a higher enrichment in the ureido group, indicating incorporation of nonspecific carbon from glutamine oxidation into carbamylphosphate used for citrulline synthesis. In conclusion, dietary glutamine is a poor carbon skeleton precursor for the synthesis of citrulline, although it contributes both nonspecific nitrogen and carbon to citrulline synthesis.
虽然谷氨酰胺被认为是瓜氨酸合成的主要前体，但目前的文献并没有区分谷氨酰胺碳骨架与非特异性氮（即氨）和谷氨酰胺氧化衍生的碳对瓜氨酸合成的贡献。为了阐明谷氨酰胺和非特异性氮在瓜氨酸合成中的作用，我们将 l-[2-(15)N]-和 l-[5-(15)N]谷氨酰胺以及 (15)N-乙酸铵经胃内输注到小鼠体内。谷氨酰胺的氨基基团几乎等量标记了瓜氨酸的三个氮基团。酰胺基团和乙酸铵标记了瓜氨酸的脲基和氨基，但不标记 δ-氮。在这项研究的这一部分还输注了 D(5)-谷氨酰胺，它追踪谷氨酰胺的碳骨架，利用率很低，仅占循环瓜氨酸的 0.2-0.4%。膳食谷氨酰胺氮（两个氮基团）的掺入量是其碳骨架掺入瓜氨酸的 25 倍。为了研究谷氨酰胺、精氨酸和脯氨酸的碳骨架和非特异性碳对瓜氨酸合成的相对贡献，我们经胃内输注了这些氨基酸的 U-(13)C(n)示踪剂。膳食精氨酸是瓜氨酸合成的主要前体，约占循环瓜氨酸的 40%。脯氨酸的贡献较小（3.4%），谷氨酰胺可以忽略不计（0.4%）。然而，谷氨酰胺示踪剂导致脲基的富集更高，表明来自谷氨酰胺氧化的非特异性碳掺入用于瓜氨酸合成的氨甲酰磷酸中。总之，尽管谷氨酰胺为瓜氨酸合成提供了非特异性氮和碳，但它作为合成瓜氨酸的碳骨架前体的效率很低。
