The metabolism of L-arginine and its significance for the biosynthesis of endothelium-derived relaxing factor: L-glutamine inhibits the generation of L-arginine by cultured endothelial cells.

The mechanism by which L-glutamine (L-Gln) inhibits the release of endothelium-derived relaxing factor from bovine aortic cultured endothelial cells was investigated. The intracellular concentration of L-arginine (L-Arg) in Arg-depleted endothelial cells was inversely related to the level of L-Gln. Removal of L-Gln from the culture medium (usually containing L-Gln at 2 mM) abolished the inhibitory effect of the culture medium on L-Arg generation. L-Gln (0.2 and 2 mM) but not D-Gln inhibited the generation of L-Arg by both Arg-depleted and nondepleted endothelial cells. L-Gln did not interfere with the uptake of L-Arg or the metabolism of L-Arg-L-Phe to L-Arg but inhibited the formation of L-Arg from L-citrulline (L-Cit), L-Cit-L-Phe, and NG-monomethyl-L-arginine. L-Gln also inhibited the conversion of L-[14C]Cit to L-[14C]Arg by Arg-depleted endothelial cells. However, L-Gln did not inhibit the conversion of L-argininosuccinic acid to L-Arg by endothelial cell homogenates. Thus, L-Gln interferes with the conversion of L-Cit to L-Arg probably by acting on argininosuccinate synthetase rather than argininosuccinate lyase. L-Gln also inhibited the generation of L-Arg by the monocyte-macrophage cell line J774 but had no effect on the conversion of L-Cit to L-Arg by these cells. As the release of endothelium-derived relaxing factor from cultured and non-cultured endothelial cells is limited by the availability of L-Arg, endogenous L-Gln may play a regulatory role in the biosynthesis of endothelium-derived relaxing factor.

本研究探讨了L-谷氨酰胺（L-glutamine, L-Gln）抑制牛主动脉培养内皮细胞释放内皮源性舒张因子（endothelium-derived relaxing factor）的具体机制。在经精氨酸剥夺处理的内皮细胞中，细胞内L-精氨酸（L-arginine, L-Arg）的浓度与L-Gln水平呈负相关。将常规添加有2 mM L-Gln的培养基中的L-Gln移除后，培养基对L-Arg生成的抑制作用完全消失。0.2 mM与2 mM浓度的L-Gln（而非D-谷氨酰胺），均可抑制经精氨酸剥夺及未剥夺处理的内皮细胞的L-Arg生成。L-Gln不会干扰L-Arg的摄取过程，也不会影响L-Arg-L-苯丙氨酸向L-Arg的代谢，但可抑制L-瓜氨酸（L-citrulline, L-Cit）、L-Cit-L-苯丙氨酸及NG-单甲基-L-精氨酸向L-Arg的转化生成。L-Gln同样可抑制经精氨酸剥夺处理的内皮细胞将L-[¹⁴C]瓜氨酸转化为L-[¹⁴C]精氨酸。但L-Gln不会抑制内皮细胞匀浆中L-精氨琥珀酸向L-Arg的转化过程。由此可见，L-Gln可能通过作用于精氨琥珀酸合成酶（argininosuccinate synthetase）而非精氨琥珀酸裂解酶（argininosuccinate lyase），干扰L-瓜氨酸向L-精氨酸的转化。L-Gln还可抑制单核细胞-巨噬细胞系J774的L-Arg生成，但对该细胞系中L-瓜氨酸向L-Arg的转化过程无影响。由于培养及未培养的内皮细胞释放内皮源性舒张因子的过程受L-Arg可用性的限制，内源性L-谷氨酰胺可能在内皮源性舒张因子的生物合成中发挥调控作用。