Jayakumar Arumugam R, Rama Rao K V, Schousboe Arne, Norenberg Michael D
Department of Pathology, University of Miami School of Medicine, Miami, Florida 33101, USA.
Glia. 2004 May;46(3):296-301. doi: 10.1002/glia.20003.
Ammonia is a neurotoxin implicated in the pathogenesis of hepatic encephalopathy, Reye's syndrome, inborn errors of the urea cycle, glutaric aciduria, and other metabolic encephalopathies. Brain ammonia is predominantly metabolized to glutamine in astrocytes by glutamine synthetase. While the synthesis of glutamine has generally been viewed as the principal means of ammonia detoxification, this presumed beneficial effect has been questioned as growing evidence suggest that some of the deleterious effects of ammonia may be mediated by glutamine rather than ammonia per se. Since ammonia is known to induce the production of free radicals in cultured astrocytes, we investigated whether such production might be mediated by glutamine. Treatment of astrocytes with glutamine (4.5 mM) increased free radical production at 2-3 min (95%; P < 0.05), as well as at 1 and 3 h (42% and 49%, respectively; P < 0.05). Similarly treated cultured neurons failed to generate free radicals. Free radical production by glutamine was blocked by the antioxidants deferoxamine (40 microM) and alpha-phenyl-N-tert-butyl-nitrone (250 microM), as well as by the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine methyl ester (500 microM). Free radical production was also blocked by 6-diazo-5-oxo-L-norleucine (1 mM), an inhibitor of glutaminase, suggesting that ammonia released by glutamine hydrolysis may be responsible for the generation of free radicals. Additionally, the mitochondrial permeability transition inhibitor, cyclosporin A, blocked free radical production by glutamine. The results indicate that astrocytes, but not neurons, generate free radicals following glutamine exposure. Glutamine-induced oxidative and/or nitrosative stress may represent a key mechanism in ammonia neurotoxicity.
氨是一种神经毒素，与肝性脑病、瑞氏综合征、尿素循环的先天性缺陷、戊二酸尿症及其他代谢性脑病的发病机制有关。脑内的氨主要在星形胶质细胞中通过谷氨酰胺合成酶代谢为谷氨酰胺。虽然谷氨酰胺的合成通常被视为氨解毒的主要方式，但随着越来越多的证据表明氨的一些有害作用可能是由谷氨酰胺而非氨本身介导的，这种假定的有益作用受到了质疑。由于已知氨可诱导培养的星形胶质细胞产生自由基，我们研究了这种产生是否可能由谷氨酰胺介导。用谷氨酰胺（4.5 mM）处理星形胶质细胞，在2 - 3分钟时自由基产生增加（95%；P < 0.05），在1小时和3小时时也增加（分别为42%和49%；P < 0.05）。同样处理的培养神经元未能产生自由基。谷氨酰胺产生的自由基被抗氧化剂去铁胺（40 microM）和α - 苯基 - N - 叔丁基硝酮（250 microM）以及一氧化氮合酶抑制剂N（ω） - 硝基 - L - 精氨酸甲酯（500 microM）阻断。自由基产生也被谷氨酰胺酶抑制剂6 - 重氮 - 5 - 氧代 - L - 正亮氨酸（1 mM）阻断，这表明谷氨酰胺水解释放的氨可能是自由基产生的原因。此外，线粒体通透性转换抑制剂环孢素A阻断了谷氨酰胺产生的自由基。结果表明，谷氨酰胺暴露后，星形胶质细胞而非神经元会产生自由基。谷氨酰胺诱导的氧化和/或亚硝化应激可能是氨神经毒性的关键机制。
