Department of Biological Science, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore.
Aquat Toxicol. 2010 Jun 1;98(1):91-8. doi: 10.1016/j.aquatox.2010.01.018. Epub 2010 Feb 4.
This study aimed to examine the hypothesis that intestinal glutamate dehydrogenase (GDH) and glutamine synthetase (GS) could be involved in ammonia detoxification in the euryhaline Bostrychus sinensis exposed to ammonia in a hyperosmotic environment, whereby drinking was essential for osmoregulation. Our results indicate that there was a significant increase in ammonia content in the intestine of B. sinensis exposed to 15 mmol l(-1) NH(4)Cl in seawater (pH 7.0) for 6 days. There were also significant increases in the amination and deamination activities and protein abundance of intestinal GDH. The GDH amination/deamination ratio remained unchanged, indicating that there could be increases in the turnover of glutamate. However, the difference between the amination and deamination activities increased 2-fold, implying that there could be an increase in glutamate formation in the intestine. Since the intestinal glutamate content remained unchanged, excess glutamate formed might have been channeled into other amino acids and/or transported to other organs. Indeed, the intestinal glutamine content increased significantly by 2-fold, with a significant increase in the activity and protein abundance of intestinal GS. Since the magnitude of glutamine accumulation in the intestine was lower than those in liver and muscle, which lacked changes in GDH activities, intestinal glutamate could have been shuttled to liver and muscle to facilitate increased synthesis of glutamine therein. By contrast, when fish were exposed to a much higher concentration (30 mmol l(-1)) of NH(4)Cl in 5 per thousand water (pH. 7.0), the magnitude of increase in ammonia content in the intestine was less prominent, and there were no changes in activities and kinetic properties of intestinal GDH. Therefore, it can be concluded that the intestine of B. sinensis was involved in the defense against ammonia toxicity during exposure to ammonia in a hyperosmotic medium.
在高渗环境中暴露于氨的广盐性 Bostrychus sinensis 中，肠谷氨酸脱氢酶 (GDH) 和谷氨酰胺合成酶 (GS) 可能参与氨解毒，其中饮水对于渗透调节是必需的。我们的结果表明，在海水中（pH 值 7.0）暴露于 15 mmol l(-1) NH(4)Cl 6 天后，B. sinensis 肠道中的氨含量显著增加。肠道 GDH 的氨化和脱氨活性以及蛋白质丰度也显著增加。GDH 氨化/脱氨比保持不变，表明谷氨酸的周转率可能增加。然而，氨化和脱氨活性之间的差异增加了 2 倍，这意味着肠道中谷氨酸的形成可能增加。由于肠道谷氨酸含量保持不变，形成的多余谷氨酸可能被转移到其他氨基酸和/或转运到其他器官。事实上，肠道谷氨酰胺含量显著增加了 2 倍，肠道 GS 的活性和蛋白质丰度也显著增加。由于肠道中谷氨酰胺的积累量低于肝脏和肌肉，而肝脏和肌肉中 GDH 活性没有变化，因此肠道谷氨酸可能被转运到肝脏和肌肉中，以促进其中谷氨酰胺的合成增加。相比之下，当鱼暴露于 30 mmol l(-1) 的更高浓度 (NH(4)Cl 在 5 千分之水中（pH 值为 7.0）时，肠道中氨含量的增加幅度不太明显，肠道 GDH 的活性和动力学特性也没有变化。因此，可以得出结论，B. sinensis 的肠道参与了在高渗介质中暴露于氨时对氨毒性的防御。
