Effect of chronic exposure to sublethal of ammonia concentrations on NADP+-dependent dehydrogenases of Nile tilapia liver

The effect of chronic exposure to sublethal ammonia (NH3) on liver NADPH producing enzymes of Nile tilapia juveniles (Oreochromis niloticus) was studied. Fish with an initial weight of 15.0±1.4 g were reared in a static system and exposed to the total ammonia nitrogen (TAN) concentrations 5 or 10 mg L^−1 for consecutive 70 days at 26±0.5°C. NADPH is mainly produced by glucose 6-phosphate dehydrogenase (G6PDH), 6- phosphogluconate dehydrogenase (6PGDH), and the cytosolic form of NADP-dependent isocitrate dehydrogenase (IDH). These enzymes have been recently considered as an important factor in cellular defense against oxidative damage as they share in maintaining reduced glutathione (GSH) level. The activity of the enzymes G6PDH, 6PGDH, and cytosolic IDH in liver tissues of fish exposed to different concentrations of NH3 showed significant increase. The degree of increase in activity was positively related to ammonia concentration. The increased activity of these enzymes resulted in higher NADPH availability. This may be interpreted as a defense to maintain a high level of GSH against the expected oxidative stress in ammonia-exposed fish.