Ammonia Toxicity in Fish

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1. Introduction
Ammonia is the major metabolic end product during the catabolism of proteins, amino acids and other nitrogen containing biomolecules in different animal tissues. Ammonia is very toxic to the fish. Its toxicity leads to reduced growth rate (Atwood et al., 2000; El-Shafai et al., 2004; Hegazi and Hasanein, 2010), disruption of ion-osmo homeostasis (Knoph and Thorud, 1996; Person-Le Ruyet et al., 2003, 1998), gill hyperplasia (Benli et al., 2008), and if present in very high concentration, it causes hyperexcitability, coma, convulsions and finally death (Ip et al., 2001b).
To survive the effect of the ammonia toxicity, fish modifies its metabolism by either decreasing the production of ammonia, increasing its excretion, or converting the ammonia to glutamine and/or urea (Ip et al., 2001b). Most of the freshwater teleosts are ammoniotelic, as they excrete ammonia as primary excretory product to the external environment mainly by diffusion through the gills (Saha and Ratha, 2007). But, several species of fish have adapted to unique environmental circumstances by expressing high levels of OUC-enzymes and thus converting more than 50% of waste nitrogen as urea-N, they are considered as ureotelic (Anderson, 2001; Saha and Ratha, 2007). Though, quite a few recent studies have proposed an alternate to ureotelism (i.e. increased OUC pathway activity) as a mechanism for responding to such environmental circumstances. For example, in marble goby (Oxyeleotris marmoratus) a facultative freshwater air-breather, which can tolerate continuous air exposure for up to a week, glutamine synthetase (GS) appears to function as ammonia trap (Jow et al., 1999). A similar observation was made in the swamp eel (Monopterus albus) (Tay et al.,...

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...) concentrations. Furthermore, water evaporation at the high temperatures of the tropics can concentrate external ammonia (EA) (Rao et al., 1994). The situation is further aggreavated for those fish living in rice fields, where agricultural fertilization can lead to high concentrations of EA (Rao et al., 1994).
In the present study presence of multiple GS mRNA transcripts and their differential expression pattern in tissues air-breathing walking catfish (C. batrachus) during exposure to high environmental ammonia (HEA) (50 mM NH4Cl) were investigated. In addition, attempts were made for characterization of different GS proteins, and for this integrated approaches of computational analysis and expression profiling were used to predict properties and features that may be important for their function and to elucidate its possible association with hyper-ammonia stress.

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