The cytochrome P450 (CYP) superfamily of heme-containing enzymes catalyzes Phase I biotransformation of endogenous and xenobiotic compounds, including fatty acids, steroids, drugs, and environmental contaminants. Common variations (polymorphisms) in cytochrome P450 genes can affect the function of the enzymes. The effects of polymorphisms are most prominently seen in the breakdown of medications. In fish, members of the CYP2 and CYP3A families play a major role in the metabolism of xenobiotics and endogenous compounds. The current study aimed to isolate and compare cytochrome P450 3A40 gene from Oreochromis niloticus (Nile tilapia) to understand its diversity and evolution in comparison with fresh water fish species available in the GenBank database. Total length of 1300 bp was obtained and its polymorphism with similar samples from O. niloticus in the GenBank was determined. Site no. 41 T>A changed the amino acid from Phenylalanine (F) to Tyrosine (Y), while sites no. 43 and 54 caused no effect (silent mutations), and site no. 74 A>G changes the amino acid from Glutamic Acid (E) to Glycine (G), in which the two later mutations formed a different protein isoform in its conformational structure. Phylogenetic analysis reflected a clear divergence of fresh water families (Cichlidae and Poecilidae) from other families, while fixed and shared mutations between families were found. Phylogenetic analysis provided strong support for the identity of the majority of CYPs in fresh water fishes. In the current study, conservative regions among the studied families were found.

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