Molecular characterization of d-amino acid oxidase from common carp Cyprinus carpio and its induction with exogenous free d-alanine

A full-length cDNA encoding d-amino acid oxidase (DAO, EC 1.4.3.3) was cloned and sequenced from the hepatopancreas of carp fed a diet supplemented with d-alanine. This clone contained an open reading frame encoding 347 amino acid residues. The deduced amino acid sequence exhibited about 60 and 19–29% identity to mammalian and microbial DAOs, respectively. The expression of full-length carp DAO cDNA in Escherichia coli resulted in a significant level of protein with DAO activity. In carp fed the diet with d-alanine for 14 days, DAO mRNA was strongly expressed in intestine followed by hepatopancreas and kidney, but not in muscle. During d-alanine administration, DAO gene was expressed quickly in hepatopancreas with the increase of DAO activity. The inducible nature of carp DAO indicates that it plays an important physiological role in metabolizing exogenous d-alanine that is abundant in their prey invertebrates, crustaceans, and mollusks.