Characterization of L-amino acid oxidase and antimicrobial activity of aplysianin A, a sea hare-derived antitumor-antimicrobial protein

:  L-amino acid oxidase (LAAO) activity, as well as mechanisms of antimicrobial action of aplysianin A, a sea hare-derived 340 kDa homotetrameric protein, was determined. Spectrophotometric and High-performance liquid chromatography analyses of aplysianin A indicated that one flavin adenine dinucleotide, a cofactor of LAAO, bound to each subunit of the homotetramer. Aplysianin A can specifically catalyze oxidation of basic amino acids (L-arginine and L-lysine), and is the first protein from marine invertebrate animals with LAAO activity. Substrate specificity of aplysianin A is markedly different from that of commonly known LAAO, such as snake venom LAAO, which prefer hydrophobic amino acids. Km value of aplysianin A was the smallest of those for all known LAAO reported. In the presence of catalase, the antibacterial activity of aplysianin A was inhibited as expected, indicating that the antibacterial action of aplysianin A results from hydrogen peroxide production during the reaction with substrates. Interestingly, aplysianin A acted as an antibacterial agent even in the presence of excess catalase. Antibacterial assays in various media suggested that this phenomenon was simply attributed to the consumption of amino acids required for bacterial growth in the media by aplysianin A.