A monoamine oxidase from scallop Chlamys farreri serving as an immunomodulator in response against bacterial challenge

Monoamine oxidase (MAO) is an essential enzyme in the catabolism of monoamines, and implicated in the immune response of vertebrates. In the present study, the full-length cDNA encoding monoamine oxidase (designated CfMAO) was cloned from Chlamys farreri by using rapid amplification of cDNA ends (RACE) approaches and expression sequence tag (EST) analysis. The open reading frame of CfMAO cDNA encoded 519 amino acids, which shared 73.9% similarity with that from oyster Crassostrea gigas, and 64.5–66.3% similarity with those from vertebrates. A conserved Amino_oxidase domain and a transmembrane domain were identified in the deduced CfMAO protein. The mRNA transcripts of CfMAO could be detected in all the tested tissues, including haemocytes, hepatopancreas, kidney, adductor muscle, mantle, gill and gonad. The mRNA expression of CfMAO was up-regulated significantly in haemocytes of scallops during 6–48 h after bacteria Vibrio anguillarum challenge, and it reached the peak (25.9-fold, P < 0.05) at 12 h. The cDNA fragment encoding the mature peptide of CfMAO was expressed in the prokaryotic expression system, and 1 mg of the recombinant protein (rCfMAO) could catalyze the deamination of 3665.59 nmol serotonin, 2061.89 nmol norepinephrine, 2104.85 nmol epinephrine or 3040.34 nmol dopamine within 1 min (nmol min−1 mg−1) in vitro. When the reaction mixture was coincubated with 0.1 mmol L−1 MAO inhibitor clorgyline, its catalyzing activity to deaminize serotonin and dopamine was decreased significantly to 1603.69 and 955.39 nmol min−1 mg−1 (P < 0.05) respectively. These results indicated that CfMAO, as the homologue of monoamine oxidase in scallop C. farreri, could modulate the immune response of scallops through the deamination of monoamines.