Blood loss induces l-amino acid oxidase gene expression in the head kidney of the red-spotted grouper, Epinephelus akaara.

In fish, the innate immune system is more important than the adaptive immune system because it responds quickly and nonspecifically to protect against pathogens. Thus, a variety of innate immune molecules have been found in fish. Recently, l-amino acid oxidases (LAOs) were discovered as a new member of the antibacterial protein from fish skin mucus and serum. In this study, we newly found an antibacterial LAO in red-spotted grouper (Epinephelus akaara) serum. It showed a broad range of substrate specificity with aromatic and hydrophobic amino acids. The grouper LAO gene had a low expression level in the kidney under normal conditions; however, it was significantly upregulated by blood loss 1 day after bleeding. In addition, the LAO activity in the serum recovered within 3 days in the same experiment. This quick recovery may indicate that the LAO is an essential innate immune molecule in the whole grouper body.