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

Seawater activates l-amino acid oxidase from the serum of the red-spotted grouper Epinephelusakaara

l-amino acid oxidases (LAOs) catalyze the oxidative deamination of l-amino acid and generate α-keto acid, ammonia, and hydrogen peroxide as byproducts. LAOs showed the variety of bioactivity by the resulting hydrogen peroxide. The serum of the red-spotted grouper Epinephelus akaara contains an LAO (Ea-LAO) with the potential to kill bacterial pathogens Aeromonas salmonicida and Vibrio anguillarum via hydrogen peroxide. However, it is unknown how the grouper tolerates the harmful effects of the serum Ea-LAO byproducts. In this study, we analyzed the kinetics of fish LAOs to understand how they escape the toxicity of byproducts. The LAO activity of grouper serum was suppressed in low-salt solutions such as NaCl, CaCl₂, MgCl₂, and diluted seawater. The activity was non-linearly increased and fitted to the four-parameter log-logistic model. The EC₅₀ of the seawater was calculated to have a 0.72-fold concentration. This result suggested that the Ea-LAO could be activated by mixing with seawater. The results of circular dichroism spectroscopy showed that the α helix content was estimated to be 12.1% and 5.3% in a salt-free buffer (inactive condition) and the original concentration of seawater (active condition), respectively, indicating that the secondary structure of the Ea-LAO in the active condition was randomized. In addition, the Ea-LAO showed reversible LAO activity regulation according to the salt concentration in the environment. Taken together, this indicates that the Ea-LAO is normally on standby as an inactive form, and it could activate as a host-defense molecule to avoid pathogen invasion via a wound when mixed with seawater.

Identification and characterization of l-amino acid oxidase 2 gene in orange-spotted grouper (Epinephelus coioides)

Recently, l-amino acid oxidases (LAAOs) have been identified in several fish species as first-line defense molecules against bacterial infection. Here, we report the cloning and characterization of a fish LAAO gene, EcLAAO2, from orange-spotted grouper (Epinephelus coioides). The full-length cDNA is 3030 bp, with an ORF encoding a protein of 511 amino acids. EcLAAO2 is mainly expressed in the fin, gill, and intestine. Its expression is upregulated in several immune organs after challenge with lipopolysaccharide (LPS) and poly (I:C). The recombinant EcLAAO2 protein (rEcLAAO2), expressed and purified from a baculovirus expression system, was determined to be a glycosylated dimer. According to a hydrogen peroxide-production assay, the recombinant protein was identified as having LAAO enzyme activity with substrate preference for L-Phe and L-Trp, but not L-Lys as other known fish LAAOs. rEcLAAO2 could effectively inhibit the growth of Vibrio parahaemolyticus, Staphylococcus aureus, and Bacillus subtilis while exhibiting less effective inhibition of the growth of Escherichia coli. Finally, protein models based on sequence homology were constructed to predict the three-dimensional structure of EcLAAO2 as well as to explain the difference in substrate specificity between EcLAAO2 and other reported fish LAAOs. In conclusion, this study identifies EcLAAO2 as a novel fish LAAO with a substrate preference distinct from other known fish LAAOs and reveals that it may function against invading pathogens.

Antimicrobial properties of L-amino acid oxidase: biochemical features and biomedical applications

Abstract

Mucus layer that covers the body surface of various animal functions as a defense barrier against microbes, environmental xenobiotics, and predators. Previous studies have reported that L-amino acid oxidase (LAAO), present in several animal fluids, has potent properties against pathogenic bacteria, viruses, and parasites. LAAO catalyzes the oxidative deamination of specific L-amino acids with the generation of hydrogen peroxide and L-amino acid metabolites. Further, the generated hydrogen peroxide is involved in oxidation (direct effect) while the metabolites activate immune responses (indirect effect). Therefore, LAAO exhibits two different mechanisms of bioactivation. Previously, we described the selective, specific, and local oxidative and potent antibacterial actions of various LAAOs as potential therapeutic strategies. In this review, we focus on their biochemical features, enzymatic regulations, and biomedical applications with a view of describing their probable role as biochemical agents and biomarkers for microbial infections, cancer, and autoimmune-mediated diseases. We consider that LAAOs hold implications in biomedicine owing to their antimicrobial activity wherein they can be used in treatment of infectious diseases and as diagnostic biomarkers in the above-mentioned diseased conditions.

Key points

•Focus on biochemical features, enzymatic regulation, and biomedical applications of LAAOs.

•Mechanisms of antimicrobial activity, inflammatory regulation, and immune responses of LAAOs.

•Potential biomedical application as an antimicrobial and anti-infection agent, and disease biomarker.