Recombinant production and evaluation of an antibacterial L-amino acid oxidase derived from flounder Platichthys stellatus

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.

l-Amino acid oxidase as a fish host-defense molecule

An l-amino acid oxidase (LAO) is an amino acid metabolism enzyme that also performs a variety of biological activities. Recently, LAOs have been discovered to be deeply involved in innate immunity in fish because of their antibacterial and antiparasitic activity. The determinant of potent antibacterial/antiparasitic activity is the H₂O₂ byproduct of LAO enzymatic activity that utilizes the l-amino acid as a substrate. In addition, fish LAOs are upregulated by pathogenic bacteria or parasite infection. Furthermore, some fish LAOs show that the target specificity depends on the virulence of the bacteria. All results reflect that LAOs are new innate immune molecules. This review also describes the potential of the immunomodulatory functions of fish LAOs, not only the innate immune function by a direct oxidation attack of H₂O₂.

Recombinant expression of an l-amino acid oxidase from the fungus Hebeloma cylindrosporum in Pichia pastoris including fermentation

l‐amino acid oxidases (LAAOs) are flavoenzymes that catalyze the oxidative deamination of l‐amino acids to the corresponding α‐keto acids, ammonia, and hydrogen peroxide. Here, we show the overexpression, purification, and the characterization of LAAO4 from the fungus Hebeloma cylindrosporum in the yeast Pichia pastoris with a 9His‐tag and compare this with the recently characterized 6His‐hcLAAO4 expressed in E. coli. The expression of the enzyme with an ER‐signal sequence in P. pastoris resulted in a glycosylated, secreted protein. The enzymatic activity without activation was higher after expression in P. pastoris compared to E. coli. Due to treatment with acidic pH, a striking increase of activity could be detected for both expression systems resulting in similar specific activities after acid activation. Regarding the substrate spectrum, temperature stability, K_m, and v_max values, hcLAAO4 showed very few differences when produced in these two expression systems. A higher yield of hcLAAO4 could be obtained by fermentation.

Antimicrobial activity and mechanism of peptide CM4 against Pseudomonas aeruginosa

Antibacterial peptide CM4 (ABP-CM4) is a small cationic peptide with broad-spectrum activities against bacteria, fungi and tumor cells and may possibly be used as an antimicrobial agent. In this study, a C-terminal amidated antibacterial peptide ABP-CM4 (ABP-CM4N) with the strongest antibacterial activity was obtained through screening the antibacterial activities of ABP-CM4 with different modifications. The minimal inhibitory concentration of ABP-CM4N was 8 μM against P. aeruginosa (ATCC 27853) which was lower than that of ABP-CM4 (16 μM). The strengthened antimicrobial activity of ABP-CM4N may be associated with the increased membrane binding capacity, being two times that of ABP-CM4 (p < 0.001). The antibacterial mechanism of ABP-CM4N to Pseudomonas aeruginosa was examined by means of cell membrane integrity analysiss, the intracellular ultrastructure change observation and E. coli genomic DNA binding assay. It was found that ABP-CM4N had the same antimicrobial mechanism as ABP-CM4, and the aim of the antimicrobial mechanism was mainly to destroy the cell membrane which caused nucleic acid or protein leakage, and secondly to interact with E. coli genomic DNA after penetrating the cell membrane. Furthermore, in vitro ABP-CM4N showed a better bacteriostatic activity in meats, with the treated samples showing two to three times less positive colonies than ABP-CM4.

Recombinant expression and functional analysis of antimicrobial Siganus oraminl-amino acid oxidase using the Bac-to-Bac baculovirus expression system

Siganus oraminl-amino acid oxidase (SR-LAAO), isolated from the serum of Siganus oramin, is an innate immune protein with significant antibacterial activity. The aim of this study was to express SR-LAAO in insect cells using a baculovirus expression system and evaluate the function of the recombinant SR-LAAO. To this end, an optimized sequence of the SR-LAAO gene was designed and synthesized, based on the codon bias of insect cells. Bacmid shuttle vectors and recombinant baculovirus were successfully constructed, and the recombinant baculovirus was transfected into Sf9 insect cells. The antibacterial activity and enzymatic characteristics of the recombinant SR-LAAO were investigated. The results showed that the pFastBac-optiSR-LAAO shuttle vectors and Bacmid-optiSR-LAAO were correctly constructed. The Sf9 insect cells exhibited significant cytopathic effects following infection with Bacmid-optiSR-LAAO and Bacmid; the specific PCR analysis proved that the recombinant baculovirus was successfully constructed. The immunofluorescence assay revealed that the recombinant baculovirus rSR-LAAO was abundantly expressed in infected Sf9 insect cells; the results of SDS/PAGE and Western blot analyses showed that a specific band appeared at about 60 kDa. Moreover, the crude rSR-LAAO enzyme displayed strong antibacterial activity against aquatic pathogens, particularly Streptococcus agalactiae and Streptococcus iniae. In addition, the results of catalase interference test implied that the antibacterial activity of rSR-LAAO was directly associated with (H₂O₂ production). The results of the rSR-LAAO enzymatic characteristics test indicated that the Km value with l-Lysine as a substrate was 16.61 mM when the temperature was under 37 °C, and the optimum pH was 7. The antibacterial activity of rSR-LAAO could be completely inhibited by 10 mg/mL of pepsin, trypsin, and proteinase K compared with both methanol and acetone. Adding an equal volume of ethanol had a minimal impact on the antibacterial activity of rSR-LAAO. The crude enzyme could maintain a high level of antibacterial activity against both Gram-positive and -negative bacteria from 4 °C to 30 °C. In the present study, SR-LAAO was successfully expressed in Sf9 cells using the baculovirus expression system, and provides basic references for further research into the role of LAAO in marine animals and the development of new antimicrobial drugs.

Recombinant expression and characterization of a L-amino acid oxidase from the fungus Rhizoctonia solani

L-Amino acid oxidases (L-AAOs) catalyze the oxidative deamination of L-amino acids to the corresponding α-keto acids, ammonia, and hydrogen peroxide. L-AAOs are homodimeric enzymes with FAD as a non-covalently bound cofactor. They are of potential interest for biotechnological applications. However, heterologous expression has not succeeded in producing large quantities of active recombinant L-AAOs with a broad substrate spectrum so far. Here, we report the heterologous expression of an active L-AAO from the fungus Rhizoctonia solani in Escherichia coli as a fusion protein with maltose-binding protein (MBP) as a solubility tag. After purification, it was possible to remove the MBP-tag proteolytically without influencing the enzyme activity. MBP-rsLAAO1 and 9His-rsLAAO1 converted basic and large hydrophobic L-amino acids as well as methyl esters of these L-amino acids. The progress of the conversion of L-phenylalanine and L-leucine into the corresponding α-keto acids was determined by HPLC and ¹H-NMR analysis of reaction mixtures, respectively. Enzymatic activity was stimulated 50-100-fold by SDS treatment. K _m values ranging from 0.9-10 mM and v _max values from 3 to 10 U mg^-1 were determined after SDS activation of 9His-rsLAAO1 for the best substrates. The enzyme displayed a broad pH optimum between pH 7.0 and 9.5. In summary, a successful overexpression of recombinant L-AAO in E. coli was established that results in a promising enzymatic activity and a broad substrate spectrum for biotechnological application.