L-amino-acid oxidase

Although LAAOs are present in a variety of eukaryotic and prokaryotic organisms,[5] snake venom is a particularly rich source of the enzyme and the LAAOs are proposed to supply toxic effects upon envenomation.[3][6][7] LAAOs that have been purified from the venoms of various snake species have proven to be the best suitors for examining this novel family of enzymes.[8] It has been determined in most cases concerning the snake families, such as Viperidae, Crotalidae, and Elapidae, that snake venom-LAAO (sv-LAAO) constitutes about 1-9% of the total protein quantity.[9]

The specific activities of sv-LAAOs with various L-amino acids have been explored. Many studies show that a number of sv-LAAOs exhibit a preference for hydrophobic L-amino acids as substrates.[12] For example, results have indicated that most sv-LAAOs demonstrate relatively high specificities toward hydrophobic amino acids such as L-Met, L-Leu, and L-Ile in addition to aromatic amino acids such as L-Phe and L-Trp.[8][13]

This enzyme participates in 8 metabolic pathways: alanine and aspartate metabolism, methionine metabolism, valine, leucine and isoleucine degradation, tyrosine metabolism, phenylalanine metabolism, tryptophan metabolism, phenylalanine, tyrosine and tryptophan biosynthesis, and alkaloid biosynthesis. It employs one cofactor, flavin adenine dinucleotide (FAD). The enzyme binds to FAD in the first step of the catalytic process, thereby reducing FAD to FADH₂. The FAD is regenerated from FADH₂ by oxidation as a result of O₂ being reduced to H₂O₂. The mechanism proceeds via oxidative deamination of the L-amino acid, which affords an imino acid intermediate. Following hydrolysis of the intermediate, the enzyme successfully affords the 2-oxo acid, as shown in Scheme 1.[8]

Scheme 1: General enzymatic reaction scheme for L-amino acid oxidase

A variety of biological activities have been discovered from isolated sv-LAAOs including cytotoxic, antibacterial, antiviral, and antiparasitic activities as well as platelet aggregation effects.[14] These activities can be defined by the enzyme's ability to provoke oxidative stress by forming oxygen radicals and hydrogen peroxide.[15] The particular mechanism of action of sv-LAAOs in terms of platelet aggregation has been determined less clear, because some sv-LAAOs are known to create aggregates and some are known to be anti-aggregating factors.[16]

In recent studies, it has been shown that LAAOs have been isolated from the skin and/or gill mucous secretions of rockfish, great sculpin, and flounder.[17] The presence of these enzymes were identified to be a unique type of antibacterial protein in the external defense employed by certain fish species.

Notably, because of its potential in relevant antimicrobial, anti-tumor cell, and/or consumption of amino acids, the interest of researching sv-LAAOs has begun to grow.[7] Many authors have investigated the mechanism of antibacterial action of sv-LAAO. It is well established that sv-LAAO kills and breaks down bacteria by the H₂O₂ that is produced as a result of the oxidation reaction occurring in the surrounding environment.[7]

In one case study, it was reported that the sv-LAAO (isolated from C. durissus cascavella venom) caused the rupture of bacteria membranes while promoting extravasation, or leakage, of plasmatic contents out of the cellular structure. They argued that the amount of hydrogen peroxide generated was sufficient to inhibit bacterial growth, and that the ability of the enzyme to bind to bacterial membranes is not important in its antibacterial activity.[18]

Cardiovascular disease can present itself in many different forms, from cardiomyopathy to peripheral arterial disease, and heart disease remains a key threat to human health. While there are means of therapy to both prevent and cure cardiovascular diseases, many drugs are unavailable for clinical use due to severe side effects in addition to high toxicity levels.[9] However, in the past decade, sv-LAAOs have shown promise in affecting platelet aggregation. It has been proposed that hydrogen peroxide is considered to play a significant role regarding the enzymes ability to both cause and prevent this platelet aggregation.[9] With this knowledge, it appears that sv-LAAOs could be evaluated as a potential cardiovascular disease therapeutic because of their biological potency.

As a consequence of the numerous activities the enzyme participates in, it could be deduced that there is likely some enzymatic function regarding the complex network of venom toxin activities.[4] However, the role of sv-LAAOs in venom-induced pathology has not reported to be sufficiently assessed.