Antiproliferative Activity of King Cobra (Ophiophagus hannah) Venom l‐Amino Acid Oxidase

King cobra (Ophiophagus hannah) venom l ‐amino acid oxidase (LAAO), a heat‐stable enzyme, is an extremely potent antiproliferative agent against cancer cells when compared with LAAO isolated from other snake venoms. King cobra venom LAAO was shown to exhibit very strong antiproliferative activities against MCF‐7 (human breast adenocarcinoma) and A549 (human lung adenocarcinoma) cells, with an IC₅₀ value of 0.04 ± 0.00 and 0.05 ± 0.00 μg/mL, respectively, after 72‐hr treatment. In comparison, its cytotoxicity was about 3–4 times lower when tested against human non‐tumourigenic breast (184B5) and lung (NL 20) cells, suggesting selective antitumour activity. Furthermore, its potency in MCF‐7 and A549 cell lines was greater than the effects of doxorubicin, a clinically established cancer chemotherapeutic agent, which showed an IC₅₀ value of 0.18 ± 0.03 and 0.63 ± 0.21 μg/mL, respectively, against the two cell lines. The selective cytotoxic action of the LAAO was confirmed by phycoerythrin (PE) annexin V/7‐amino‐actinomycin (AAD) apoptotic assay, in which a significant increase in apoptotic cells was observed in LAAO‐treated tumour cells than in their non‐tumourigenic counterparts. The ability of LAAO to induce apoptosis in tumour cells was further demonstrated using caspase‐3/7 and DNA fragmentation assays. We also determined that this enzyme may target oxidative stress in its killing of tumour cells, as its cytotoxicity was significantly reduced in the presence of catalase (a H₂O₂ scavenger). In view of its heat stability and selective and potent cytotoxic action on cancer cells, king cobra venom LAAO can be potentially developed for treating solid tumours.