Non-thermal plasma-activated methionine-containing Ringer’s solution enhances cytotoxicity by increasing oxidative stress

Non-thermal plasma (NTP) is an ionized gas composed of electrons, ions, radicals, and UV. NTP-irradiated solutions induce reactive oxygen species (ROS)-dependent cell death. NTP irradiation of solutions containing methionine (Met) has been reported to enhance the cytotoxic effects of NTP; however, the exact mechanism by which NTP-irradiated Met solutions exhibit cytotoxicity is not fully understood. In this study, we aimed to elucidate the mechanism using the human breast cancer cell line MDA-MB-231 cells. NTP-irradiated Met-containing acetate Ringer’s solution (Met/PAA) potently induced cytotoxicity compared with NTP-irradiated acetate Ringer’s solution (PAA). Catalase-treated Met/PAA induced cytotoxicity to the same extent, suggesting that NTP irradiation might generate a novel cytotoxic substance distinct from ROS. Catalase-treated Met/PAA decreased intracellular glutathione (GSH) levels, increased ROS production, and caused mitochondrial dysfunction. When catalase-treated Met/PAA was pretreated with antioxidants, GSH and N-acetylcysteine, its cytotoxicity was suppressed. These phenomena indicate that oxidative stress is involved in cell death induced by catalase-treated Met/PAA. Dehydromethionine and methionine sulfoxide were identified as oxidation products of Met in the Met/PAA solution, but neither of the products exhibited cytotoxicity. Taken together, the irradiation of Met-containing solutions likely generates unidentified Met oxidation products other than ROS, resulting in cell death through oxidative stress-induced energy production failure.