Identification of Cytotoxicity Intermediate Products and Degradation Pathways for Microcystins Using Low-Frequency Ultrasonic Irradiation

Abstract

In this study, two microcystins (MCs) were examined. MC-LR and MC-RR were degraded with low frequencies (20, 40, 60, 100, and 200 kHz) of ultrasonic waves. The optimal ultrasonic frequency for treating MCs was 200 kHz. Kinetic reaction results indicated that the degradation reactions of MC-LR and MC-RR from low-frequency ultrasound obeyed the pseudo-first-order kinetic model. The structural characterizations of the intermediate products were identified via liquid chromatography-mass spectrometry, which elucidated the mechanism of the degradation pathways. The main degradation pathways in the LC-MS analysis were Adda and Mdha. The cytotoxic effects on human oral epidermoid carcinoma (KB) cells were evaluated through cell viability assay using 3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide. The inhibitory effect of ≥5 μg/mL MC-LR on KB cells was significantly enhanced in a dose-dependent manner. Compared with the control, the viability of the KB cells cultured with 15 and 5 μg/mL MC-LR was increased by 23 % and 29 %, respectively, after the application of MC-LR and the optimization of the frequencies' ultrasonic irradiated treatment for 48 h. A similar trend was observed in the degradation of ultrasonic irradiated MC-RR. Compared with the control, the viability of the KB cells cultured with 15 and 5 μg/mL MC-RR increased by 12 % and 22 %, respectively (P < 0.05). Results showed that low-frequency ultrasonic irradiation effectively reduced the cytotoxicity of the intermediate products of MCs. Therefore, low-frequency ultrasound is a feasible method for the detoxification of drinking water.