Degradation of methyl and ethyl mercury into inorganic mercury by oxygen free radical-producing systems: Involvement of hydroxyl radical

Abstract

Degradation of methyl mercury (MeHg) and ethyl Hg (EtHg) with oxygen free radicals was studied in vitro by using three well-known hydroxyl radical (•OH)-producing systems, namely Cu²⁺-ascorbate, xanthine oxidase (XOD)-hypoxanthine (HPX)-Fe(III)EDTA and hydrogen peroxide (H₂O₂)-ultraviolet light B. For this purpose, the direct determination method for inorganic Hg was employed. MeHg and EtHg were readily degraded by these three systems, though the amounts of inorganic Hg generated from MeHg were one half to one third those from EtHg. Degradation activity of XOD-HPX-Fe(III)EDTA system was inhibited by Superoxide dismutase, catalase and the •OH scavengers and stimulated by H₂O₂. Deletion of the •OH formation promoter Fe(III)EDTA from XOD-HPX-Fe(III)EDTA system resulted in the decreased degradation of MeHg and EtHg, which was enhanced by further addition of the iron chelator diethylenetriamine pentaacetic acid. In all these cases, a good correlation was observed between alkyl Hg degradation and deoxyribose oxidation determining •OH. By contrast, their degradation appeared to be unrelated to either Superoxide anion$(O_{2^ - } )$ production or H₂O₂ production alone. We further confirmed that H₂O₂ (below 2 mM) itself did not cause significant degradation of MeHg and EtHg. These results suggested that •OH, but not$O_{2^ - }$ and H₂O₂, might be the oxygen free radical mainly responsible for the degradation of MeHg and EtHg.