Superoxide produced in the matrix of mitochondria enhances methylmercury toxicity in human neuroblastoma cells

The mechanism of intracellular metabolism of methylmercury (MeHg) is not fully known. It has been shown that superoxide (O₂·⁻), the proximal reactive oxygen species (ROS) generated by mitochondria, is responsible for MeHg demethylation. Here, we investigated the impact of different mitochondrial respiratory inhibitors, namely rotenone and antimycin A, on the O₂·⁻ mediated degradation of MeHg in human neuroblastoma cells SH-K-SN. We also utilized paraquat (PQ) which generates O₂·⁻ in the mitochondrial matrix. We found that the cleavage of the carbon-metal bond in MeHg was highly dependent on the topology of O₂·⁻ production by mitochondria. Both rotenone and PQ, which increase O₂·⁻ in the mitochondrial matrix at a dose-dependent manner, enhanced the conversion of MeHg to inorganic mercury (iHg). Surprisingly, antimycin A, which prompts emission of O₂·⁻ into the intermembrane space, did not have the same effect even though antimycin A induced a dose dependent increase in O₂·⁻ emission. Rotenone and PQ also enhanced the toxicity of sub-toxic doses (0.1 μM) MeHg which correlated with the accumulation of iHg in mitochondria and depletion of mitochondrial protein thiols. Taken together, our results demonstrate that MeHg degradation is mediated by mitochondrial O₂·⁻, specifically within the matrix of mitochondria when O₂·⁻ is in adequate supply. Our results also show that O₂·⁻ amplifies MeHg toxicity specifically through its conversion to iHg and subsequent interaction with protein cysteine thiols (R-SH). The implications of our findings in mercury neurotoxicity are discussed herein. - Highlights: • Superoxide produced in the matrix of mitochondria degrades MeHg. • Superoxide produced in intermembrane space does not degrade MeHg. • Matrix-generated superoxide enhances Hg toxicity by converting MeHg to iHg.