Cytolethality of Glutathione Conjugates with Monomethylarsenic or Dimethylarsenic Compounds

Abstract

Arsenicals are known to be toxic and carcinogenic in humans. Inorganic arsenicals are enzymatically methylated to monomethylarsonic acid (MMAs^V) and dimethylarsinic acid (DMAs^V), which are the major pentavalent methyl arsenic metabolites. Recent reports indicate that trivalent methyl arsenicals are produced through methylation of inorganic arsenicals and participate in arsenic poisoning. Trivalent methyl arsenicals may be generated as arsenical–glutathione conjugates, such as monomethylarsonous diglutathione (MMAs^IIIDG) and dimethylarsinous glutathione (DMAs^IIIG), during the methylation process. It has been well known that reduced glutathione (GSH) reduces MMAs^V and DMAs^V in vitro, and produces MMAs^IIIDG and DMAs^IIIG. Some studies have shown that exogenous GSH increased cytolethality of MMAs^V and DMAs^V in vitro, while other studies have suggested that exogenous GSH decreased them. In this study, we examined the true effects of exogenous GSH on the cytolethality of MMAs^V and DMAs^V by investigating reactions between various concentrations of MMAs^V or DMAs^V and GSH. GSH significantly increased the cytolethality and cellular uptake of pentavalent methyl arsenicals when GSH over 25 mM was pre-incubated with mM levels of arsenicals, and this cytolethality might have been caused by arsenical–GSH conjugate generation. However, GSH at less than 25 mM did not affect the cytolethality and cellular uptake of pentavalent methyl arsenicals. These findings suggest that high concentrations of arsenicals and GSH are needed to form arsenical–GSH conjugates and to show significant cytolethality. Furthermore, we speculated that MMAs^IIIDG and DMAs^IIIG may separate into trivalent methyl arsenicals and glutathione, which are then transported into cells where they show significant cytolethality.