Cytotoxicity of the redox cycling compound diquat in isolated hepatocytes: Involvement of hydrogen peroxide and transition metals

Abstract

Diquat is a hepatotoxin whose toxicity in vivo and in vitro is mediated by redox cycling and greatly enhanced by pretreatment with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of glutathione reductase. The mechanism by which redox cycling mediates diquat cytotoxicity is unclear, however. Here, we have attempted to examine the roles of three potential products of redox cycling, namely superoxide anion radical (O ⨪ 2), hydrogen peroxide (H 2O 2), and hydroxyl radical (.OH), in the toxicity of diquat to BCNU-treated isolated hepatocytes. Addition of high concentrations of catalase, but not superoxide dismutase, to the incubations provided some protection against the toxic effect of diquat, but much better protection was observed when catalase was added in combination with the iron chelator desferrioxamine. Addition of desferrioxamine alone also provided considerable protection, whereas the addition of copper ions enhanced diquat cytotoxicity. Taken together, these results indicate that both H 2O 2 and the transition metals iron and copper could play major roles in the cytotoxicity of diquat. The role of (O ⨪ 2) remains less clear, however, but studies with diethylenetriaminepentaacetic acid indicate that (O ⨪ 2) is unlikely to significantly contribute to the reduction of Fe 3+ to Fe 2+. The hydroxyl radical or a related species seems the most likely ultimate toxic product of the H 2O 2/Fe 2+ interaction, but hydroxyl radical scavengers afforded only minimal protection.