Prooxidant and antioxidant activity of vitamin E analogues and troglitazone

The order of antioxidant effectiveness of low concentrations of vitamin E analogues, in preventing cumene hydroperoxide-induced hepatocyte lipid peroxidation and cytotoxicity, was 2,2,5,7,8-pentamethyl-6-hydroxychromane (PMC) > troglitazone > Trolox C > alpha-tocopherol > gamma-tocopherol > delta-tocopherol. However, vitamin E analogues, including troglitazone at higher concentrations, induced microsomal lipid peroxidation when oxidized to phenoxyl radicals by peroxidase/H2O2. Ascorbate or GSH was also cooxidized, and GSH cooxidation by vitamin E analogue phenoxyl radicals was also accompanied by extensive oxygen uptake and oxygen activation. When oxidized by nontoxic concentrations of peroxidase/H2O2, vitamin E analogues except PMC also caused hepatocyte cytotoxicity, lipid peroxidation, and GSH oxidation. The prooxidant order of vitamin E analogues in catalyzing hepatocyte cytotoxicity, lipid peroxidation, and GSH oxidation was troglitazone > Trolox C > delta-tocopherol > gamma-tocopherol > alpha-tocopherol > PMC. A similar order of effectiveness was found for GSH cooxidation or microsomal lipid peroxidation but not for ascorbate cooxidation. Except for troglitazone, the toxic prooxidant activity of vitamin E analogues was therefore inversely proportional to their antioxidant activity. The high troglitazone prooxidant activity could be a contributing factor to its hepatotoxicity. We have also derived equations for three-parameter quantitative structure-activity relationships (QSARs), which described the correlation between antioxidant and prooxidant activity of vitamin E ananlogues and their lipophilicity (log P), ionization potential (E(HOMO)), and dipole moment.