Antioxidant and prooxidant actions of gallic acid derivatives: Effect on metal-dependent oxidation of lipids and low density lipoprotein

Antioxidant and prooxidant activities of gallic acid and its alkylesters such as ethyl-, propyl-, butyl-, octyl- and laurylgallates were analyzed. Increased length of the alkyl chain in gallate esters enhanced the inhibitory effect on the iron-mediated lipid peroxidation. The concentrations of alkylgalltes required for the inhibition of the formation of thiobarbituric acid-reactive substances were dependent on the hydrophobicity of the compounds. Effectiveness of gallate compounds as antioxidants can be explained by hydrophobic binding of the compounds to membrane lipids. Gallic acid compounds also showed a prooxidant effect by stimulating the copper-dependent oxidation of low density lipoprotein. The most potent prooxidant was gallic acid followed by ethyl-, propyl-, and butylgallates in this order, and octyl- and laurylgallates did not stimulate the oxidation of low density liporotein. These prooxidant effects of gallate compounds correlated with the copper reducing activity. Addition of superoxide dismutase protected low density lipoprotein from the copper-dependent oxidation by gallate compounds, indicating that superoxide radical formed by the cuprous ion-mediated reduction of oxygen may participate in the oxidation of low density lipoprotein. Prooxidant action of gallate compounds may explain the cytotoxic effect of these compounds.