FORMATION AND REACTIVITY OF TOXIC QUINONE METHIDES

羟基化和多羟基化芳烃在植物中广泛存在

Hydroxylated and polyhydroxylated aromatics are widespread in the environment, especially as constituents of edible plants. Many of these phenolic compounds can be oxidized enzymatically to electrophilic quinone methides, but, with a few exceptions, the involvement of quinone methides in mediating the adverse effects of substituted phenols has not been considered. Both cytochrome P450 and peroxidase activity can catalyze quinone methide formation, so the alkylation of cellular components by this pathway may occur in many different tissues. Preliminary data demonstrate wide variation both in the formation and reactivity of quinone methides due to the structure of the phenolic precursor. The present application addresses relationships between phenol structure and P450- catalyzed quinone methide formation, reactivity, and cytotoxicity. To accomplish these goals, the following specific aims are proposed. (1) Determine the influence of chemical structure on the oxidation of phenolic compounds to quinone methides. (a) The effects of specific ortho oxygen- containing substituents, and unsaturated para alkyl substituents will be examined, as these are commonly found in naturally-occurring phenols, and are expected to affect quinone methide formation and reactivity. These studies will be extended to investigate quinone methide formation from mutagenic and non-mutagenic flavonoids. (b) Influences of substrate structure on cytochrome P450 isozyme selectivities will be studied, as previous results have demonstrated isozymic differences in the catalysis of quinone methide formation. Mechanistic information on this oxidative pathway will be obtained with deuterium-labeled analogs and free radical scavengers. For comparison with P450 activity, quinone methide formation by a model peroxidase system also will be investigated. (2) Investigate the effects of quinone methide structure on electrophilic reactivity. Rates of reactions of quinone methides with water, glutathione, nucleophilic amino acids, and purine deoxynucleosides will be measured. This data will be correlated with quinone methide structures to deduce general principles underlying their reactivity, and gain insight into intracellular binding selectivities. Reactions of selected quinone methides with pure proteins and DNA will also be investigated to generate qualitative and quantitative data concerning the particular residues affected. (3) Investigate the roles of quinone methides in mediating the toxicity of phenolic compounds. Isolated rat hepatocytes will be incubated with substituted phenols which produce quinone methides of varying reactivities with nucleophiles. The effects of quinone methide formation on cell viability, and on biochemical parameters related to toxicity will be determined. The covalent binding of quinone methides to proteins and DNA will be investigated to determine intracellular sites of alkylation. The resulting data will provide substantial insight into the roles of quinone methides in the cytotoxicity of phenolic compounds.