Evidence for the nonenzymatic and irreversible binding of cytembena to rat liver microsomes in vitro

Characteristics of the irreversible binding of cytembena (CYT) to rat liver microsomal proteins have been investigated in vitro. Binding of [14C]CYT to rat liver microsomal proteins remained unchanged in the presence of SKF-525A or after heat denaturation and was not dependent upon the presence of pyridine nucleotide (NADPH or NADH). Ethacrynic acid, cysteine, dithiothreitol, and lysine were found to block the irreversible binding of [14C]CYT to microsomal proteins in a dose-dependent manner. The rank order of effectiveness as inhibitors of CYT binding was ethacrynic acid greater than cysteine greater than dithiothreitol greater than lysine. In other studies, CYT was shown to preferentially form adducts with cysteine rather than lysine or glycine. Using structural analogs and metabolites of CYT, it was found that 4-methoxybenzoylacrylic acid and beta-benzoylacrylic acid were effective competitors of [14C]CYT binding to liver microsomal proteins. By contrast, 4-methoxybenzoylpropionic acid, 5-(4-methoxyphenyl)dihydro-2(3H)furanone and 4-hydroxy-4-(4-methoxyphenyl)butyric acid were ineffective as inhibitors of the irreversible binding of CYT. These data suggest that sulfhydryl groups are involved in the nonenzymatic binding of CYT and that the presence of a carbon-carbon double bond in CYT, in debrominated metabolites or structural analogs, is requisite for an interference of the binding of CYT to microsomal proteins.