Structure-activity relationships for substrates and inhibitors of mammalian liver microsomal carboxylesterases

Purpose. Carboxylesterases are important in the detoxification of drugs, pesticides and other xenobiotics. This study was to evaluate a series of substrates and inhibitors for characterizing these enzymes. Methods. A series of novel aliphatic esters and thioesters were used in spectral assays to monitor human, murine and porcine esterases. A series of transition state mimics were evaluated as selective esterase inhibitors. Results. Several alpha-alkyl thioacetothioates were found to be similar to 2 to 11-fold superior to commonly used substrates for monitoring carboxylesterase activity. Insertion of a heteroatom in the acid portion of these esters in the beta or gamma position relative to the carbonyl had a dramatic effect on enzyme activity with S or O substituents often improving the k(CAT)/K-M ratio of the substrate and PI decreasing it. Several alpha,alpha'-(2-oxo-3,3,3-trifluoropropylthio(alkanes proved to be potent selective transition state mimics of the esterase activity with IC50's from 10(-5) to 10(-9) M. Conclusions. This library of substrates and inhibitors are useful research tools for characterizing the numerous isozymes of carboxylesterases present in mammalian tissues.