Epoxide Hydratase: Purification to Apparent Homogeneity as a Specific Probe for the Relative Importance of Epoxides among Other Reactive Metabolites
Aromatic and olefinic compounds can be metabolized by microsomal monooxygenases to epoxides which chemically represent electrophilic species (for reviews, see refs. 1–5). Spontaneous binding of such epoxides to DNA, RNA, and protein has been observed (6–10). Accordingly, such metabolites have been suggested and, in some instances, shown to disturb the normal functions of cells, leading to such effects as mutagenesis (11–14), malignant transformation (15–19), or cell necrosis (20). However, aromatic and olefinic compounds are biotransformed to a vast array of metabolites (cf. refs. 21–27), possibly including a considerable number of reactive metabolites other than epoxides. The relative importance of epoxides among other reactive metabolites is at present unknown. With respect to the model compound used in this study, benzo[a]pyrene, our previous studies had shown that the 4,5- (K-region-) epoxide metabolite was a potent mutagen for the frameshift-sensitive Salmonella strains TA 1537 and TA 1538 (28), that the premutagenic hydrocarbon required a NADPH-supported microsomal monooxygenase system to become mutagenically active, and that the mutagenic response was potentiated by the presence of epoxide hydratase inhibitors at concentrations where no interference with other systems has been observed (28). Yet no conclusion could be reached whether the relative contribution of epoxide metabolites to the overall muta-genic effect of bioactivated benzo[a]pyrene was of any significance since the potentiation of the mutagenic effect by epoxide hydratase inhibitors could simply mean that blocking this pathway led to an accumulation of epoxides, making them important in this situation, while in absence of such inhibitors their contribution to the overall mutagenic effect may have been negligible.
KeywordsReactive Metabolite Styrene Oxide Polycyclic Hydrocarbon Apparent Homogeneity Cyclohexene Oxide
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