We have previously reported that the diuretic thiosteroid spironolactone (SPL) inactivates rat liver microsomal cytochromes P450 [P450 (P450 3A and P450 2C11)] in a in a mechanism-based fashion, and we have identified two polar SPL metabolites (SPL-sulfinic acid and -sulfonic acid), formed in a partition ratio of approximately 20:1 in such rat liver microsomal incubations [Decker et al. (1989) Biochemistry 28, 5128-5136]. We proposed at the time that these metabolites were most likely derived from further enzymatic (or nonenzymatic) oxidations of the one-electron oxidation product [SPL-thiyl radical (SPL-S.)] and/or the two-electron-oxidized species [SPL-sulfenic acid (SPL-SOH)]. In those studies, glutathione (GSH) was found to attenuate both SPL-mediated P450 loss as well as polar metabolite formation by approximately 40%. We have now reexamined this in greater detail and report that it is due to GSH trapping of an electrophilic oxidized SPL species to form an adduct that we have isolated and unambiguously characterized by mass spectral analyses as the glutathionyl-SPL adduct (SPL-SSG). Moreover, we have found not only that rat liver microsomal formation of this adduct is enhanced at pH 9.0, the pH optimum for flavin-containing monooxygenase (FMO), but also that such adduct formation was indeed efficiently catalyzed by purified hog liver FMO. Because FMO oxidations of thiols are thought to entail a two-electron process to form the corresponding sulfenic acids, we infer that such a SPL-SSG adduct most likely reflects FMO-catalyzed oxidation of SPL to SPL-SOH, which on leaving the FMO active site is then trapped by GSH.(ABSTRACT TRUNCATED AT 250 WORDS)