Human kidney flavin-containing monooxygenases and their potential roles in cysteine s-conjugate metabolism and nephrotoxicity.

The potential roles of human hepatic and renal flavin-containing monooxygenases (FMOs) in the metabolism of the cysteine S-conjugates S-allyl cysteine (SAC) and S-(1,2-dichlorovinyl)-L-cysteine (DCVC) were investigated. Incubations of human cDNA-expressed FMO1, FMO3, FMO4, and FMO5 with SAC resulted in detection of SAC sulfoxide, with FMO3 exhibiting approximately 3-, 4-, and 10-fold higher activity than FMO1, FMO4, and FMO5, respectively. DCVC sulfoxide formation was only detected with FMO3 and was 59-fold lower than SAC sulfoxide formation. Incubations of human liver microsomes with SAC or DCVC resulted in detection of the corresponding sulfoxides and provided evidence for the involvement of FMO3. Incubations of SAC or DCVC with human kidney microsomes, however, led only to the detection of SAC sulfoxide. Immunoblots with monospecific antibodies to FMO1, FMO3, and FMO5 in kidney microsomes from 26 humans showed that the average expression levels for FMO1, FMO3, and FMO5 were 5.8 +/- 2.3, 0.5 +/- 0.4, and 2.4 +/- 1.4 pmol/mg (means +/- S.D.), respectively. Interestingly, African-American kidney samples (n = 8) exhibited significantly higher FMO1 levels than Caucasian samples (n = 17), whereas no difference in expression level between males and females was observed with any of the examined FMO isoforms. Collectively, the results provide evidence for the expression of three FMO isoforms in the human kidney and show that the contribution of renal FMOs in cysteine S-conjugate metabolism is likely to vary depending upon the cysteine S-conjugate and the relative expression levels of the active FMOs.