Human Kidney Flavin-Containing Monooxygenases and Their Potential Roles in CysteineS-Conjugate Metabolism and Nephrotoxicity

Abstract

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 Ϯ 2.3, 0.5 Ϯ 0.4, and 2.4 Ϯ 1.4 pmol/mg (means Ϯ S.D.), respectively. Interestingly, African-American kidney samples (n ϭ 8) exhibited significantly higher FMO1 levels than Caucasian samples (n ϭ 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 Sconjugate metabolism is likely to vary depending upon the cysteine S-conjugate and the relative expression levels of the active FMOs.Flavin-containing monooxygenases (FMOs) are microsomal enzymes that catalyze oxidation of sulfur-, selenium-, and nitrogen-containing compounds (Ziegler, 1993). Five active isoforms have been identified and are present in most mammalian tissues. However, species-, sex-, tissue-, and agedependent differences exist in expression levels of these isoforms (Hines et al., 1994;Dolphin et al., 1996;

Roles of Necrosis, Apoptosis, and Mitochondrial Dysfunction inS-(1,2-Dichlorovinyl)-L-cysteine Sulfoxide-Induced Cytotoxicity in Primary Cultures of Human Renal Proximal Tubular Cells

“…Moreover, the relatively low amount of total ␤-lyase activity in the human kidney (Lash et al, 1990) and the lack of or marginal ability of aminooxyacetic acid to protect hPT cells from DCVC-induced necrosis or apoptosis, suggest further that FMO-dependent bioactivation is important. We also recently demonstrated expression of FMO isozymes in human kidney (Krause et al, 2003). These studies, although derived from a limited number of samples, also provided data suggesting that humans exhibit a wide range of levels of expression of FMOs in the kidneys.…”

“…Furthermore, this suggests that FMO-dependent bioactivation may account for some of the species-dependent differences in the nephrotoxicity of TRI and DCVC. We recently showed that human kidney expresses multiple isoforms of FMO (Krause et al, 2003), supporting the potential for this pathway to function in hPT cells.…”

“…S-(1,2-Dichlorovinyl)-L-cysteine (DCVC), the cysteine conjugate of TRI, was initially considered to be the primary, if not sole, penultimate nephrotoxic metabolite that produces renal cellular injury after bioactivation by the cysteine conjugate ␤-lyase (␤-lyase). However, DCVC can also be metabolized by the flavincontaining monooxygenase (FMO) to produce DCVC sulfoxide (DCVCS) (Sausen and Elfarra, 1990;Ripp et al, 1997;Krause et al, 2003), which is highly reactive and is a potent nephrotoxicant in rats and is cytotoxic in isolated rat proximal tubular (rPT) cells (Lash et al, 1994).…”

Metabolism and Disposition Kinetics of Nicotine

“…Human FMO3 is mostly expressed in liver, but also in brain, especially in substantia nigra (Cashman and Zhang, 2002). Low levels of FMO3 have been detected in 96 human kidney (Krause et al, 2003). The expression of amine N-methyltransferase is highest in human thyroid, adrenal gland, and lung (Thompson et al, 1999).…”

Sulfoxides as Urinary Metabolites ofS-Allyl-l-Cysteine in Rats: Evidence for the Involvement of Flavin-Containing Monooxygenases

“…Previously, we have shown that SAC is metabolized to the greatest extent by cDNA-expressed rabbit FMO3 followed by FMO1 and then FMO2, but SAC showed no activity with rabbit FMO5 (Ripp et al, 1997). More recently and using cDNA-expressed human FMOs, SAC has been shown to be a good substrate for FMO1, FMO3, and FMO4, and although the activity is low, it is also a substrate for human FMO5 (Ripp et al, 1999a,b;Krause et al, 2002).…”