Mammalian flavin-containing monooxygenases: structure/function, genetic polymorphisms and role in drug metabolism

Abstract

Flavin-containing monooxygenase (FMO) oxygenates drugs and xenobiotics containing a "soft-nucleophile", usually nitrogen or sulfur. FMO, like cytochrome P450 (CYP), is a monooxygenase, utilizing the reducing equivalents of NADPH to reduce 1 atom of molecular oxygen to water, while the other atom is used to oxidize the substrate. FMO and CYP also exhibit similar tissue and cellular location, molecular weight, substrate specificity, and exist as multiple enzymes under developmental control. The human FMO functional gene family is much smaller (5 families each with a single member) than CYP. FMO does not require a reductase to transfer electrons from NADPH and the catalytic cycle of the 2 monooxygenases is strikingly different. Another distinction is the lack of induction of FMOs by xenobiotics. In general, CYP is the major contributor to oxidative xenobiotic metabolism. However, FMO activity may be of significance in a number of cases and should not be overlooked. FMO and CYP have overlapping substrate specificities, but often yield distinct metabolites with potentially significant toxicological/pharmacological consequences. The physiological function(s) of FMO are poorly understood. Three of the 5 expressed human FMO genes, FMO1, FMO2 and FMO3, exhibit genetic polymorphisms. The most studied of these is FMO3 (adult human liver) in which mutant alleles contribute to the disease known as trimethylaminuria. The consequences of these FMO genetic polymorphisms in drug metabolism and human health are areas of research requiring further exploration.

Fig. 1

Catalytic cycle of flavin-containing monooxygenase (adapted from Ziegler, 1991). (1) FAD reduced by NADPH (fast). (2) FADH₂ reacts with O₂ (fast). Flavin-hydroperoxide is stable: thought to be the form in which FMO exists in the cell. (3) FAD-OOH reacts with any suitable nucleophile gaining access to active site. No substrate binding required. (4) One atom of O₂ is incorporated into substrate and the other into H₂O-FMO is a monooxygenase. (5) FAD-OH is converted to FAD via release of H₂O (slowest step in the cycle determines V_max). The final step in the cycle is the release of NADP⁺ (slow).

Fig. 2

Human FMO2 substrate specificity toward phenothiazines with tertiary amine substituents of differing length.

Table 2

Postulated function of conserved sequence motifs^a