Flavin-containing monooxygenases (FMOs) constitute a family of xenobiotic-metabolising enzymes []. Using an NADPH cofactor and FAD prosthetic group,these microsomal proteins catalyse the oxygenation of nucleophilic nitrogen,sulphur, phosphorous and selenium atoms in a range of structurally diversecompounds. Five mammalian forms of FMO are now known and have been designatedFMO1-FMO5 [, , , , ].The deduced amino acid sequence of human FM05 includes the putative FAD- (GxGxxG) and NADP+pyrophosphate-binding (GxGxxA) sites characteristic of mammalian FMOs [], a 'FATGY' motif that has also been observed in a rangeof siderphore biosynthetic enzymes [], and a C-terminal hydrophobic segmentthat is believed to anchor the monooxygenase to the microsomal membrane [].Human and guinea pig FMO5, like other FMOs, are encoded by multipletranscripts. FMO5 has been identified in livers of adult humans, rabbitsand guinea pigs, and foetal livers of humans []. Neither the human nor guinea pig enzyme effectively catalyse the metabolism of methimazole, ageneral FMO substrate; however, both are active with n-octylamine []. Theresponses to detergent, ions and elevated temperature are all similar to those observed in rabbit FMO5, suggesting that these properties are species-independent and that this form of FMO is not readily classified as a drug-metabolising enzyme [].
