Molecular and functional characterization of flavin-containing monooxygenases (FMO1-6) in tree shrews

Flavin-containing monooxygenases (FMOs) are a family of important drug oxygenation enzymes that, in humans, consist of five functional enzymes (FMO1-5) and a pseudogene (FMO6P). The tree shrew is a non-rodent primate-like species that is used in various biomedical studies, but its usefulness in drug metabolism research has not yet been investigated. In this study, tree shrew FMO1-6 cDNAs were isolated and characterized by sequence analysis, tissue expression, and metabolic function. Compared with human FMOs, tree shrew FMOs showed sequence identities of 85-90 % and 81-89 %, respectively, for cDNA and amino acids. Phylogenetic analysis showed that each tree shrew and human FMO were closely clustered. The genomic and genetic structures of the FMO genes were conserved in tree shrews and humans. Among the five tissue types analyzed (lung, heart, kidney, small intestine, and liver), FMO3 and FMO1 mRNAs were most abundant in liver and kidney, respectively. Recombinant tree shrew FMO1-6 proteins expressed in bacterial membranes all mediated benzydamine and trimethylamine N-oxygenations and methyl p-tolyl sulfide S-oxygenation. The selective human FMO3 substrate trimethylamine was predominantly metabolized by tree shrew FMO3. Additionally, tree shrew FMO6 was active toward trimethylamine, as is cynomolgus macaque FMO6, in contrast with the absence of activity of the human FMO6P pseudogene product. Tree shrew FMO1-6, which are orthologous to human FMOs (FMO1-5 and FMO6P) were identified, and tree shrew FMO3 has functional and molecular features generally comparable to those of human FMO3 as the predominant FMO in liver.