Flavin-containing monooxygenase (FMO, EC1.14.13.8) comprises a family of xenobiotic-metabolizing enzymes that catalyze the oxygenation of a wide variety of xenobiotics, most commonly nitrogen and sulfur. Mammals express five different FMOs in a species- and tissue- specific manner. FMO2, is expressed predominantly in lung and differs from other FMOs in that it can catalyze the N-oxygenation of certain primary alkylamines. From its initial discovery as an unique form of FMO in lung, FMO2 has been studied primarily using a rabbit animal model. The initial goal of this research was to characterize this protein in a primate animal model. To understand FMO2 protein structure at the molecular level, we first cloned cDNA from a monkey lung cDNA library. Monkey FMO2 is expressed predominantly in lung. The high expression levels and broad substrate specificity in monkey, suggests that FMO2 plays a role in xenobiotic metabolism in this primate model. We then established a heterologous expression system to generate FMO2 with biological functionality in vitro. FMO2 from baculovirus-mediated expression resembled monkey and rabbit microsomal FMO2 immunochemically and catalytically. The successful FMO2 expression in the baculovirus system will serve as a valid tool for structure studies of protein functional domains, as well as, the amino acids responsible for enzyme properties of chimeras. Human FMO2 encodes a truncated protein containing 471 amino acid residues, 64 amino acids shorter in its C-terminal than orthologs in other species. We characterized human FMO2 in terms of gene polymorphism (genotyped by Dr. Hines), protein levels and catalytic activity with human lung microsomes. An ethnically related polymorphism was observed, in which all Caucasians genotyped to date are homozygous for a truncated, enzymatically inactive enzyme which may not even be translated. Approximately 15% of humans of African descent are heterozygous for full-length FMO2, but the level of expression may not be sufficient to significantly effect drug metabolism in the lung. The results of truncated FMO2 produced from baculovirus expression suggest that the C-terminal of FMO2 might be responsible for enzyme stability and/or proper structure necessary to exert fully enzyme activity. We conclude that the human FMO2 possesses unique features compared to all other mammals examined to date including other primates. / Graduation date: 1999