The flavin-containing monooxygenase gene family (FMO1-6) in humans encodes five functional isoforms that catalyze the monooxygenation of numerous N-, P- and S-containing drugs and toxicants. A previous single nucleotide polymorphism (SNP) analysis of FMO1 in African-Americans identified seven novel SNPs. To determine the functional relevance of the coding FMO1 variants (H97Q, I303V, I303T, R502X), they were heterologously expressed using a baculovirus system. Catalytic efficiency and stereoselectivity of N- and S-oxygenation was determined in the FMO1 variants using several substrates. The I303V variant showed catalytic constants equal to wild-type FMO1 for methimazole and methyl p-tolyl sulfide. Catalytic efficiency (V(max)/K(m)) of methyl p-tolyl sulfide oxidation by R502X was unaltered. In contrast, methimazole oxidation by R502X was not detected. Both H97Q and I303T had elevated catalytic efficiency with regards to methyl p-tolyl sulfide (162% and 212%, respectively), but slightly reduced efficiency with regards to methimazole (81% and 78%). All the variants demonstrated the same stereoselectivity for methyl p-tolyl sulfide oxidation as wild-type FMO1. FMO1 also metabolized the commonly used insecticide fenthion to its (+)-sulfoxide, with relatively high catalytic efficiency. FMO3 metabolized fenthion to its sulfoxide at a lower catalytic efficiency than FMO1 (27%) and with less stereoselectivity (74% (+)-sulfoxide). Racemic fenthion sulfoxide was a weaker inhibitor of acetylcholinesterase than its parent compound (IC(50) 0.26 and 0.015 mM, respectively). The (+)- and (-)-sulfoxides were equally potent inhibitors of acetylcholinesterase. These data indicate that all the currently known FMO1 variants are catalytically active, but alterations in kinetic parameters were observed.