Assessment of methyl p-tolyl sulfoxidation by human FMO2.1, p.S195L and p.N413K flavin-containing monooxygenase variant.

Previous work identified two common ethnically-dependent allelic variants of human flavin-containing monooxygenase 2 (FMO2) that encode protein with monooxygenase activity that could occur in combination with the functional FM02*1 allele (Q472 protein); the encoded proteins were S195L and N413K. To understand how these variants alter enzyme activity, we performed assays with the FMO substrate, methyl p-tolyl sulfide (MTS) under a range of conditions with S195L, N413K and Q472 produced by over-expression in Sf9 insect cells. MTS sulfoxidation was assayed by HPLC. All protein variants demonstrated the same pattern of response to the presence of magnesium (increased activity) and to heat (decreased activity). Increased activity was observed with Q472 and N413K in response to increasing pH and to cholate, while S195L had diminished activity. The major effect of the N413K substitution was to increase velocity compared with Q472, while S195L had reduced activity and altered patterns of response to incubation conditions. In contrast to Q472 and N413K, S195L activity was negligible unless NADPH was added before MTS. Also, the K,, for S195L is at least 10-fold higher than for Q472 and N413K. Although these allelic variants are expected to occur infrequently as mutations to the FM02*1 allele, they contribute to our overall understanding of mammalian FMO structure and function. [ABSTRACT FROM AUTHOR]