N- and S-oxygenation activity of truncated human flavin-containing monooxygenase 3 and its common polymorphic variants.

Human flavin-containing monooxygenase 3 (FMO3) is a membrane-bound, phase I drug metabolizing enzyme. It is highly polymorphic with some of its variants demonstrating differences in rates of turnover of its substrates: xenobiotics including drugs as well as dietary compounds. In order to measure its in vitro activity and compare any differences between the wild type enzyme and its polymorphic variants, we undertook a systematic study using different engineered proteins, heterologously expressed in bacteria, purified and catalytically characterized with 3 different substrates. These included the full-length as well as the more soluble C-terminal truncated versions of the common polymorphic variants (E158K, V257M and E308G) of FMO3 in addition to the full-length and truncated wild-type proteins. In vitro activity assays were performed with benzydamine, tamoxifen and sulindac sulfide, whose products were measured by HPLC. Differences in catalytic properties between the wild-type FMO3 and its common polymorphic variants were similar to those observed with the truncated, more soluble versions of the enzymes. Interestingly, the truncated enzymes were better catalysts than the full-length proteins. The data obtained point to the feasibility of using the more soluble forms of this enzyme for in vitro drug assays as well as future biotechnological applications possibly in high throughput systems such as bioelectrochemical platforms and biosensors.

Flavin-Containing Monooxygenase 3 Polymorphic Variants Significantly Affect Clearance of Tamoxifen and Clomiphene.

Human flavin-containing monooxygenase 3 (hFMO3) is a drug-metabolising enzyme that oxygenates many drugs and xenobiotics in the liver. This enzyme is also known to exhibit single nucleotide polymorphisms (SNPs) that can alter the rates of monooxygenation of therapeutic agents. The purpose of this study was to investigate the effect of the three common polymorphic variants of hFMO3 (V257M, E158K and E308G) on the metabolism and clearance of three structurally similar compounds: tamoxifen (breast cancer medication), clomiphene (infertility medication) and GSK5182 (antidiabetic lead molecule). For GSK5182, none of the three variants showed any significant differences in its metabolism when compared to the wild-type enzyme. In the case of clomiphene, two of the variants, V257M and E308G, exhibited a significant increase in all the kinetic parameters measured with nearly two times faster clearance. Finally, for tamoxifen, a mixed behaviour was observed; E158K variant showed a significantly higher clearance compared to the wild type, whereas V257M mutation had the opposite effect. Overall, the data obtained demonstrate that there is no direct correlation between the SNPs and the metabolism of these three hFMO3 substrates. The metabolic capacity is both variant-dependent and substrate-dependent and therefore when testing new drugs or administering already approved therapies, these differences should be taken into consideration.