Abstract
Only limited information about the function of equine cytochrome P450 enzymes (CYPs) is available. Several studies reported species differences in drug metabolism when equine and human CYPs were compared. In this study, the equine isoenzymes CYP3A95 and CYP3A97, the equine flavoprotein NADPH-P450 oxidoreductase (POR), and the cytochrome b5 (CYB5) of horses were amplified, cloned, sequenced, and heterologously expressed in insect cells. The protein expression was evaluated by western blotting, and spectrophotometry and HPLC were used for functional analysis. All four proteins were demonstrated to be functionally active. CYP3A95 and CYP3A97 formed 6β-hydroxytestosterone from testosterone, but they did not metabolize the fluorescent substance 7-benzyloxy-4-trifluoromethylcoumarin and a derivative of beetle luciferin, whereas the human orthologue CYP3A4 metabolized both compounds. Different ratios of POR:CYP:CYB5 did not result in a change of the pattern of testosterone metabolites. Heterologous expression of equine CYPs of the 3A subfamily allow characterization of the respective CYPs. This in vitro system is also suitable to investigate drug metabolism of therapeutics used in equine therapy to predict adverse effects and prevent reduced drug efficacy in polypharmacy.
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