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Stereoselective Oxidation Kinetics of Deoxycholate in Recombinant and Microsomal CYP3A Enzymes: Deoxycholate 19-hydroxylation is an In Vitro Marker of CYP3A7 Activity.
The primary bile acids (BAs) synthesized from cholesterol in the liver are converted to secondary BAs by gut microbiota. It was recently disclosed that the major secondary BA, deoxycholate (DCA) species, is stereoselectively oxidized to tertiary BAs exclusively by CYP3A enzymes. This work subsequently investigated the in vitro oxidation kinetics of DCA at C-1β, C-3β, C-4β, C-5β, C-6α, C-6β and C-19 in recombinant CYP3A enzymes and naive enzymes in human liver microsomes (HLMs). The stereoselective oxidations of DCA fit well with Hill kinetics at 1-300 μM in both recombinant CYP3A enzymes and pooled HLMs. With none or trace contributions from CYP3A5, CYP3A7 favors the oxidations at C-19, C-4β, C-6α, C-3β and C-1β, whereas CYP3A4 favors the oxidations at C-5β and C-6β compared to each other. Correlation between DCA oxidations and testosterone 6β-hydroxylation in 14 adult single-donor HLMs provided a proof-of-concept evidence that DCA 19-hydroxylation is an in vitro marker reaction for CYP3A7 activity, whereas oxidations at other sites are mixed indicators for CYP3A4 and CYP3A7 activities. Deactivation caused by DCA induced P450-P420 conversion, as shown by the spectral titrations of isolated CYP3A proteins, was observed when DCA levels were near or higher than its critical micelle concentration (about 1500 μM). Unlike CYP3A4, CYP3A7 showed abnormally elevated activities at 500 μM and 750 μM, which might be associated with an altered affinity for DCA multimers. The disclosed kinetic and functional roles of CYP3A isoforms in disposing the gut bacteria-derived DCA may help understand the structural and functional mechanisms of CYP3A.
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