A Naturally Occurring Isoform-Specific Probe for Highly Selective and Sensitive Detection of Human Cytochrome P450 3A5.

Cytochrome P450 (CYP) 3A5 characterized with polymorphic and extensive expression in multiple tissues is the most important P450 enzyme among the minor CYP3A isoforms. However, a selective and sensitive probe for CYP3A5 remains unavailable. In this study, we identified and characterized a naturally occurring lignan 12 (schisantherin E) as an isoform-specific probe for selective detection of CYP3A5 activity in complex biological samples. With thorough characterization including LC-MS and NMR, we found that 12 can be metabolized by CYP3A5 to generate a major metabolite 2-O-demethylated 12. Meanwhile, both reaction phenotyping and chemical inhibition experiments further revealed that CYP3A5 selectively catalyzed the 2-O-demethylation of 12. Specifically, the interactions between the Phe210 residue of CYP3A5 and methyl benzoate of 12 might play key roles in 12-O-demethylation, which was revealed by docking simulation and site-directed mutagenesis studies. These findings are beneficial for exploring the role of CYP3A5 in drug metabolism and pathologic process.

Assessment of the inhibition potential of Licochalcone A against human UDP-glucuronosyltransferases.

Licochalcone A (LCA) is a major bioactive compound in Licorice, a widely used herbal medicine. In this study, the inhibitory effects of LCA against human UDP-glucuronosyltransferases (UGTs) and LCA associated herb-drug interactions were systematically investigated. Our results demonstrated that LCA displayed broad-spectrum inhibition against human UGTs. LCA exhibited strong inhibitory effects against UGT1A1, 1A3, 1A4, 1A6, 1A7, 1A9, and 2B7 (both IC50 and Ki values lower than 5 µM), while showing moderate inhibitory effects against UGT1A8, 1A10, 2B4, 2B15, and 2B17. The inhibitory effects of LCA against two major UGTs, including UGT1A1 and 1A9, were further investigated in human liver microsomes (HLMs), where the potential risks of LCA via inhibition of UGT1A1 and 1A9 were predicted by combining the in vitro inhibitory data and physiological data. The results from this study also showed that several LCA-containing products were able to increase the area under the curve (AUC) of the substrates that were predominantly metabolized by UGT1A1 or 1A9. These findings together demonstrate that LCA has a potent and broad-spectrum inhibitory effect against most human UGTs and thus suggest that much caution should be exercised when high-dose LCA is co-administered with UGT substrates.

Deoxyschizandrin, a naturally occurring lignan, is a specific probe substrate of human cytochrome P450 3A.

To accurately predict the modifications done during metabolic processes by cytochrome P450 (P450) 3A enzyme, selecting substrates that best represent a broad range of substrate substitutions and that follow the Michaelis-Menten kinetic properties is highly necessary. In the present study, the oxidative pathways of deoxyschizandrin (DS), the most abundant lignan in Fructus Schisandrae fruit extract, were characterized with liver microsomes from human (HLM) and rat (RLM). Only one monohydroxylated metabolite 7(S)-hydroxylated metabolite (isoschizandrin, ISZ), was identified using liquid chromatography-mass spectrometry and nuclear magnetic resonance techniques. CYP3A4 and CYP3A5 were found to be the major isoforms involved in the monohydroxylation of DS. Also, the kinetic studies showed that DS hydroxylation obeyed Michaelis-Menten kinetics both in HLM and in RLM. However, the subsequent metabolism of ISZ was nearly nonexistent when DS was present. More importantly, the interactions between DS and three well characterized CYP3A probe substrates, testosterone (TST), midazolam (MDZ), and nifedipine (NIF), were studied. TST and MDZ were shown to compete with DS for the mutual binding site, causing Km to be increased. The presence of DS also lowered the binding affinities for MDZ and TST. However, DS showed only slight inhibitory effects on nifedipine (NIF) oxidation even though NIF was able to inhibit DS hydroxylation in a noncompetitive fashion. These results show that DS is a good representative substrate of MDZ and TST primarily due to their shared, large binding regions on CYP3A. Therefore, DS is an attractive candidate as a novel CYP3A probe substrate for predicting the metabolic modifications in CYP3A activity.