Discovery and characterization of naturally occurring potent inhibitors of catechol-O-methyltransferase from herbal medicines.

Human catechol-O-methyltransferase (hCOMT) is considered a therapeutic target due to its crucial roles in the metabolic inactivation of endogenous neurotransmitters and xenobiotic drugs. There are nevertheless few safe and effective COMT inhibitors and there lacks a diversity in structure. To discover novel safe and effective hCOMT inhibitors from herbal products, in this study, 53 herbal products were collected and their inhibitory effects against hCOMT were investigated. Among them, Scutellariae radix (SR) displayed the most potent inhibitory effect on hCOMT with an IC50 value of 0.75 µg mL-1. To further determine specific chemicals as COMT inhibitors, an affinity ultrafiltration coupled with liquid chromatography-mass spectrometry method was developed and successfully applied to identify COMT inhibitors from SR extract. The results demonstrated that scutellarein 2, baicalein 9 and oroxylin A 12 were potent COMT inhibitors, showing a high binding index (>3) and very low IC50 values (32.9 ± 3.43 nM, 37.3 ± 4.32 nM and 18.3 ± 2.96 nM). The results of inhibition kinetics assays and docking simulations showed that compounds 2, 9 and 12 were potent competitive inhibitors against COMT-mediated 3-BTD methylation, and they could stably bind to the active site of COMT. These findings suggested that affinity ultrafiltration allows a rapid identification of natural COMT inhibitors from a complex plant extract matrix. Furthermore, scutellarein 2, baicalein 9 and oroxylin A 12 are potent inhibitors of hCOMT in SR, which could be used as promising lead compounds to develop more efficacious non-nitrocatechol COMT inhibitors for biomedical applications.

Identification and characterization of naturally occurring inhibitors against human carboxylesterase 2 in White Mulberry Root-bark.

White Mulberry Root-bark (WMR) is an edible Chinese herbal used for the treatment of inflammation, nephritis and asthma. This study aimed to investigate the inhibitory effects of ethanol extract from WMR against human carboxylesterase 2 (hCE2), as well as to identity and character natural hCE2 inhibitors in this herbal. Our results demonstrated that the ethanol extract of WMR displayed potent inhibitory effects against hCE2, while three major bioactive constitutes in WMR were identified on the basis of LC fingerprinting combined with activity-based screening of LC fractions. Three bioactive compounds including SD, KG and SC were efficiently identified by comparison of LC retention times, UV and MS spectral data, with the help of authentic standards. The inhibition potentials and inhibition types of these natural compounds against hCE2 were further investigated in human liver microsomes. The results demonstrated that these bioactive compounds are potent non-competitive inhibitors against hCE2, with the Ki values ranging from 0.76µM to 1.09µM. All these findings suggested that three abundant natural compounds in WMR displayed potent inhibitory effects against hCE2, which could be used as lead compounds to develop more potent hCE2 inhibitors for the alleviation of hCE2-mediated severe delayed-onset diarrhea.

Identification and characterization of naturally occurring inhibitors against UDP-glucuronosyltransferase 1A1 in Fructus Psoraleae (Bu-gu-zhi).

As an edible traditional Chinese herb, Fructus psoraleae (FP) has been widely used in Asia for the treatment of vitiligo, bone fracture and osteoporosis. Several cases on markedly elevated bilirubin and acute liver injury following administration of FP and its related proprietary medicine have been reported, but the mechanism in FP-associated toxicity has not been well investigated yet. This study aimed to investigate the inhibitory effects of FP extract and its major constituents against human UDP-glucuronosyltransferase 1A1 (UGT1A1), the key enzyme responsible for metabolic elimination of bilirubin. To this end, N-(3-carboxy propyl)-4-hydroxy-1,8-naphthalimide (NCHN), a newly developed specific fluorescent probe for UGT1A1, was used to evaluate the inhibitory effects of FP extract or its fractions in human liver microsomes (HLM), while LC-UV fingerprint and UGT1A1 inhibition profile were combined to identity and characterize the naturally occurring inhibitors of UGT1A1 in FP. Our results demonstrated that both the extract of FP and five major components of FP displayed evident inhibitory effects on UGT1A1 in HLM. Among these five identified naturally occurring inhibitors, bavachin and corylifol A were found to be strong inhibitors of UGT1A1 with the inhibition kinetic parameters (Ki) values lower than 1 µM, while neobavaisoflavone, isobavachalcone, and bavachinin displayed moderate inhibitory effects against UGT1A1 in HLM, with the Ki values ranging from 1.61 to 9.86µM. These findings suggested that FP contains natural compounds with potent inhibitory effects against human UGT1A1, which may be one of the important reasons for triggering FP-associated toxicity, including elevated bilirubin levels and liver injury.

Fructus Psoraleae contains natural compounds with potent inhibitory effects towards human carboxylesterase 2.

Fructus Psoraleae (FP) is an edible Chinese herbal which is widely used in Asia for the treatment of various diseases including asthma, diarrhea, and osteoporosis. This study aimed to investigate the inhibitory effects of the crude ethanol extract from FP on human carboxylesterase 2 (hCE2), as well as to identity and characterize the naturally occurring inhibitors of hCE2 in FP. Our results demonstrated that the ethanol extract of FP displayed potent inhibitory effects towards hCE2, while five major bioactive constitutes in FP were efficiently identified by LC-DAD-ESI-MS/MS, with the aid of LC-based activity profiling. The identified bioactive compounds including neobavaisoflavone, isobavachalcone, bavachinin, corylifol A and bakuchiol were found to be naturally occurring potent inhibitors of hCE2, with low Ki values ranging from 0.62µM to 3.89µM. This is the first report of the chemical constitutes in FP as potent inhibitors of hCE2.