Glucuronidation mediated by UDP-glucuronosyltransferases (UGTs) is a significant metabolic pathway that facilitates efficient elimination of numerous endo- and xenobiotics including phenolics. UGT genetic deficiency and polymorphisms or inhibition of glucuronidation by concomitant use of drugs are associated with inherited physiological disorders or drug induced toxicities. Moreover, extensive glucuronidation can be a barrier to oral bioavailability as the first-pass glucuronidation (or premature clearance by UGTs) of orally administered agents usually results in the poor oral bioavailability and lack of efficacies. This review focused on the first-pass glucuronidation of phenolics including natural polyphenols and pharmaceuticals. The complexity of UGT-mediated metabolism of phenolics is highlighted with species-, gender-, organ- and isoform-dependent specificity, as well as functional compensation between UGT1A and 2B subfamily. In addition, recent advances are discussed with respect to the mechanisms of enzymatic actions including the important properties such as binding pocket size and phosphorylation requirements.

Genistein, one of the most active natural flavonoids, exerts various biological effects including chemoprevention, antioxidation, antiproliferation and anticancer. More than 30 clinical trials of genistein with various disease indications have been conducted to evaluate its clinical efficacy. Based on many animals and human pharmacokinetic studies, it is well known that the most challenge issue for developing genistein as a chemoprevention agent is the low oral bioavailability, which may be the major reason relating to its ambiguous therapeutic effects and large interindividual variations in clinical trials. In order to better correlate pharmacokinetic to pharmacodynamics results in animals and clinical studies, an in-depth understanding of pharmacokinetic behavior of genistein and its ADME properties are needed. Numerous in vitro/in vivo ADME studies had been conducted to reveal the main factors contributing to the low oral bioavailability of genistein. Therefore, this review focuses on summarizing the most recent progress on mechanistic studies of genistein ADME and provides a systemic view of these processes to explain genistein pharmacokinetic behaviors in vivo. The better understanding of genistein ADME property may lead to development of proper strategy to improve genistein oral bioavailability via mechanism-based approaches.

The purpose of this research was to develop a sensitive and reproducible UPLC-MS/MS method to simultaneously quantify genistein, genistein-7-O-glucuronide (G-7-G), genistein-4’-O-glucuronide (G-4’-G), genistein-4’-O-sulfate (G-4’-S) and genistein-7-Osulfate (G-7-S) in mouse blood samples. After the method was fully validated over a wide linear range, it was applied to quantify the levels of genistein and its metabolites in a mouse bioavailability study. The linear response range were 19.5–10,000 nM for genistein, 12.5–3,200 nM for G-7-G, 20–1280 nM for G-4’-G, 1.95–2,000 nM for G-4’-S, and 1.56–3,200 nM for G-7-S, respectively. The lower limit of quantification (LLOQ) was 4.88, 6.25, 5, 0.98 and 0.78 nM for genistein, G-7-G, G-4’-G, G-4’-S and G-7-S, respectively. Only 20 µl mouse blood sample from i.v. and p.o. administration were needed for analysis because of the high sensitivity of the method. The intra- and inter-day variance is less than 15% and accuracy is within 85–115%. The analysis was finished within 4.5 min. The applicability of this assay was demonstrated and successfully applied for bioavailability study in FVB mouse after i.v. and p.o. administration of 20 mg/kg of genistein, and its oral bioavailability was ~24%.

In the present study, we investigated the pharmacological action of hydrogen sulfide (H2S, using sodium hydrosulfide, NaHS, and/or sodium sulfide, Na2S as donors) on sympathetic neurotransmission from isolated, superfused porcine iris-ciliary bodies. We also examined the effect of H2S on norepinephrine (NE), dopamine and epinephrine concentrations in isolated porcine anterior uvea. Release of [3H]NE was triggered by electrical field stimulation and basal catecholamine concentrations was measured by high performance liquid chromatography (HPLC). Both NaHS and Na2S caused a concentration-dependent inhibition of electrically evoked [3H]NE release from porcine iris-ciliary body without affecting basal [3H]NE efflux. The inhibitory action of H2S donors on NE release was attenuated by aminooxyacetic acid (AOA) and propargyglycine (PAG), inhibitors of cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CSE), respectively. With the exception of dopamine, NaHS caused a concentration-dependent reduction in endogenous NE and epinephrine concentrations in isolated iris-ciliary bodies. We conclude that H2S can inhibit sympathetic neurotransmission from isolated porcine anterior uvea, an effect that is dependent, at least in part, on intramural biosynthesis of this gas. Furthermore, the observed action of H2S donors on sympathetic transmission may be due to a direct action of this gas on neurotransmitter pools.

We characterized the in vitro glucuronidation of prunetin, a prodrug of genistein that is a highly active cancer prevention agent. Metabolism studies were conducted using expressed human UGT isoforms and microsomes/S9 fractions prepared from intestine and liver of rodents and humans. The results indicated that human intestinal microsomes were more efficient than liver microsomes in glucuronidating prunetin, but rates of metabolism were dependent on time of incubation at 37°C. Human liver and intestinal microsomes mainly produced metabolite 1 (prunetin-5-O-glucuronide) and metabolite 2 (prunetin-4'-O-glucuronide), respectively. Using 12 human UGT isoforms, we showed that UGT1A7, UGT1A8 and 1A9 were mainly responsible for the formation of metabolite 1 whereas UGT1A1, UGT1A8 and UGT1A10 were mainly responsible for the formation of metabolite 2. This isoform specific metabolism was consistent with earlier results obtained using human liver and intestinal microsomes, as the former (liver) is UGT1A9-rich whereas the latter is UGT1A10-rich. Surprisingly, we found that thermo stability of the microsomes were isoform-and organ-dependent. For example, human liver microsomal UGT activities were much more heat (37°C ) stable than intestinal microsomal UGT activities, consistent with the finding that human UGT1A9 is much more thermo stable than human UGT1A10 and UGT1A8. The organ-specific thermo stability profiles were also evident in rat microsomes and mouse S9 fractions, even though human intestinal glucuronidation of prunetin differ significantly from its rodent intestinal glucuronidation. In conclusion, prunetin glucuronidation is species-, organ-and UGT-isoform dependent, all of which may be impacted by thermo stability of specific UGT isoforms involved in the metabolism.