Quercetin competitively inhibited BFC activity in human reco
Quercetin competitively inhibited BFC activity in human recombinant cDNA-expressed CYP3A4 with Ki=15.4±1.52μM, and myricetin noncompetitively with Ki=74.6±7.99μM (Fig. 4). The degree of inhibition by quercetin was not affected by a pre-incubation step and was similar in the microsomes from both genders (Fig. 5). Analysis of enzyme kinetics in the presence of quercetin indicated uncompetitive inhibition concentrations with alpha Ki values below 100μM (Fig. 5). Alpha Ki values did not differ significantly between pools from male and female donors (P=0.113).
Myricetin showed somewhat stronger inhibition in female pools (Fig. 6). However, more specific analysis of enzyme kinetics in the presence of myricetin indicated that Ki values were above 100μM which is much higher than physiological concentrations (Conquer et al., 1998) and was not considered as relevant.
Isorhamnetin did not affect BFC activity either in human recombinant cDNA-expressed CYP3A4 (Fig. 5) or in the microsomes (Fig. 7). Thus no further analyses with isorhamnetin were performed.
Discussion Modulations of CYP450 activity are a primary cause of herb–drug interactions. Occurrence of herb–drug interactions highlighted the importance of understanding the mechanism and factors responsible for variations in the activities of drug-metabolizing enzymes. Potency of several flavonoid compounds as an inhibitor of human CYP450 has been demonstrated, but investigations on gender-related differences are lacking. The anatomical and physiological differences between males and females influence the action of many drugs and thus interactions between drugs and bioactive compounds. There are also gender-related differences in herpes simplex virus type 1 and activity of some CYP450 isoforms. A number of studies showed that CYP3A activity is higher in females than in males (El-Eraky and Thomas, 2003, Waxman and Holloway, 2009). The purpose of the present study was to determine the inhibitory effects of quercetin, myricetin, and isorhamnetin on human CYP2E1 and CYP3A in the microsomes from male and female donors. Both CYP2E1 and CYP3A are involved in the metabolism of pharmaceuticals; thus, modulation of these enzyme activities can generate effects of pharmacological and toxicological importance. One of the most abundant natural flavonols quercetin has previously been shown to potently inhibit CYP2E1 and CYP3A4 activity in vitro in different species including humans (Rastogi and Jana, 2014), pigs (Ekstrand et al., 2015), and rats (Zhou and Tang, 2005). Gender-related differences in the interactions between several CYP450 enzymes and quercetin were studied only in porcine microsomes (Ekstrand et al., 2015). In that study, quercetin reduced CYP2E1 activity in the hepatic microsomes from male, but not female, pigs. Given that pig and minipig hepatic microsomes metabolize p-nitrophenol at rates comparable to the human CYP2E1 (Baranova et al., 2005, Puccinelli et al., 2011), we hypothesized that human CYP2E1 might also express a gender-related response to quercetin. If so, patient gender should be considered when healthcare providers inform patients on drug interactions due to CYP2E1. To facilitate comparison of the results on porcine and human microsomes, we used the same concentrations of flavonols and the same incubation procedure as in the study by Ekstrand et al. (2015). Selection of the flavonols concentrations by Ekstrand et al. (2015) was based on the concentration of quercetin in the liver from pigs fed a high-quercetin diet (de Boer et al., 2005). In humans, the liver concentration of flavonoids is unknown. Plasma concentrations of quercetin was reported to be 1.5μmol/L after 28days of supplementation with quercetin dose above 1g/day (Conquer et al., 1998). After intake of 80–100mg quercetin equivalent in the form of plant products, flavonols concentrations were reported to be on the order of 0.3–0.75μmol/L. Approximately 20–40% of quercetin is metabolized to isorhamnetin (Day et al., 2001).