Naringenin is the one of the many components in

Naringenin is the one of the many components in grapefruit juice. It was assumed that administration of 20mL grapefruit juice per kilogram of body weight is completely glucuronidated in the hepatic or gut wall for first pass metabolism [38]. It has been detected that one liter of grapefruit juice contains between 10 and 176mg naringenin [50], which corresponds to concentrations ranging between 36 and 646μM. Accordingly, 155mL fresh grapefruit juice has a naringenin concentration that was used in this study, and it is able to inhibit CYP3A enzyme activity in mammals. Also, these concentrations can be achieved 4–6h after consuming a flavonoid-rich diet [51], [52], [53] or after ingestion of supplements [54]. This estimation suggests that the naringenin concentrations that we used in vitro might be relevant to the in vivo situation. However, the relationship between the concentration of a emca mg reaching the organism and its concentration reaching the active site on the enzyme is complex. There is no information about the concentrations of naringenin that reach the liver; thus, the use of concentrations in the micromolar range might not accurately reflect the in vivo situation. In further studies, the amounts of naringenin reaching the liver should be determined. Several limitations might have influenced our results. First, sexes were equally represented in pools from human hepatic microsomes only. This did not affect the degree of inhibition by naringenin, but might have affected Vmax and Km values. In further studies, the microsomes from each sex should be evaluated separately. We recently reported sex-related differences in the ability of flavonoids to inhibit the activities of porcine CYP1A, CYP3A, and CYP2E1 [55]. However, we found no evidence of sex-related differences in human or mice microsomes in response to bioactive compounds. Second, our choice of substrate for CYP3A may not have been ideal. While characterization of BFCOD as a marker of CYP3A activity was performed for humans [56], pigs, and fish [24], further validation work for mice is needed. Finally, a known limitation in all in vitro studies is that they can be used only as a first phase screening to identify compounds with the potential to affect CYP450 activity, but cannot entirely reflect in vivo situation. In summary, naringenin was identified as an inhibitor of the drug-metabolizing CYP3A enzyme. We provided evidence about species-specific similarities and differences in in vitro CYP3A inhibition by naringenin. Fish showed a mode of inhibition of the CYP3A enzyme that was different from that found in mammals. The presence of diosmin and naringin did not affect BFCOD activity. Our results suggest that pigs and mice, but not fish, might be appropriate choices for animal models that could supply hepatic microsomes for studying human CYP3A activity. However, it should be noted that animal models cannot truly reflect the human CYP450-mediated metabolism or interactions with bioactive compounds, but it is recommended for screening and selecting compounds for further research.
Conflict of interest statement
Acknowledgements This study was financially supported by the Swedish University of Agricultural Sciences, NL faculty, Uppsala, Sweden and by the Ministry of Education, Youth and Sports of the Czech Republic – projects “CENAKVA” (No. CZ.1.05/2.1.00/01.0024) and “CENAKVA II” (No. LO1205 under the NPU I program). We also thank American Manuscript Editors for editing the English manuscript.
Introduction Flavonols are a group of flavonoids ubiquitously present in plants and plant-products, such as fruits, vegetables, grains, tea, and wine. These substances have received considerable attention due to their widespread appearance in nature and human diet as well as their positive biological effects on living organisms that are often related to their antioxidant properties (Tai et al., 2012) and antibacterial properties (Urzua et al., 2012). Flavonols are found in various foods in the form of glycosides, i.e., sugar O-conjugated at C3 position. Following ingestion, sugar moieties are cleaved from the phenolic backbone in the small intestine, and aglycones are partly absorbed by epithelial cells (Gee et al., 2000, Marin et al., 2015). The major sites of metabolism of these aglycones are the small intestine and liver where they form sulfate-, glucuronide-, and methyl- conjugates. Aglycones are also degraded by colonic microbiota (Rechner et al., 2004). Data on bioavailability of aglycones considerably vary between different species and within the same species in different studies (Guo and Bruno, 2015, Lesser and Wolffram, 2006). Moreover, different flavonols have different degrees of bioavailability determined by chemical structure.