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Abstract
Tyrosinase is the rate-limiting enzyme for the production of melanin and other pigments via the oxidation of l-tyrosine. The methanol extract from Humulus lupulus showed potent inhibition against mushroom tyrosinase. The bioactivity-guided fractionation of this methanol extract resulted in the isolation of seven flavonoids (1–7), identified as xanthohumol (1), 4′-O-methylxanthohumol (2), xanthohumol C (3), flavokawain C (4), xanthoumol B (5), 6-prenylnaringenin (6) and isoxanthohumol (7). All isolated flavonoids (1–7) effectively inhibited the monophenolase (IC50s = 15.4–58.4 µM) and diphenolase (IC50s = 27.1–117.4 µM) activities of tyrosinase. Kinetic studies using Lineweaver–Burk and Dixon-plots revealed that chalcones (1–5) were competitive inhibitors, whereas flavanones (6 and 7) exhibited both mixed and non-competitive inhibitory characteristics. In conclusion, this study is the first to demonstrate that the phenolic phytochemicals of H. lupulus display potent inhibitory activities against tyrosinase.
Declaration of interest
All students were supported by a scholarship from the BK21 plus. This research was supported by a grant from the Young Scientists personnel expenditure (490009-1297) of National Research Council of Science & Technology (NST) and a grant from the Korea Institute of Oriental Medicine (Grant No. K15290). The authors report no declarations of interest.
Supplementary material available online