Phenols displaying tyrosinase inhibition from Humulus lupulus

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.