The application of hops could be extended to obtain higher commercial values. Tannins from hops were assessed for their tyrosinase inhibition ability, and the associated mechanisms were explored. Nuclear magnetic resonance (NMR) and high-performance liquid chromatography–electrospray ionization–tandem mass spectrometry (HPLC–ESI–MS/MS) revealed that the hop tannins were characterized as condensed tannins with (epi)catechin and (epi)gallocatechin as subunits and an average polymerization degree of 10.32. Tyrosinase inhibition assay indicated that hop tannins had an IC₅₀ = 76.52 ± 6.56 μM. Kinetic studies of the inhibition processes indicated the tannins provided inhibition through competitive–uncompetitive mixed reactions. In silico molecule docking showed that tannins were bound to the active site of tyrosinase via hydrogen and electrovalent bonds. Circular dichroism (CD) observed the structural variation in the tyrosinase after reacting with the tannins. Fluorescence quenching analysis and free radical scavenging assays indicated that the tannins had copper ion chelating and antioxidant activities, which may also contribute to inhibition. The intracellular inhibition assay revealed that the melanin was reduced by 34.50% in B16F10 cells. These results indicate that these tannins can be applied as whitening agents in the cosmetics industry.

The hop is rich in tannins used as a conventional additive in the beer industry, but other applications are limited. This study investigated the tyrosinase inhibition activity of extracted hop tannins and the associated structure–function activity. The tannins were extracted and subjected to a gel permeation chromatography (GPC), a nuclear magnetic resonance (NMR) and an acid-cleavage coupled HPLC-ESI-MS/MS analysis to obtain the structural information of the tannins. Then, tyrosinase inhibition kinetic assays, inductively coupled plasma optical emission spectrometer and antioxidant (ICP-OES), circular dichroism (CD) as well as molecular docking analysis were applied to investigate the inhibition mechanism. Furthermore, the intracellular inhibition ability of hop tannins was assessed with B16-F10 cells. The results indicated that hop tannins were composed of (epi)catechin as extensional units and (epi)gallocatechin as terminal units and can be classified as prodelphenidins. The tyrosinase inhibition assays showed the hop tannin had a IC₅₀ = 76.52 ± 6.56 µM, meanwhile it inhibited the tyrosinase through a competitive–noncompetitive mixed way. The tannins were found to bind on the surface of tyrosinase via forming hydrogen bonding and consequently changed the secondary structure of tyrosinase. The fluorescence and antioxidant assay indicated the tannin had both copper ion chelating and antioxidant ability, which may also contribute to the inhibition. The intracellular inhibition analysis showed that activity of tyrosinase was reduced by 66.67% and melanin production was found to be reduced by 34.50% while 10 µM hop tannins were applied. These results indicated that the hops are not only important in the beer industry, but that hop tannins can be also applied as whitening agents in the cosmetic industry.