Flavonoids as Mushroom Tyrosinase Inhibitors:  A Fluorescence Quenching Study

Abstract

Flavonoids, a group of naturally occurring antioxidants and metal chelators, can be used as tyrosinase inhibitors due to their formation of copper-flavonoid complexes. Thus, to investigate the underlying inhibition mechanism, a large group of flavonoids from several major flavones and flavonols were tested using fluorescence quenching spectroscopy. In addition, large differences in the tyrosinase inhibitory activities and chelating capacities according to the location of the hydroxyl group(s) in combination with the A and B rings in the flavonoids were confirmed. Accordingly, the major conclusions from this work are as follows: (i) The tyrosinase inhibitory activity is not only dependent on the number of hydroxyl groups in the flavonoids, (ii) the enzyme is primarily quenched by the hydroxyl group(s) of A and B rings on the ether side of the flavonoids, and (iii) the tyrosinase inhibitory activity of 7,8,3',4'-tetrahydroxyflavone is supported by a virtual model of docking with the mushroom tyrosinase, which depicts the quenching of the enzyme. The results also demonstrated that the dihydroxy substitutions in the A and B rings are crucial for Cu2+-chelate formation, thereby influencing the tyrosinase inhibitory activity.

Inhibitory Mechanism of Salidroside on Tyrosinase

“…In a word, The results based on molecular docking further validated that salidroside didn't react with copper ions located in the active center of tyrosinase, which was in accordance with the conclusion achieved by UV spectrum. In addition, Donghyun Kim et al had found that some flavonoids had a high inhibition to tyrosinase, which was related to the ability to form chelate with copper ions [35]. However, in this experiment, the inhibition of salidroside on tyrosinase was only up to 33.04%.…”

“…Keep temperature of 30°C for 10 min. Finally, Scan ultraviolet-visible spectrum of each mixture respectively in the wavelength of 200-600 nm in the reaction of 0, 10, 20, 30, and 40 min [30,31].…”

“…Particularly the active center had a high conservation, thus it played an important role in the catalytic process of tyrosinase [33]. In ultraviolet visible range of wavelengths, small molecule inhibitors with characteristic absorption would chelate with tyrosinase, which could enhance internal conjugation of inhibitors and ultraviolet absorption peak of inhibitors got redshifted [30,31]. The paper studied the ultraviolet absorption spectrum of salidroside when CuSO 4 solution or tyrosinase was added.…”

“…Thus, it could be speculated the reason that salidroside didn't have too high inhibition to tyrosinase was that the inhibitor couldn't react with copper ions located in the active center of tyrosinase. So far many researches had showed that flavonoids had a high inhibition to tyrosinase such as quercetin due to the similar structures to L-DOPA, and others such as rutin also had a great inhibition to tyrosinase which mainly reacted with copper ions located in the active center of enzyme [35,36]. In conclusion, it could be conjectured that the inhibition ability to inhibitors of tyrosinase was not only related to the similarity of substrates, but also associated with the extent of chelation between inhibitor and copper ions located in the active center of tyrosinase.…”

Inhibitory Effect of the Ethyl Acetate Fraction of Ethanol Extract from Rhus verniciflua Stokes Wood on the Activity of Mushroom Tyrosinase

“…In the ultraviolet chromatogram, the wavelength of maximum absorption of the inhibitors will exhibit redshift. The excessive Cu 2+ added in the flavonol, the inhibitor chelated with Cu 2+ , the characteristic absorption peaks of the flavonol were moved toward longer wavelength (Kim et al 2006b). According to this feature, the effect of Cu 2+ and tyrosinase on the UV-visible spectrum of the ethyl acetate extract are shown in Fig.…”