Promotion effects of flavonoids on browning induced by enzymatic oxidation of tyrosinase: structure–activity relationship

Tyrosinase, widely distributed in nature, is a copper-containing polyphenol oxidase involved in the formation of melanin. Flavonoids are most often considered as tyrosinase inhibitors but have also been confirmed to be tyrosinase substrates. Four structure-related flavonoids including flavones (apigenin and luteolin) and flavonols (kaempferol and quercetin) are found to promote not inhibit browning induced by tyrosinase catalyzed oxidation both in model systems and in mushrooms under aerobic conditions. A comparison with enzymatic oxidation and autooxidation of flavonoids alone has helped to clarify why flavonoids function as a substrate rather than an inhibitor. Flavonoids almost do not affect the kinetics of melanin formation from enzymatic oxidation of L-dopa in excess. In addition, a new brown complex formed during the reaction of flavonoid quinone and dopaquinone is suggested to enhance the browning effects by competing with isomerization and autooxidation. Structure–activity relationships of the four flavonoids in melanin formation leading to browning induced by autooxidation and enzymatic oxidation confirm the enzymatic nature of the browning.