Flavonols from saffron flower: Tyrosinase inhibitory activity and inhibition mechanism

A common flavonol, kaempferol, isolated from the fresh flower petals of Cro cus sativus L. (Iridaceae) was found to inhibit the oxidation of L-3,4-dihy droxyphenylalanine (L-DOPA) catalyzed by mushroom tyrosinase with an ID50 O f 67 mu g/mL (0.23 mM). Interestingly, its 3-O-glycoside derivatives did no t inhibit this oxidation. The inhibition kinetics analyzed by a Lineweaver- Burk plot found kaempferol to be a competitive inhibitor, and this inhibito ry activity presumably comes from its ability to chelate copper in the enzy me. This copper chelation mechanism can be applicable for all of the flavon ols as long as their 3-hydroxyl group is free. However, quercetin, kaempfer ol, and galangin each affect the oxidation of L-tyrosine in somewhat differ ent ways.