Flavonols from Heterotheca inuloides: Tyrosinase inhibitory activity and structural criteria

Tyrosinase inhibitory activity of flavonols, galangin, kaempferol and querc etin, was found to come from their ability to chelate copper in the enzyme. In contrast, the corresponding flavones, chrysin, apigenin and luteolin, d id not chelate copper in the enzyme. The chelation mechanism seems to be sp ecific to flavonols as long as the 3-hydroxyl group is free. Interestingly, flavonols affect the enzyme activity in different ways. For example, querc etin behaves as a cofactor and does not inhibit monophenolase activity. On the other hand, galangin inhibits monophenolase activity and does not act a s a cofactor. Kaempferol neither acts as a cofactor nor inhibits monophenol ase activity. However, these three flavonols are common to inhibit diphenol ase activity by chelating copper in the enzyme. (C) 2000 Elsevier Science L td. All rights reserved.