Inhibitory effects of salicylic acid family compounds on the diphenolase activity of mushroom tyrosinase

Mushroom tyrosinase (EC 1.14.18.1) catalyzes both the hydroxylation of tyrosine into o-diphenols and the oxidation of o-diphenols into o-quinones that form brown or black pigments. In the present paper, inhibitory effects on the diphenolase activity of 10 compounds of the salicylic acid-family on the diphenolase activity of mushroom tyrosinase have been studied. The results show that some of these compounds behave as reversible inhibitors. Salicylic acid is a competitive inhibitor while 4-methoxysalicylic acid is non-competitive, 5-methoxysalicylic acid is mixed-I type and 4-methylsalicylic acid and 5-methylsalicylic acid are mixed-II type. The inhibition constants of these five compounds were evaluated. The inhibition strength follows the order: 4-methylsalicylic acid > 5-methylsalicylic acid > 4-methoxysalicylic acid > salicylic acid > 5-methoxysalicylic acid. Models of the interaction between the enzyme and the inhibitors are further discussed and compared.