Ethyl Xanthate and Propyl Xanthate as Activators and Inhibitors of Mushroom Tyrosinase in Different Concentrations

The effect of ethyl xanthate(I) and propyl xanthate(II) on the kinetics of hydroxylation by mushroom tyrosinase (MT) has been investigated at 20 C in 10 mMphosphate buffer solution, pH 6.8. 4-[(4-Methyl-phenyl)azo]-phenol (MePAPh) was used as a synthetic substrate for the enzyme for cresolase reaction. The results show that ethyl xanthate and propyl xanthate can activate or inhibit the cresolase activity of mushroom tyrosinase depending on the concentration of these effectors. Both I and II act uncompetitive at relatively high concentrations (20-50 M). The inhibition constant (Ki) values for I and II are 13.8 and 11 M, respectively. However, both I and II act as activators at relatively low concentrations (0-11.5 M). Activation of the enzyme in low concentrations of xanthates arises from increasing the affinity of binding for the substrate as well as increasing the enzyme catalytic constant. The activation constant (Ka) values for I and II are 1.88 and 2.68 M, respectively. The enzyme has two distinct sites for both effectors. The first one is a high-affinity activation site and the other is a low-affinity inhibition site