2.7. Mushroom tyrosinase activity determination
The mushroom tyrosinase was used for the bioassay. The tyros- inase inhibitory activity was determined with the degree of inhibi- tion on tyrosinase-catalysed oxidation of L-DOPA (Chu, Wang, & Duh, 2009) Diphenolase inhibitory activity was determined by measuring the dopachrome accumulation at 475 nm using spectro- photometer. All the experiments were performed in 25 mM so dium phosphate buffer (pH 6.8). The reaction mixture consisting of 0.1 ml of samples, 0.1 ml of mushroom tyrosinase (1000 Unitsl ml) and L-DOPA (3.8 mM) was added in this order to read the absorbance at 475 nm for 5 min. The reaction was performed at 25 °C. The value in the absence of samples was represented as the control. The inhibition of tyrosinase activity was calculated with the following formula: Inhibition a (1 47s in sam- pleoD47s in control)) x 100%. For determination of inhibition type, various concentrations of L-DOPA as the substrate, the tyrosinase inhibitory activity was measured according to the method de- scribed above. Inhibitory kinetics of samples was analysed by Lineweaver-Burk plots. The kinetic data were plotted as 1Jactivity (1/) versus 1/substrate concentration (1/S) according to the method of Lineweaver-Burk, and the Michaelis-Menten constant (Km) and maximum velocity (Vmax) were determined with variable substrate concentrations in the standard reaction mixture.