Abstract: The kinetics of inhibitory effect of Poria cocos extract on monophenolase and diphenolase activity of mushroom tyrosinase was studied.Test results show that Poria cocos extract efficiently inhibits both monophenolase and diphenolase activities of mushroom tyrosinase.Concentrations of Poria cocos extract leading to 50% activity lost (IC₅₀) on monophenolase and diphenolase were calculated to be 0.008 and 0.040 g/L respectively.There is a lag time in the catalytic reaction process of the monophenolase.Inhibition of Poria cocos extract on activity of diphenolase of tyrosinase indicates a reversible effect.Lineweaver-Burk plots indicates that the inhibitor binds the enzyme reversibly other than reacts with it destructively.Lineweaver-Burk plots show that the inhibition type is a composite of competitive and non-competitive inhibition.On this basis,Poria cocos extract stimulus preliminary assessment was conducted with RBC hemolysis test,and results show that Poria cocos extract with a mass concentration of 2.000 g/L is non-stimulus while such concentration is far higher than IC₅₀ of Poria cocos extract on monophenolase and diphenolase activity.It further indicates that Poria cocos extract displays whitening effect under the condition of the security.This suggests that Poria cocos extract with mass concentration range of 0.200~0.800 g/L is safe,for whitening cosmetic use.

Key words: additive of whitening cosmetics, Poria cocos extract, tyrosinase, mixed inhibition, stimulus