L-半胱氨酸对酪氨酸酶的抑制动力学研究

用酶动力学方法考察了L-半胱氨酸对酪氨酸酶单酚酶和二酚酶活性的抑制效应。结果表明,L-半胱氨酸对酪氨酸酶单酚酶和二酚酶活性均有抑制作用,导致单酚酶活力和二酚酶活力下降50%的L-半胱氨酸浓度(IC50)分别为20.3μmol/L和52.0μmol/L。在降低蘑菇酪氨酸酶酶活的同时,L-半胱氨酸能明显延长单酚酶和二酚酶的延滞时间。探讨了二酚酶延滞时间产生的原因:L-半胱氨酸与酪氨酸酶催化氧化产物多巴醌反应,形成了无色的L-DOPA的L-半胱氨酸衍生物,从而阻断了多巴色素的形成,直至体系中的L-半胱氨酸反应完全后,多巴醌才开始转化为多巴色素。L-半胱氨酸对二酚酶的抑制作用表现为不可逆的竞争性抑制。

Inhibitory Kinetics of L-Cysteine on Tyrosinase Activity

The inhibitory effects of L-cysteine on the monophenolase and diphenolase activities of tyrosinase were studied using the enzymological kinetic method.It was found that L-cysteine can inhibit both the monophenolase and diphenolase activities of mushroom tyrosinase.The L-cysteine concentrations leading to 50% activity loss (IC50) were 20.3 μmol/L for monophenolase and 52.0 μmol/L for diphenolase,respectively.Increasing L-cysteine concentrations provoked longer lag periods of monophenolase and diphenolaseas.It is concluded in the diphenolase investigation that the lag period corresponded to the time in which L-cysteine reacted with the enzymatically generated o-quinone.This study demonstrates that L-cysteine is an irreversible competitive inhibitor of the diphenolase activity of mushroom tyrosinase.