pH-dependent inhibition of mushroom tyrosinase by N-substituted N-nitrosohydroxylamines

Several synthetic N-substituted N-nitrosohydroxylamines were found to inhibit mushroom tyrosinase in a pH-dependent manner regardless of the N-substituent. The inhibitory activity, or pI50 (-log [IC50, M]) value, linearly decreased as the pH of the media increased. The inhibitory activities of tested N-substituted N-nitrosohydroxylamines at pH 6.8 and 5.8 were found to be almost 10 times and 100 times greater than at pH 7.8, respectively. The types of inhibition were different at pH 6.8 and 5.8. These results suggest that the inhibitory effect of N-substituted N-nitrosohydroxylamines is caused by the non-ionized form of the inhibitor. Furthermore, the mechanism of inhibition depends on the interaction between the inhibitor and the active site of tyrosinase at different pH values.