Mechanism of selective toxicity of 4-S-cysteinylphenol and 4-S-cysteaminylphenol to melanocytes

Our previous studies showed that 4-S-cysteinylphenol (4-S-CP) and 4-S-cysteaminylphenol (4-S-CAP) inhibit the growth of malignant melanoma and cause depigmentation of black skin. In this study we examined kinetic constants of CP and CAP as substrates for tyrosinases and their properties as sulphydryl scavengers. 4-S-CP and 4-S-CAP were found to be much better substrates for mushroom tyrosinase than l-tyrosine while their 2-S isomers were not the substrates. 4-S-CP and 4-S-CAP were also good substrates for mammalian tyrosinase. Upon tyrosinase oxidation the two phenols conjugated with cysteine to form the cysteinyl derivatives of the corresponding catechols via o-quinone forms. The tyrosinase oxidation product of 4-S-CP had a poor ability to conjugate with alcohol dehydrogenase, a sulphydryl enzyme, while that of 4-S-CAP had a much higher ability. These results suggest that in melanocytes these phenols are oxidised by tyrosinase to the corresponding o-quinone forms, some of which conjugate with sulphydryl enzymes through cysteine residues, thus exerting cytotoxic effects.