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The inhibitory effects of sulfur compounds, namely cysteine and α-mercaptopropionylglycine (α-MPG) (thiol type), and N-methyl-3-mercaptoimidazole (MMI), 6-propylthiouracil (PTU) and ergothioneine (ESH) (thione type) on melanin formation reaction by tyrosinase, in the presence of tyrosine as a substrate, were assessed. Tyrosine oxidation was monitored both by radioassay using L-[3,5-^3H]tyrosine, L-[2,6-^3H]tyrosine and L-[carboxyl-^<14>C]tyrosine as substrates, and by spectrometry to quantitate the dopachrome formed from L-tyrosine. The rated compounds, ranked as to inhibitory effect in descending order are : cysteine ≃α-MPG>MMI>PTU>ESH. Cysteine and α-MPG, both noncyclic thiol compounds, formed a 3,4-dihydroxyphenylalanine (dopa) conjugate to inhibit melanin formation when added at low concentrations. At high concentrations, they inctivated the enzyme by interacting with tyrosinase. In MMI, PTU and ESH of heterocyclic thione structure, dopa conjugate formation was more inhibitory than tyrosinase inactivation on melanin formation.