N-(3,5-dihydroxybenzoyl)-6-hydroxytryptamine as a novel human tyrosinase inhibitor that inactivates the enzyme in cooperation with l-3,4-dihydroxyphenylalanine.

N-(3,5-Dihydroxybenzoyl)-6-hydroxytryptamine (2) was a novel inhibitor of L-3,4-dihydroxyphenylalanine (DOPA) oxidase activity of human HMV-II melanoma tyrosinase. The IC₅₀ values for 2 and three reference compounds, N-(3,5-dihydroxybenzoyl)serotonin, 6-hydroxyindole, and kojic acid, were 9.1, 842, 22, and 310 µM, respectively, indicating that the 6-hydroxyindole moiety was more effective than 5-hydroxyindole as the pharmacophore of polyphenolic tyrosinase inhibitors and that the inhibitory activity of 6-hydroxyindole was strengthened by the link with a resorcinol group. Furthermore, compound 2 exhibited a unique property of inactivating the human tyrosinase in the presence of low concentrations of DOPA. This inactivation was attenuated by high concentrations of DOPA and for the most part was irreversible as confirmed by activity stain in native polyacrylamide gel electrophoresis and by removal of 2 and DOPA using gel permeation chromatography. Tyrosinase is the enzyme that oxidizes tyrosine to DOPA and further oxidizes DOPA to the melanin precursor dopaquinone. A compound such as 2 that inactivates the enzyme in the presence of a small amount of DOPA is therefore attractive as a new type of tyrosinase inhibitor. Unfortunately, 2 hardly suppressed the melanogenesis in melanoma cell culture. However, the above strong inhibitory activity and the unique property in the combination with DOPA suggest that this compound is a useful lead in designing new antimelanogenic agents.