The Photo-oxidation of Dihydroxyphenylalanine in the Presence of 8-Methoxypsoralen*

8-Methoxypsoralen (8-MOP) is well known for its ability to stimulate melanin formation in vitiligo or normal skin if administration is coupled with exposure to sunlight or ultraviolet light. As an approach to studying the mechanism of action of 8-MOP, the latter was incubated with DOPA and exposed to various types of light. Sunlight, white light, or long wave ultraviolet light (3660 Å.) stimulated the oxidation of DOPA in the presence of 8-MOP but not in the absence of the latter. 8-MOP protected DOPA against short wavelength ultraviolet(2540 Å.) although DOPA was oxidized by the latter in the absence of 8-MOP. The photo-oxidation of DOPA in the presence of long wavelength ultraviolet light and 8-MOP was more rapid at pH values above 7 and was completely inhibited by ascorbic acid, cysteine, cysteamine, β-aminoethylisothiuronium bromide HBr, and partially inhibited by sodium thioglycollate. The possible implication of these in vitro observations to the mode of action of 8-MOP in vivo is discussed.