Melanin formation starts with tyrosine, which tyrosinase converts into dopa (dihydroxyphenylalanine). Thereafter the pathway of biosynthesis is non-enzymatic; probably various polycyclic quinone-like compounds are formed and combine with protein to form the final melanin pigment. The rate-limiting step in this pathway is the action of tyrosinase. This enzyme is normally more or less inhibited, but it can be released and this leads to melanogenesis. One important releasing factor is ultraviolet (UV) radiation. The very shortest wavelengths in sunlight are responsible, and these wavelengths also cause the normal UV erythema reaction. Thus sunburn and suntan are usually seen together.
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