Dopachrome conversion: A possible control point in melanin biosynthesis

Abstract

Melanin biosynthesis involves the enzymatic conversion of tyrosine, via dihydroxyphenylalanine (dopa), to melanin through a number of intermediate steps. It is generally assumed that the major rate-limiting factor in this process is the amount of active tyrosinase (monophenol monoxygenase; monophenol, dihydroxyphenylalanine: oxygen oxidoreductase, EC 1.14.18.1) available since when dopa has been oxidized by tyrosinase to dopa quinone the succeeding steps can occur spontaneously through autooxidation. The authors now present evidence that a later step, namely the conversion of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid may also be under regulatory control. They have found a factor(s) from 3 different melanomas (Cloudman, B16, and Greene) which catalyzes this step. This activity is also present in mushrooms, which contain a potent pigment-synthesizing system, but not in a variety of cell lines of nonmelanocytic origin. They have not yet identified the factor but it is of relatively low molecular weight (less than 1000 daltons by Sephadex G10 gel filtration), negatively charged at pH 6.8, and stable to boiling. The factor contains no detectable sulfhydryl groups. In crude cell extracts the factor appears to be complexed to tyrosinase since it cochromatographs with tyrosinase through the early steps of purification. The factor binds tightly to QAE-Sephadex anion exchange resin and can be separated from tyrosinase by differential salt elution. It is not yet demonstrated that the factor has a physiological role in melanogenesis but the authors feel such a role is likely since the activity was found only in cells with melanin-synthesizing systems.