DOPACHROME CONVERSION - A POSSIBLE CONTROL POINT IN MELANIN BIOSYNTHESIS

[摘要] Melanin biosynthesis involves the enzymatic conversion of Tyr, via dopa, to melanin through a number of intermediate steps. It is 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. A later step, namely the conversion of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid may also be under regulatory control. A factor(s) from 3 different melanomas (Cloudman [mouse S91], B16 [mouse], and Greene [Syrian golden hamster]) was found 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 [NIE 115 mouse neuroblastoma, mouse Friend leukemia, mouse mycloma and Lan 2 fibroblasts (derivatives of mouse A9 L cells)], of nonmelanocytic origin. The factor is not yet identified but it is of relatively low MW (less than 1000 daltons by Sephadex G10 gel filtration), negatively charged at pH 6.8, and stable to boiling. The factor contains no detectable SH 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 has not been demonstrated that the factor has a physiological role in melanogenesis but apparently such a role is likely, since the activity was found only in cells with melanin-synthesizing systems.