Polymerization of 5,6-dihydroxyindole-2-carboxylic acid to melanin by the pmel 17/silver locus protein

Abstract

Recent advances in melanogenesis have focused on the role of dihydroxyindole-2-carboxylic acid [(HO)₂IndCOOH]. For example, it has been shown that formation of (HO)₂IndCOOH from depachrome is catalyzed by dopachrome tautomerase, that the melanogenic protein tyrosinase-related protein (TRP)-1 can oxidize (HO)₂IndCOOH to its indole quinone, that (HO)₂IndCOOH-melanins can be synthesized chemically, that mammalian melanins are naturally rich in (HO)₂IndCOOH subunits, and that (HO)₂IndCOOH is incorporated into melanins are naturally rich in (HO)₂IndCOOH subunits, and that (HO)₂IndCOOH is incorporated into melanins of melanomas in mice. The question thus emerges as to the mechanims(s) by which (HO)₂IndCOOH and other precursors become incorporated into melanins in vivo. Accordingly, an activity was partially purified that catalyzed melanin formation with (HO)₂IndCOOH as a substrate. Analyses of the (HO)₂IndCOOH polymerization factor from Cloudman melanoma cells revealed the following: it was proteinaceous in that it was heat labile and destroyed by proteinase K; it was a glycoprotein in that it adhered to wheat germ agglutinin and was eluted with N-acetyl glucosamine; it was located predominantly in the melanosomal fraction of cell homogenates; the activity was reduced by exposure to the metal chelators EDTA and EGTA, but not by phenylthiourea, a tyrosinase inhibitor; activity was found with the mouse pmel 17/silver locus protein immunopurified from human melanoma cells, and was significantly reduced in extracts of mouse melanocytes cultured from silver (si/si) mice compare to extracts from Si/Si melanocytes. In summary, an activity has been identified in human to mouse melanoma cells that catalyzes the superoxide-dependent polymerization of (HO)₂IndCOOH to melanin in vitro, and appears to be a function of the pmel 17/silver protein of the human pmel 17 gene and the mouse silver locus