Microheterogeneity of melanosome-bound tyrosinase from the Harding-Passey murine melanoma

This study was directed towards the characterization of the origin of the microheterogeneity displayed by mammalian tyrosinase, the enzyme responsible for pigmentation in mammals. Tyrosinase was purified from the Harding Passey murine melanoma, fractionated into a continuous series of subisozymic forms, and analyzed using various chemical and immunological probes. Treatment with neuraminidase revealed that all the forms had similar amounts of sialic acid, and reactivity with various carbohydrate specific lectins showed that the isozymes also contained subterminal galactose, N-acetylglucosamine, and mannose, but lacked alpha-fucose. Amino acid composition data indicated that the polypeptides of all the forms had identical residue contents. The sum of the evidence further supports the theory that the isozymic forms demonstrable for mammalian tyrosinase represent intermediate processing stages of the enzyme from the nascent protein chain to the fully glycosylated, high molecular weight form of tyrosinase that is localized within melanin granules.