The Multiple Forms of Mushroom Tyrosinase
نویسندگان
چکیده
The four isozymes of mushroom tyrosinase can be partially resolved by analytical electrophoresis on acrylamide gel. Three of the four have similar sedimentation coefficients around 7.2 S, while that of or-tyrosinase is 6.7 S. The molecular weight of /34yrosinase, calculated from sedhnentation velocity and diiIusion data, is 123,800, in good agreement with other published values for mushroom tyrosinase. p-, y-, and &Tyrosinase show similar amino acid wmpositions, while that of a-tyrosinase is different in several respects. Comparison of tryptic peptide maps point to a close similarity among the isozymes, though the or-isozyme displays some differences. The number of tryptic peptides from each of the isozymes, 22 to 25, is consistent with a tetrameric structure composed of very similar or identical subunits, each containing a total of about 23 base (lysine and arginine) residues. Only one NH*-terminal amino acid, isoleucine, is detected from each isozyme by the dansylation procedure and only one COOH-terminal amino acid, valine, by carboxypeptidase A digestion. Ion exchange chromatography of a tryptic digest of ,& tyrosinase has led to the isolation and analysis of 28 peptide fragments, among them the presumed COOH-terminal tripeptide, phenylalanine-alanine-valine.
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