The Multiple Forms of Mushroom Tyrosinase

Abstract

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 α-tyrosinase is 6.7 S. The molecular weight of β-tyrosinase, calculated from sedimentation velocity and diffusion data, is 123,800, in good agreement with other published values for mushroom tyrosinase. β-, γ-, and δ-Tyrosinase show similar amino acid compositions, while that of α-tyrosinase is different in several respects. Comparison of tryptic peptide maps point to a close similarity among the isozymes, though the α-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 NH2-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 β-tyrosinase has led to the isolation and analysis of 28 peptide fragments, among them the presumed COOH-terminal tripeptide, phenylalanine-alanine-valine.