title: Tyrosinase isozyme heterogeneity in differentiating B16/C3 melanoma.
abstract: The B16/C3 murine melanoma is a pigmented tumor that is rich in the copper-containing enzyme, tyrosinase. This enzyme, which converts tyrosine to melanin precursors, is largely associated with membrane fractions of cells and exists in a number of discrete isozymic forms ranging in molecular mass from 58,000 to 150,000 daltons and pI from 3.4 to 5.2. One of these isozymes (Mr = 58,000, pI 3.4) has been purified to homogeneity. The purified enzyme catalyzes the hydroxylation of L-tyrosine to L-dihydroxyphenylalanine (L-DOPA) and the conversion of L-DOPA to dopaquinone. Ascorbic acid, tetrahydrofolate, and dopamine can serve as cofactors in the hydroxylase reaction. The Michaelis constants for the purified enzyme were 7 X 10(-4) M for L-tyrosine and 6 X 10(-4) M for L-DOPA. The Vmax for L-DOPA was much greater than the Vmax for L-tyrosine indicating that tyrosine hydroxylation is rate-limiting in melanin precursor biosynthesis. Two putative copper chelators, phenylthiourea and diethyldithiocarbamide inhibited both the tyrosine hydroxylase and L-DOPA oxidase activities of the enzyme. Phenylthiourea was a noncompetitive inhibitor while diethyldithiocarbamide was a competitive inhibitor indicating that these agents act by different mechanisms. When digested with proteases and glycosidases, higher molecular weight forms of tyrosinase co-migrated with the purified enzyme in isoelectric focusing and sodium dodecyl sulfate-polyacrylamide gel electrophoresis suggesting that the isozyme was derived from larger precursors. Thus, post-translational processing of tyrosinase may underlie isozyme diversity and this may be important in the control of melanogenesis in this tumor model.
mesh_terms: D000596:Amino Acids / Q000032:analysis; D000818:Animals; D004156:Catechol Oxidase / Q000378:metabolism*; D007527:Isoenzymes / Q000302:isolation & purification / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008546:Melanoma, Experimental / Q000201:enzymology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008970:Molecular Weight; D014442:Monophenol Monooxygenase / Q000302:isolation & purification / Q000378:metabolism*; D009439:Neuraminidase; D014357:Trypsin
chemical_list: D000596:Amino Acids; D007527:Isoenzymes; D004156:Catechol Oxidase; D014442:Monophenol Monooxygenase; D009439:Neuraminidase; D014357:Trypsin