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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)
 
 
 
 
 

Tyrosinase isozyme heterogeneity in differentiating B16/C3 melanoma.

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.[1]

References

  1. Tyrosinase isozyme heterogeneity in differentiating B16/C3 melanoma. Laskin, J.D., Piccinini, L.A. J. Biol. Chem. (1986) [Pubmed]
 
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