Peroxidase, "protyrosinase," and the multiple forms of tyrosinase in mice.

Mammalian tyrosinase as currently defined has two catalytic functions: 1) the oxidation of tyrosine to L-3,4-dihydroxyphenylalanine (dopa) and 2) the oxidation of dopa to dopa quinone (1-9). Although an imposing body of evidence has been advanced in support of this interpretation, Okun and associates have recently challenged the prevailing concept of the action of tyrosinase ( 10, 11 ). They have proposed that within mammalian melanocytes, the oxidation of tyrosine to dopa is mediated by peroxidase whereas the oxidation of dopa to dopa quinone is regulated by dopa oxidase. In their view it is the coupling of peroxidase and dopa oxidase which accounts for "tyrosinase" activity in mammals; i.e., a tyrosinase with catalytic action directed toward both tyrosine and dopa does not exist. Our previous studies have demonstrated that polyacrylamide gel electropherograms of supernatant fractions from homogenates of mammalian pigmented hair bulbs reveal the presence of multiple molecular forms of tyrosinase (12, 13). Extensive examination of hair bulbs from various coat-color mutants of the mouse indicate that the multiple molecular forms of tyrosinase are under genetic control (12, 13). Depending on genic constitution, a maximum of three electrophoretically separable forms of tyrosinase (T1, T,-T3) are demonstrable.2 In keeping with general practice (14, 15), dopa has been the melanogenic substrate used in our laboratory to demonstrate tyrosinase within polyacrylamide gels. Accordingly, there have been two basic assumptions: first, that tyrosinase revealed by dopa also is able