The tyrosinase-protyrosinase system in frog epidermis.

The formation of melanin in vertebrate skin has been the subject of controversy since Bloch found that black pigment was formed in melanocytes when human skin was incubated with dopa (dihydroxyphenylalanine). No such pigment was formed when the skin was incubated with tyrosine or epinephrine. (1) In view of subsequent observations that extracts of human and mouse melanomas can oxidize tyrosine as well as dopa, Lerner recommended that the single term tyrosinase be used to describe the enzyme responsible for catalyzing the oxidation of tyrosine and dopa. (2) The demonstration of tyrosine conversion to melanin by mammalian skin required special effort. It was not until 1950 that Fitzpatrick et al. irradiated human volunteers with ultraviolet light for 8 days and found that slices of skin incubated in tyrosine became dark. Microscopically, dark granules were found in the cytoplasm of the cell body and dendritic processes of the melanocytes. The interpretation of these observations was that tyrosinase does exist in human epidermis but in an inactive or partially inhibited state. (3) Several years ago in experiments designed to study the induction of frog skin tyrosinase by melanocyte-stimulating hormone, proteolytic enzymes were found to activate epidermal tyrosinase of Rana pipiens. The nature of the activation suggested a proenzyme-enzyme conversion. (4) The inactive tyrosinase has been purified and compared with active tyrosinase produced by chymotrypsin proteolysis. In this report, we conclude that the inactive tyrosinase is a true proenzyme and that activation consists of the proteolytic cleavage of a low molecular weight peptide probably from the N-terminal portion of the molecule. The active enzyme possesses both tyrosinase and dopa oxidase functions that remain in constant relationship throughout purification. No evidence for participation of peroxidase could be found in the oxidation of either tyrosine or dopa.