DEMONSTRATION OF TYROSINASE IN THE VITILIGO SKIN OF HUMAN-BEINGS BY A SENSITIVE FLUOROMETRIC METHOD AS WELL AS BY C-14-LABELED (U)-L-TYROSINE INCORPORATION INTO MELANIN

[摘要] Tyrosinase activity (monophenol, dihydroxyphenylalanine: oxygen oxidoreductase EC 1.14.18.1) in vitiligo and normal epidermal homogenates of human skin was measured by estimating dopa by a highly sensitive fluorometric method. The tyrosinase activity in the vitiligo skin was .apprx. 4-37% of corresponding normal skin. The activity of tyrosinase in normal human skin from different individuals and from different regions of the body was in the range of 4-140 pm of dopa formed per min per mg protein of epidermal homogenate. The enzyme from vitiligo and normal skin was severely inhibited by substance(s) of low MW. The enzyme exhibits a lag of .apprx. 4 h in the absence of added dopa and 1 h in presence of 5 .mu.M dopa. Tyrosinase from the normal and vitiligo skin was inhibited by excess concentration of tyrosine. The homogenates from vitiligo skin could synthesize melanin from uniformly labeled 14C-tyrosine. The rate of tyrosine incorporation into melanin by the epidermal homogenates is increased by dopa disproportionate to its effect on tyrosinase activity. Melanocytes are present in the vitiligo skin. A tentative hypothesis is put forward to explain the lack of melanin synthesis by the vitiigo skin under in vivo conditions, altough melanocytes are present.