Mammalian Tyrosinase: Biosynthesis, Processing, and Modulation by Melanocyte-stimulating Hormone

We have examined the rate of synthesis and degradation of tyrosinase (monophenol, 3,4-dihydroxyphenylalanine:oxygen oxidoreductase, EC 1.14.18.1), the critical enzyme involved in mammalian pigmentation, using pulse-chase metabolic labeling of murine melanoma cells and immunoprecipitation of protein extracts with antibodies directed specifically against the enzyme. We have found that tyrosinase is synthesized and glycosylated within melanocytes rapidly, since significant quantities of pulse-labeled enzyme could be detected within 30 min. The maximum amount of enzyme was processed within 4 hr, and the t1/2 of tyrosinase in vivo was 10 hr (compared to 120 hr with purified enzyme), suggesting that tyrosinase activity in melanocytes is at least in part regulated by rapid synthesis and active degradation. We also have examined the melanogenic stimulation caused by melanocyte-stimulating hormone, using metabolic labeling, radiometric assays, and immunofluorescence techniques; responding cells increased their melanogenic potential more than 7-fold within 4 days without increasing their levels of tyrosinase synthesis. The results demonstrate that a pool of inactive tyrosinase exists in melanocytes and that rapid increases in enzyme activity elicited by melanocyte-stimulating hormone reflect an alteration in the activity of a preexisting pool of intracellular tyrosinase.