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Dopa is found in the wing coverings of cockchafers Melolontha melotontha L. and M. hippocosfdtii F. (r3), and in the cocoons of one of the Satumidae, Samba cecropia L. (14). It also occurs in Vit-irt .faba (IS) and the Georgia velvet bean, Stizolobim~ deeringianum (16).

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Tyrosinase, the enzyme that controls the synthesis of melanin, is a unique product of melanocytes. Normal and malignant human melanocytes grown in culture were used to study the factors that regulate the expression of tyrosinase. Immunoprecipitation experiments showed that newly synthesized tyrosinase appeared as a protein with an apparent molecular weight of 70,000 that was processed to a protein with an apparent molecular weight of 80,000. Neither tunicamycin nor 2-deoxy-D-glucose inhibited this conversion, suggesting that Oglycosylation is the major biochemical event in the posttranslational modification of tyrosinase. Agents that stimulated the proliferation of normal melanocytes also stimulated tyrosinase activity. Melanocytes with low levels of tyrosinase activity synthesized less tyrosinase, processed the enzyme more slowly, and degraded it more rapidly than melanocytes with high levels of tyrosinase activity. We conclude that tyrosinase activity in cultures of human melanocytes derived from different donors is determined predominantly by its abundance.Epidermal melanin is synthesized by melanocytes via oxidation of tyrosine. The amount of melanin in the epidermis is determined, at least in part, by the activity of tyrosinase. For example, the activity of tyrosinase is higher in foreskins taken from black babies than in foreskins taken from caucasian babies (31); and melanocytes in patients with some forms of albinism have extremely low or no detectable tyrosinase activity (17). Knowledge of factors that regulate tyrosinase is important because melanin protects the skin from harmful solar radiation. In murine melanoma cells grown in vitro the activity of tyrosinase is at least partially regulated by the intracellular levels of cyclic adenosine monophosphate (cAMP). Exposure of these cells to substances that increase the intracellular levels of cAMP also causes an increase in tyrosinase activity and melanin content (1, 6-9, 22, 33, 38).Recently, a method for propagating normal human melanocytes in vitro (5) was developed. This achievement made it possible to study the control of tyrosinase activity in nonmalignant human melanocytes and to compare their responses with those of murine and human melanoma cells. Our experiments show that the activity of tyrosinase in cultured melanocytes is stimulated by agents that stimulate the growth of the cells and is determined by the rate of synthesis, processing, and degradation of the enzyme.