Tyrosinase synthesis in different skin types and the effects of alpha-melanocyte-stimulating hormone and cyclic AMP.

Tyrosinase synthesis and its regulation in human melanocytes was studied by measuring the incorporation of [35S] methionine into incubated skin biopsies. Tyrosinase was detected in all skin samples with the highest levels in skin type IV and the lowest levels in skin type I. Following psoralen ultraviolet A (PUVA) therapy for several weeks, significant increases in the amounts of tyrosinase were found in skin types III and IV. The presence of alpha-melanocyte-stimulating hormone (alpha-MSH) (100 mumol/l) or the long-acting analogue [Nle4, DPhe7] alpha-MSH (1-10 mumol/l) in the incubation medium failed to alter tyrosinase levels in the skin biopsies taken from patients both before and after receiving PUVA therapy. Bromo-adenosine 3,5-cyclic monophosphate sodium salt (8-bromo-cAMP) (10 mmol/l), on the other hand, increased the amounts of tyrosinase both before and after PUVA, but these effects were only seen in biopsies of type III and IV skin. These results indicate that MSH fails to stimulate tyrosinase synthesis in human melanocytes. Nevertheless, tyrosinase synthesis and its regulation by cyclic AMP-dependent mechanisms could be important control points in the pigmentary response.