Melanin biosynthesis patterns following hormonal stimulation.

The effect of melanocyte-stimulating hormone (MSH) on the differentiation of mammalian melanocytes has been widely studied since the early 1950s. There have been many reports about the stimulatory effect of MSH on melanin production and specifically on the activity of tyrosinase, the critical enzyme in the melanogenic pathway. However, few and variable results have been obtained concerning the effect of this hormone on the regulation of DOPAchrome tautomerase (TRP2), another melanogenic enzyme which functions later in the melanogenic pathway, or on other melanogenic activities, such as TRP1. In this study, we show that the MSH-induced stimulation of tyrosinase is accompanied by no significant change in the synthesis or catalytic activities of other melanogenic enzymes such as TRP1 or TRP2. This in turn elicits a dramatic increase in melanin production accompanied by a significant decrease in the incorporation of carboxylated precursors into that melanin biopolymer, although the biological implication of that is still unclear.