L-tyrosine, a precurosr to melanin, has recently been shown to be a regulator of the melanogenic pathway in some cultured melanoma cell lines. In this paper we demonstrated that L-tyrosine, besides increasing binding capacity for MSH, decreased cooperativity between MSH receptors and increased the level of tyrosinase induction by MSH. Apparently, regulation of MSH receptor activity by L-tyrosine involves specific changes in the interactions between the receptors and modification of the cellular

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... responsiveness to MSH. KEY WORDS: MSH receptors; L-tyrosine; L-dopa; melanoma. INTRODUCTION Malanogenesis is highly complex and controlled by multiple regulatory factors (1). One controlling element, the MSH (melanocyte stimulating hormone, melanotropin) receptor system is a positive regulator of pigmentation throughout the vertebrates, and in mammals regulates proliferation, pigmentation and morphology of melanin-producing cells (2). The activity of MSH receptors is in turn under multiple controls, being regulated by such diverse factors as ultraviolet radiation (3), interleukin I (4), interferons (5), insulin (6), dopa phosphates (7), and the phase of cell cycle (8, 9). Recent reports have shown that precursors to the pathway of melanogenesis, namely L-tyrosine and L-dopa, can also act as regulators of the pathway (10-13). The effects of L-tyrosine and L-dopa on melanogenesis were seen dramatically in