Receptor binding affinities and biological activities of linear and cyclic melanocortins in B16 murine melanoma cells expressing the native MC1 receptor.

Cyclic alpha-melanocyte-stimulating hormone (alpha-MSH) analogues produced by disulphide bridging (e.g. [Cys4,Cys10] alpha-MSH) are known to be almost equipotent to the native hormone in amphibian skin bioassays and as a consequence have been proposed as a paradigm for the active conformation of native MSH at the pigment cell MC1 receptor. However this proposal has been somewhat speculative as there is no published data comparing biological activity of cyclic MSH analogues with data on receptor binding. This study addresses this problem by comparing tyrosinase stimulatory activity with their receptor binding affinity in B16 murine melanoma cells expressing the native MC1 melanocortin receptor. Cyclic [Cys4,Cys10] alpha-MSH showed almost the same affinity for the MC1 receptor as alpha-MSH, but the linear analogue [Cys4,Cys10] alpha-MSH bound less strongly. Both had biological activities similar to that of the natural ligand. Introduction of D-Phe into the ring in position 7 increased both affinity and activity of the cyclic compound. The study suggests that the intrinsic efficacy of cyclic [Cys4,Cys10] alpha-MSH analogues is similar to native alpha-MSH. Our studies support the proposal that the cyclic structure serves as a good model for the active conformation of linear alpha-MSH.