Binding of cyclic and linear MSH core peptides to the melanocortin receptor subtypes

We report here the binding of 5-, 6- and 7-amino-acid-long linear and cyclic core peptides of MSH (melanocyte-stimulating hormone) to cells transiently expressing the human melanocortin MC1, MC3, MC4 and MC5 receptors. The results show that, in contrast to the natural peptides, the core peptides did not differentiate between the melanocortin MC3 and MC4 receptors. All tested cyclic peptides had much lower affinities than their corresponding linear homologues. Interestingly, the relative loss of binding due to the cyclisation did not change as the ring size decreased. Therefore, decreasing the ring size does not seem to force the peptide into a more unfavourable conformation.