Peptides containing multiple photolabels: a new tool for the analysis of ligand-receptor interactions. Reversible long-lasting stimulation and inhibition of MSH receptors by multiple photocrosslinks with alpha-MSH.

Crosslinking of MSH receptors on melanophores of the lizard Anolis carolinensis with analogues of alpha-MSH containing a photoreactive group in position 1, 7, 9 or 13 leads to long-lasting receptor stimulation. Reversibility of this long-lasting stimulation is obtained by employing a disulfide-containing photoreactive group which can be cleaved from the receptor by thiol reagents [Ref. 3]. When two photoreactive groups are simultaneously present on the alpha-MSH molecule (e.g in positions 1 + 9; 1 + 13; 7 + 13, or 9 + 13), identical results were obtained and long-lasting receptor stimulation was not altered after cleavage of one single crosslink. alpha-MSH analogues with three photoreactive groups in positions 1 + 7 + 13 led to irreversible receptor stimulation whereas one compound with the photoreactive groups in positions 1 + 9 + 13 induced reversible receptor inactivation which could be changed into long-lasting stimulation by cleaving the crosslink at position 1 of alpha-MSH. These results demonstrate that one and the same peptide ligand may contain structural information for both receptor activation and inhibition and that the receptor may become arrested in an activated or inhibited state by multiple photocrosslinking, depending on the relative positions of these crosslinks.