ArticleBinding and Biological Activity of C-Terminally Modified Melanocortin Peptides: A Comparison Between Their Actions at Rodent MC1 and MC3 Receptors
Section snippets
Peptide Synthesis
α-MSH and γ1-MSH were obtained commercially (Bachem, Switzerland). All other peptides were synthesized on a MilliGen 9050 fully automatic synthesizer as their N-acetyl carboxyamide form employing Fmoc strategy. Amino acids with the exception of d-Phe were used as their pentafluorophenylester; d-Phe was activated in situ using diisopropylcarbodiimide and 1-hydroxybenzotriazol. Side-chain protecting groups were as follows: arginine, methoxytrimethylsulphonyl (Mtr); glutamic acid, t-butoxy (Obut);
Results
All peptides tested were able to completely inhibit the binding of [125I-Tyr2,Nle4,dPhe7]α-MSH to both receptors and exhibit full biological activity at appropriate concentrations (Table 1). To facilitate the comparison of affinity and activity at both receptors, the data are presented relative to α-MSH in Table 2. Data from representative experiments are shown in Fig. 2, Fig. 3.
At the MC1 receptor, all compounds derived from [Nle4,dPhe7]α-MSH, a potent synthetic analogue of α-MSH-26, showed
Discussion
When analyzing the data it was observed that modification of the melanocortin peptides often led to analogues where either affinity or activity, but not both, were significantly changed. This is of interest in establishing the requirements for stimulation of each receptor. It was therefore useful to find a comparative measure for the peptides’ efficacy at each receptor by calculating the ratio between their activity and their affinity for each ligand [Fig. 4(a,b)]; therefore, in this
Acknowledgements
The authors wish to thank Dr. R. D. Cone, Vollum Institute for Advanced Biomedical Research, Portland, OR, for supplying the 293 cells transfected with the gene encoding the rat MC3 receptor. U.G.S. was supported by a grant from the Medical Research Council.
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