Synthesis and biological activities of photoaffinity labeling analogs of substance P

Abstract

Substance P (Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-MetNH2, SP) in an undecapeptide with important properties as a neurotransmitter and with other functions. No specific antagonists and no long-acting analogs of this peptide hormone are known to date. To reach these goals, analogs of SP were prepared which contain potential affinity, and photoaffinity labeling functions, suitable for irreversible attachment to SP receptors. The synthesis is reported of SP analogs which have the Phe residues in positions 7 or 8 replace with (4''-NO2)Phe, (4''-NH2)Phe, (4''-N2+)Phe, and (4''-N3)Phe. Some of these peptides are used for photoaffinity labeling studies using various bioassays. The synthesis of the (NO2)Phe-containing peptide was carried out on solid phase using Nle instead of Met and the Boc strategy up to residue 4; the remaining amino acids were added using an Fmoc strategy. The protected undecapetide was cleaved by ammonolysis, purified by chromatography on silica gel with chloroform/methanol, and deprotected afterwards. The amino, diazonium, and azido peptides were obtained in this sequence by chemical modification of the nitro peptides. On guinea pig ileum the modified peptides in position 8 had close to maximal activity, whereas modifications in position 7 produced some reduced activity, especially the nitro modification. No diazonium peptide produced any irreversible effects on guinea pig ileum. Photoinactivation studies were carried out on strips of guinea pig trachea, but no irreversible effects were observed, neither permanent stimulation nor permanent inactivation. The biological activities and effects are discussed in view of the molecular properties of the synthesized analogs.