Preparation and biological activities of potential vasopressin photoaffinity labels

Several potential photoaffinity analogues of the peptide hormone vasopressin (VP) were prepared by classical solid-phase peptide synthesis using two different pathways. Peptide sequences were built by introduction of (a) Nar-protected aminophenylalanine or (b) nitrophenylalanine in the photolabeling position. Conversion to the azido peptide was completed in pathway a after cleavage and before purification and in pathway b from small quantities of purified nitrophenylalanine-containing precursor peptides. V1 receptor binding properties were measured using membranes prepared from rat liver cells. The binding potential of agonistic VP structures was abolished by the introduction of an azido or a nitro group into the aromatic side chain at position 3. Cyclo desamino-beta,beta-dialkyl-Cys1-type VP antagonist structures were prepared with the photoactivable moiety in position 2 and an iodination residue in position 9. One particular compound, [Dmpa1, Phe(N3)2, Val4, Lys8,D-Tyr9]VP (8), containing beta,beta-dimethyl-beta-mercaptopropionic acid in position 1, had excellent binding properties, both in the radioiodinated (Kd = 4.8 +/- 1.9 x 10(-10) M) and noniodinated form (Kd = 6.4 +/- 0.98 x 10(-10) M). The analogues with long-chain beta-alkylation (diethyl and pentamethylene) and the linear antagonist photolabel showed significantly less affinity. Optimal binding properties were obtained within a very narrow range of hydrophobicity; greater or lesser hydrophobicity was correlated to less potent binding. The precursor analogues, containing nitrophenylalanine, displayed a structure-activity relationship similar to that of the azido peptides. The most potent analogues will be used for receptor labeling studies. A linear antagonist structure having a photosensitive group in position 1, has also been prepared, but this compound displayed much less affinity than the cyclic antagonists. The most potent compounds were also highly selective for the V1 receptor and did not recognize the V2 receptor from other preparations.