Six stereochemically pure analogues of somatostatin (SS), D- and L-5F-Trp8-SS, D- and L-6F-Trp8-SS, and D- and L-5Br-Trp8-SS, were synthesized and found to be more potent than somatostatin in suppressing the release of growth hormone from cultured rat pituitary cells. Two of the analogues, D-5F-Trp8- and D-5Br-Trp8-SS, were respectively 25 and 30 times more active than somatostatin in that assay. The analogues were prepared by solid phase synthesis of their corresponding diastereomeric mixtures, followed by their complete resolution by preparative partition chromatography. Reversed phase high pressure liquid chromatography (HPLC) was used to monitor the resolution and also to check the final purity of each peptide. Positive identification of each diastereoisomer was determined by amino acid analyses of their enzymatic digests, direct comparison with a known all-L standard in the case of the 5F-Trp8 analogues, and chromatographic separation of dansylated amino acids following enzymatic digestion of D- and L-5Br-Trp8-SS. The role of tryptophan in somatostatin is discussed and it is suggested that maintenance of physiological activity in somatostatin peptides, at least on the pituitary, is partially dependent upon the degree of resonance in the indole nucleus in position 8.
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