Chemical synthesis and receptor binding of catfish somatostatin: a disulfide-bridged β-d-Galp-(1→3)-α-d-GalpNAc O-glycopeptide

: The glycopeptide hormone catfish somatostatin (somatostatin‐22) has the amino acid sequence H‐Asp‐Asn‐Thr‐Val‐Thr‐Ser‐Lys‐Pro‐Leu‐Asn‐Cys‐Met‐Asn‐Tyr‐Phe‐Trp‐Lys‐Ser‐Arg‐Thr‐Ala‐Cys‐OH; it includes a cyclic disulfide connecting the two Cys residues, and the major naturally occurring glycoform contains d‐GalNAc and d‐Gal O‐glycosidically linked to Thr5. The linear sequence was assembled smoothly starting with an Fmoc‐Cys(Trt)‐PAC‐PEG‐PS support, using stepwise Fmoc solid‐phase chemistry. In addition to the nonglycosylated peptide, two glycosylated forms of somatostatin‐22 were accessed by incorporating as building blocks, respectively, Nα‐Fmoc‐Thr(Ac3‐α‐D‐GalNAc)‐OH and Nα‐Fmoc‐Thr(Ac4‐β‐D‐Gal‐(1→3)‐Ac2‐α‐D‐GalNAc)‐OH. Acidolytic deprotection/cleavage of these peptidyl‐resins with trifluoroacetic acid/scavenger cocktails gave the corresponding acetyl‐protected glycopeptides with free sulfhydryl functions. Deacetylation, by methanolysis in the presence of catalytic sodium methoxide, was followed by mild oxidation at pH 7, mediated by Nα‐dithiasuccinoyl (Dts)‐glycine, to provide the desired monomeric cyclic disulfides. The purified peptides were tested for binding affinities to a panel of cloned human somatostatin receptor subtypes; in several cases, presence of the disaccharide moiety resulted in 2‐fold tighter binding.