Structure of the 22-residue somatostatin from catfish. An O-glycosylated peptide having multiple forms.

The 22-residue somatostatin (SST-22) from channel catfish, purified by an improved method, is shown to be a glycopeptide. This represents the first report of a glycosylated somatostatin. Multiple forms of SST-22 exist with the major form containing 1 mol of galactose and 1 mol of N-acetylgalactosamine/mol of peptide attached via an O-glycosidic linkage to Thr-5. The position of the carbohydrate was determined by trapping the reactive peptide following beta-elimination of the carbohydrate with [35S]beta-mercaptoethanol followed by sequencing of the radiolabeled protein. All forms of SST-22 that have been purified are identical in amino acid composition. The heterogeneity resides in the carbohydrate portion of the glycopeptide with at least one of the minor forms containing sialic acid. The sequence for SST-22 obtained by automated Edman degradation is Asp X Asn X Thr X Val X Thr X Ser X Lys X Pro X Leu X Asn X Cys X Met X Asn X Tyr X Phe X Trp X Lys X Ser X Arg X Thr X Ala X Cys. This sequence differs at positions 5 and 19 from that published by Oyama et al. (Oyama, H., Bradshaw, R. A., Bates, O.J., and Permutt, A. (1980) J. Biol. Chem. 255, 2251-2254). The amino acid sequence reported here is identical to that deduced from the cDNA. The mass ion of SST-22 was determined by fast atom bombardment/mass spectrometry and shown to be 2943 +/- 1 (m/z). The observed mass ion is consistent with the molecular weight predicted from the amino acid sequence plus 1 mol of galactose and 1 mol of N-acetylgalactosamine.