[Ser⁵]-somatostatin-14: Isolation from the pancreas of a holocephalan fish, the Pacific ratfish (Hydrolagus colliei)

The holocephalan fishes were the first class of vertebrate in evolution to develop a pancreatic gland with both endocrine and exocrine parenchyma. An extract of the pancreas of one such fish, the Pacific ratfish (Hydrolagus colliei) contained somatostatin-like immuno-reactivity (141 pmol/g wet wt), measured with an antiserum raised against mammalian somatostatin-14. Automated Edman degradation and fast atom bombardment-mass spectrometry established the primary structure of the major molecular form as Ala-Gly-Cys-Lys- Ser-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys. A minor component of somatostatin-like immunoreactivity, constituting 8% of the total, was of approximate molecular weight 6000. Thus, in the ratfish pancreas prosomatostatin-I is processed predominantly to somatostatin-14, as in the mammalian pancreas, but the resulting tetradecapeptide contains the substitution Ser for Asn at position 5.