Evidence for the Presence of Somatostatin 28 in Plasma

Somatostatin-like immunoreactivity (SLI) from dog and rat plasma eluted from Biogel P-6 columns as three distinct peaks. A large-molecular-weight peak was present in the void volume of the column, an interme-diate-sized peak (SLI28) coeluted with synthetic soma-tostatin 28 (S-28), and a small-molecular-weight peak (SLI14) coeluted with SRIF. Material from the SLI28 peak diluted in parallel to the S-28 standard in the ra-dioimmunoassay and behaved identically to S-28 on high pressure liquid chromatography (HPLC). Levels of SLI28 in the portal vein were consistently greater than the simultaneously measured peripheral levels (portal peripheral ratio 2.2 ± 0.2). Venous samples drawn from multiple sites suggested that SLI28 is se-creted by the duodenum and/or pancreas and the in-testine. This data is consistent with the possibility that S-28 is a hormone distinct from SRIF. DIABETES 37: 474-477, May 1982. A28-amino-acid form of somatostatin S-28 has re-cently been isolated from porcine intestine1 andovine2 and porcine hypothalamus.3 It is also re-leased by isolated pancreatic islets in perifusion experiments.4 Its structure is that of the tetradecapeptide so-matostatin (SRIF) with a 14-amino-acid amino terminal ex-tension. The biologic significance of this peptide is un-known. It has been suggested that S-28 is a precursor of SRIF.3 However, since S-28 is more active than SRIF in some biologic systems,5 it is also possible that S-28 may repre-sent a naturally occurring form of somatostatin discrete from SRIF. S-28 has not yet been identified in the circulation. In this study, we describe the presence of a form of somatosta-tin-like immunoreactivity (SLI) indistinguishable from S-28 by gel filtration chromatography, high pressure liquid chro