Evidence for the Existence of a Biosynthetic Precursor for Somatostatin

The purpose of this study was to determine if the somatostatin in pancreatic islets is synthesized via a precursor-product pathway. Anglerfish islets were subjected to pulse-chase incubations with ³H-tryptophan ¹⁴C-isoleucine. Islet extracts were subjected to gel filtration on Bio-Gel P-10. During chase incubations in the presence of cycloheximide, a linear increase of ³H-radioactivity in the somatostatin-containing portion of the eluate was observed while ¹⁴C-label did not accumulate in this region. A concomitant diminution of both ³H- and ¹⁴C- radioactivity in the 8-15-kilodalton region of the eluate occurred during the chase periods, suggesting a transfer of label from larger to smaller peptides. The labeled peptides found in the 8-15-kilodalton range were subjected to treatment with 8 M urea, 8 M urea−5% thioglycolic acid, 6 M guanidine hydrochloride, and 8 M guanidine hydrochloride−5% thioglycol. No appreciable loss of radioactivity was apparent after any of these treatments. Incubation of these same peptides with cell-free supernate from unlabeled islet tissue or with trypsin yielded a tryptophan-labeled peptide having the molecular size, electrophoretic mobility, and immunoreactivity of anglerfish and synthetic somatostatin. These results suggest that a precursor peptide having a molecular size near that of proinsulin is used in the biosynthesis of somatostatin.