In vitro biosynthesis of fish islet preprosomatostatin: evidence of processing and segregation of a high molecular weight precursor.

Evidence is presented for a precursor to somatostatin that is 10-12 times larger than the authentic secreted hormone. mRNA from angler fish (Lophius americanus) islets of Langerhans was translated in the wheat germ cell-free system and the products were identified by immunoprecipitation with specific antibodies to somatostatin followed by sodium dodecyl sulfate gel electrophoresis. One 18,000-dalton polypeptide was specifically immunoprecipitable. Competition experiments showed that authentic somatostatin competed with the 18,000-dalton molecule for antibody binding. When dog pancreas microsomal membranes were present during translation, an additional polypeptide of 16,000 daltons was also immunoprecipitable. Comparison of their tryptic peptides demonstrated that the 16,000-dalton polypeptide was derived from the 18,000-dalton one. Tryptic peptide analysis of somatostatin and the 18,000-dalton precursor demonstrated that the 18,000-dalton polypeptide contains the authentic somatostatin amino acid sequence and suggests that it is located at the carboxyl terminus of the precursor molecule and is preceded by a basic amino acid.