Cell-free synthesized precursor to somatostatin-28 from the rat hypothalamus.

Cell-free translation of rat hypothalamic mRNA and specific immunoprecipitation were used to identify a polypeptide of 16,000 apparent molecular weight as prepro-somatostatin. Quantifying these results suggested that the somatostatin-specific mRNA represented less than 0.01% of the total hypothalamic mRNA. Co-translational addition of microsomal membranes led to the in vitro synthesis of a pro-form of 14,500 molecular weight. By using antisera specifically recognizing 3 different but overlapping segments of somatostatin-28 (SRIF-28), the rat prepro-somatostatin was shown to contain antigenic determinants of this N-terminally extended somatostatin as well as of the tetradecapeptide (SRIF-14). Sequential immunoprecipitation experiments implied the existence of only a single somatostatin precursor among the rat hypothalamic translation products, which would have to be differentially processed to allow release of both SRIF-28 and SRIF-14.