Isolation and characterization of somatostatin from pigeon pancreas

Biosynthesis of somatostatin-like immunoreactivity by frog retinas in vitro

Somatostatin-like immunoreactivity has been localized to a wide variety of central nervous system neurons, including the retina. We utilized the unique advantages the retina provides for in vitro studies of nerves to examine the biosynthesis of somatostatin. Extracts of frog retinas pulse-labeled with [35S]cysteine for various time periods revealed uptake of radioactivity into material adsorbable by anti-somatostatin antibody linked to affinity beads. This uptake increased in a curvilinear fashion for 4 h and was inhibited by cycloheximide (0.2 mM) or by boiling the retinas prior to labeling. Pulse-chase experiments revealed that affinity-adsorbable radioactivity from retinal extracts decreased with time of incubation in chase medium; 89% of this decrease could be accounted for by increased in the affinity-adsorbable radioactivity of the chase medium. Chromatography of the retinal extracts on Sephadex G50 (superfine) revealed four elution peaks, whereas only one peak, co-eluted with somatostatin-14, could be identified in the medium. Chromatographic elution patterns of affinity-adsorbable radioactivity from extracts of pulse-labeled retinas incubated in chase medium for various times showed a gradual shift of radioactivity from the earlier-eluting peaks to the later ones. These studies indicate that biosynthesis of somatostatin occurs in frog retinas in vitro. The retina may be a useful model for further study of peptidergic neurons.

Somatostatin immunoreactive C cells in thyroid glands from various mammalian species

The relative distribution of somatostatin- and calcitonin-containing cells in thyroid glands from various mammalian species was investigated by immunoperoxidase staining, and the concentration of immunoreactive somatostatin by radioimmunoassay. In the thyroid glands of guinea pigs and rabbits, most of the calcitonin cells were also immunoreactive to the somatostatin antiserum, and high concentration of immunoreactive somatostatin was obtained. On the other hand, in the thyroids of other animal species--rats, dogs, pigs, cows, goats, cats, monkeys, mice, and hamsters--only a few C cells revealed the immunoreaction for somatostatin, and the concentration of somatostatin was low. In all animal species studied, the somatostatin was present in the same cells that contain calcitonin, though in guinea pigs and rats there were some C cells containing a large number of reaction products for somatostatin but very few for calcitonin. Thus, it was concluded that there was a considerable variation in somatostatin immunoreactivity of thyroid C cells from species to species.

Characterization of the somatostatin-like immunoreactivity extracted from an adrenal medullary tumour

Significant amounts of somatostatin-like immunoreactivity (SLI) were detected in the extract of a human catecholamine-secreting adrenal medullary tumour. After salt fractionation and reconstitution the major portion of SLI was purified by gel filtration and two HPLC steps; in all three systems it eluted in the position of somatostatin-14. The purified somatostatin-like peptide inhibited, in a dose-related manner, growth hormone release from stimulated perfused rat anterior pituitary cells in vitro. Amino acid analysis showed the purified peptide to have an identical composition to somatostatin found in other species.

Sequence analysis of a cDNA coding for a pancreatic precursor to somatostatin

A synthetic oligonucleotide having the sequence d(T-T-C-C-A-G-A-A-G-A-A) deduced from the amino acid sequence Phe-Phe-Trp-Lys of somatostatin-14 was used to prime the synthesis of a cDNA from channel catfish (Ictalurus punctatus) pancreatic poly(A)-RNA. The major product of this reaction was a cDNA fragment of 565 nucleotides. Chemical sequence analysis of the cDNA fragment revealed that it was complementary to a mRNA coding for somatostatin. The 565-nucleotide cDNA hybridizes strongly with a poly(A)-RNA estimated to be 1000 nucleotides in length. An amino acid sequence of the somatostatin precursor was predicted from the nucleotide sequence. Oyama et al. [Oyama, H., Bradshaw, R. A., Bates, O. J. & Permutt, A. (1980) J. Biol. Chem. 255, 2251-2254] have reported the isolation of a somatostatin from the catfish that is 22 residues in length (somatostatin-22). This peptide differs from somatostatin-14 in amino acid sequence. The cDNA sequence obtained by this laboratory codes for somatostatin-14 and predicts another somatostatin gene product from this species. Thus it would appear that there are at least two somatostatin gene products.

Primary structure of corticotropin-releasing factor from ovine hypothalamus

Sequence analysis was performed of an ovine hypothalamic 41-residue polypeptide that had been postulated to be a putative corticotropin-releasing factor (CRF) because of its high intrinsic corticotropin releasing activity. The NH2-terminal 39 residues of CRF were determined by Edman degradation of 0.6-3.5 nmol of peptide in a Wittmann-Liebold modified Beckman 890C spinning cup sequencer with reverse-phase high-pressure liquid chromatography for the identification of amino acid phenylthiohydantoins (direct micro-sequence analysis). Evidence for residue 40 (isoleucine) was provided by direct micro-sequence analysis of 2.0 nmol of acetylated CRF selectively cleaved at its arginine residues by trypsin prior to analysis. The thermolytic COOH-terminal fragment isoleucyl-alanineamide was characterized as its dansyl derivative. Based on the analytical data, the following primary structure is proposed for ovine hypothalamic CRF: H-Ser-Gln-Glu-Pro-Pro-Ile-Ser-Leu-Asp-Leu-Thr-Phe-His-Leu-Leu-Arg-Glu-Val-Leu-Glu-Met-Thr-Lys-Ala-Asp-Gln-Leu-Ala-Gln-Gln-Ala-His-Ser-Asn-Arg-Lys-Leu-Leu-Asp -Ile-Ala-NH2. In agreement with this proposal, the synthetic replicate of CRF is highly potent in stimulating secretion of both corticotropin and beta-endorphin-like immunoactivities.

Isolation and sequence analysis of a somatostatin-like polypeptide from ovine hypothalamus

A large somatostatin-like polypeptide of apparent molecular weight 3000-4500 [4K somatostatin (SS)] was isolated from ovine hypothalamus. The polypeptide was obtained in the methionine sulfoxide form. Two microsequence analyses of 0.6 and 1.8 nmol of 4K SS were performed with a modified 890 C spinning cup sequencer. The sequencing data together with results of amino acid analysis and C-terminal end-group determination indicated that 4K SS was identical with somatostatin-28 (SS-28) isolated from procine upper small intestine and sequenced by Pradayrol et al. [Pradayrol, L., Jörnvall, H., Mutt, V., & Ribet, A. (1980) FEBS Lett. 109, 55-58]. No free cysteine sulfhydryl group could be detected, so that it was assumed that the two cysteine residues of ovine SS-28 formed an intramolecular disulfide bond. Besides the structure of SS-28, the N-terminal first 30 residues of an unknown polypeptide from ovine hypothalamus were sequenced as follows: H-Ile-Pro-Ile-Tyr-Glu-Lys-Lys-Tyr-Gly-Gln-Val-Pro-Met-Cys-Asp-Ala-Gly-Glu-Gln- Cys-Ala-Val-Arg-Lys-Gly-Ala-Arg-Ile-Gly-Lys. Trypsin cleaved the somatostatin (SS) entity less selectively from ovine hypothalamic SS-28 than from rat hypothalamic 12 000-dalton SS-like polypeptide (12K SS). Native ovine hypothalamic SS-28 was found to be highly potent in inhibit growth hormone release from cultured rat anterior pituitary cells. The results raised doubts that ovine SS-28 would be an SS precursor and indicated that SS-28 itself may possess regulatory functions.

Primary structure of ovine hypothalamic somatostatin-28 and somatostatin-25

The primary structure of the NH2-terminally extended somatostatins isolated from ovine hypothalamic extracts, one containing 28 residues and the other 25, has been determined. The structure of somatostatin-28 is Ser-Ala-Asn-Ser-Asn-Pro-Ala-Met-Ala-Pro-Arg-Glu-Arg-Lys-Ala-Gly-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys-OH; the shorter one, somatostatin-25, has the same sequence as somatostatin-28 except that the first three NH2-terminal residues are deleted. The two peptides as isolated were found to be oxidized at the methionine residue to the methionine sulfoxide. Their structures were established by subjecting the native peptides to direct sequence analysis in a Beckman 890C sequencer and identifying the released phenylthiohydantoin derivatives by high-performance liquid chromatography. Their structures were confirmed by trypsin digestion and isolation of all the tryptic peptides, followed by amino acid analysis of the tryptic fragments. Moreover, some of the tryptic peptides were matched with their respective synthetic replicates on high-performance liquid chromatography.

Nucleotide sequence of a cloned structural gene coding for a precursor of pancreatic somatostatin

We have constructed and cloned, in bacteria, recombinant plasmids containing DNA complementary to mRNA coding for a pancreatic pre-prosomatostatin, a product of the cell-free translation of pancreatic islet mRNAs shown previously by immunoprecipitation to be a precursor of somatostatin. A clone containing a nearly full-length cDNA insert of 550 base pairs was identified and appeared to contain the entire coding sequence for the somatostatin precursor in addition to portions of the 5' and 3' untranslated regions. mRNA coding for the pre-prosomatostatin is 600-630 bases long as determined by agarose gel electrophoresis and hybridization with labeled cDNA. Analyses of the nucleotide sequence of the cDNA revealed a protein of 119 amino acid beginning with methionine followed by a typical leader sequence containing 18 hydrophobic amino acids. The tetradecapeptide somatostatin, identical in sequence to mammalian hypothalamic somatostatin, is located at the carboxy terminus followed immediately by a stop codon. An ARg-Lys sequence immediately preceding the sequence of somatostatin is typical of a prohormone cleavage site. A sequence Ala-Pro-Arg-Glu preceding the Arg-Lys cleavage site is identical to that found in porcine prosomatostatin. The evolutionary conservation of the identical amino acid sequence of the somatostatin tetradecapeptide from fish to mammals is remarkable. In addition, similar conservation, in fish and mammals, of the sequence Ala-Pro-Arg-Glu-Arg-Lys preceding the coding region for somatostatin suggets that this particular sequence may have biologic importance in cellular processing of the somatostatin precursor.

Somatostatin: an endogenous peptide in the toad urinary bladder inhibits vasopressin-stimulated water flow

Somatostatin (somatotropin release-inhibiting factor; SRIF) is a tetradecapeptide present in brain, pancreas, gastrointestinal tract, and thyroid that inhibits the secretion or action of several hormones in these tissues. We observed that the toad urinary bladder contains concentrations of endogenous somatostatin (8.0 pg/micrograms of protein) comparable to those found in the mammalian pancreas and gastrointestinal tract. To determine if somatostatin directly alter the action of vasopressinn we studied the effects of this polypeptide on vasopressin-stimulated transport processes in the toad urinary bladder in vitro. Somatostatin produced a dose-dependent, reversible inhibition of vasopressin-stimulated osmotic water flow; it inhibited theophylline-stimulated osmotic water flow but not the water flow stimulated by 8-p-chlorophenylthioadenosine 3',5'-cyclic monophosphate. These data are consistent with an inhibition of both basal and hormone-stimulated adenylate cyclase. Vasopressin-stimulated short circuit current was not inhibited by somatostatin. These studies provide direct evidence for an effect of somatostatin on hormone-modulated epithelial transport in tissues other than the gastrointestinal tract. We propose that endogenous somatostatin may function as a local regulator of the cellular action of vasopressin on osmotic water flow.

Isolation and characterization of immunoreactive somatostatin from fish pancreatic islets

Using a radioimmunoassay with labeled synthetic tetradecapeptide somatostatin, a large amount of immunoreactive somatostatin was found in the principal pancreatic islet of the channel catfish (Ictalurus punctata). The purpose of these experiments was to isolate and characterize the somatostatin-like material. Extracts of islets were chromatographed on a Bio-Gel P-30 column, and over 90% of the immunoreactive somatostatin migrated with proteins at least twice the size of synthetic tetradecapeptide somatostatin. This fraction was further purified by ion-exchange chromatography on carboxymethyl-cellulose and DEAE-cellulose columns. Two peptides were obtained with identical immunoreactivity, which was approximately 25% that of the synthetic somatostatin. Each peptide was judged to be >95% pure by thin-layer electrophoresis, polyacrylamide gel electrophoresis at pH 8.9, and highpressure liquid chromatography. Further criteria of purity included amino-terminal analysis of fraction IV yielding only aspartic acid. A total of 1.3 mg of fraction II, and 3.8 mg of fraction IV somatostatin-like peptides were obtained from 10 g of fresh frozen islets. Characterization of the two peptides revealed both peptides slightly more acidic than synthetic tetradecapeptide somatostatin. Fraction II had an isoelectric point of 8.0-8.3, and fraction IV 8.3-9.0. Molecular weight estimation by sodium dodecyl sulfate-urea polyacrylamide gel electrophoresis revealed similar mobility of both peptides, between pancreatic polypeptide (mol wt 4,500) and glucagon (mol wt 3,500). The mobility was not altered by reduction, and was approximately twice the size of synthetic tetradecapeptide somatostatin (mol wt 1,800). This confirmed that the peptides were single polypeptide chains and not aggregates, or somatostatin bound to larger proteins. Molecular weight determination by gel filtration chromatography on Bio-Gel P-6 in 8 M urea gave an estimated mol wt of 3,700. Amino acid analysis of the two immunoreactive somatostatins indicated that they were very similar in composition. Both pancreatic somatostatins (1 muM) had full biological activity relative to synthetic somatostatin measured as inhibition of growth hormone release from rat anterior pituitary cells.