Structure of the 22-residue somatostatin from catfish. An O-glycosylated peptide having multiple forms

Targeted Mass Spectrometry Approach Enabled Discovery of O-Glycosylated Insulin and Related Signaling Peptides in Mouse and Human Pancreatic Islets

O-Linked glycosylation often involves the covalent attachment of sugar moieties to the hydroxyl group of serine or threonine on proteins/peptides. Despite growing interest in glycoproteins, little attention has been directed to glycosylated signaling peptides, largely due to lack of enabling analytical tools. Here we explore the occurrence of naturally O-linked glycosylation on the signaling peptides extracted from mouse and human pancreatic islets using mass spectrometry (MS). A novel targeted MS-based method is developed to increase the likelihood of capturing these modified signaling peptides and to provide improved sequence coverage and accurate glycosite localization, enabling the first large-scale discovery of O-glycosylation on signaling peptides. Several glycosylated signaling peptides with multiple glycoforms are identified, including the first report of glycosylated insulin-B chain and insulin-C peptide and BigLEN. This discovery may reveal potential novel functions as glycosylation could influence their conformation and biostability. Given the importance of insulin and its related peptide hormones and previous studies of glycosylated insulin analogues, this natural glycosylation may provide important insights into diabetes research and therapeutic treatments.

Response of the somatotropic axis to alterations in feed intake of channel catfish (Ictaluruspunctatus)

To better understand the effects of reduced feeding frequency on the GH-IGF-I axis, channel catfish (Ictaluruspunctatus), were either fed (Fed control, commercial diet fed daily), fed every other day (FEOD, commercial diet fed every other day), or not fed (Unfed, no feed). Pituitary GH mRNA increased whereas hepatic growth hormone receptor (GHR), IGF-I mRNA, and plasma IGF-I decreased in the FEOD and Unfed fish (P<0.05). In another study, fish were either continually fed (Fed) or fasted and then re-fed (Restricted) to examine the physiological regulation of somatostatin-14 (SS-14) and SS-22 mRNA. Fasting increased (P<0.05) levels of SS-14 mRNA in the hypothalamus and pancreatic islets (Brockmann bodies) at d 30 while re-feeding decreased SS-14 mRNA to control values in all tissues examined by d 45. Fasting had no effect on levels of SS-22 mRNA in the pancreatic islets whereas SS-22 mRNA was not detected in the stomach or hypothalamus. The results demonstrate that feeding every other day has similar negative impacts on components of the GH-IGF-I axis as fasting. The observed increase in SS-14 mRNA in the hypothalamus and pancreatic islets suggests a role for SS-14 in modulating the GH-IGF-I axis in channel catfish.

New insight into the molecular evolution of the somatostatin family

The present review describes the molecular evolution of the somatostatin family and its relationships with that of the urotensin II family. Most of the somatostatin sequences collected from different vertebrate species can be grouped as the products of at least four loci. The somatostatin 1 (SS1) gene is present in all vertebrate classes from agnathans to mammals. The SS1 gene has given rise to the somatostatin 2 (SS2) gene by a segment/chromosome duplication that is probably the result of a tetraploidization event according to the 2R hypothesis. The somatostatin-related peptide cortistatin, first identified in rodents and human, is the counterpart of SS2 in placental mammals. In fish, the existence of two additional somatostatin genes has been reported. The first gene, which encodes a peptide usually named somatostatin II (SSII), exists in almost all teleost species investigated so far and is thought to have arisen through local duplication of the SS1 gene. The second gene, which has been characterized in only a few teleost species, encodes a peptide also named SSII that exhibits a totally atypical structure. The origin of this gene is currently unknown. Nevertheless, because the two latter genes are clearly paralogous genes, we propose to rename them SS3 and SS4, respectively, in order to clarify the current confusing nomenclature. The urotensin II family consists of two genes, namely the urotensin II (UII) gene and the UII-related peptide (URP) gene. Both UII and URP exhibit limited structural identity to somatostatin so that UII was originally described as a "somatostatin-like peptide". Recent comparative genomics studies have revealed that the SS1 and URP genes, on the one hand, and the SS2 and UII genes, on the other hand, are closely linked on the same chromosomes, thus confirming that the SS1/SS2 and the UII/URP genes belong to the same superfamily. According to these data, it appears that an ancestral somatostatin/urotensin II gene gave rise by local duplication to a somatostatin ancestor and a urotensin II ancestor, whereupon this pair was duplicated (presumably by a segment/chromosome duplication) to give rise to the SS1-UII pair and the SS2-URP pair.

Chemical synthesis and receptor binding of catfish somatostatin: a disulfide-bridged beta-D-Galp-(1-->3)-alpha-D-GalpNAc O-glycopeptide

The glycopeptide hormone catfish somatostatin (somatostatin-22) has the amino acid sequence H-Asp-Asn-Thr-Val-Thr-Ser-Lys-Pro-Leu-Asn-Cys-Met-Asn-Tyr-Phe-Trp-Lys-Se r-Arg-Thr-Ala-Cys-OH; it includes a cyclic disulfide connecting the two Cys residues, and the major naturally occurring glycoform contains D-GalNAc and D-Gal O-glycosidically linked to Thr5. The linear sequence was assembled smoothly starting with an Fmoc-Cys(Trt)-PAC-PEG-PS support, using stepwise Fmoc solid-phase chemistry. In addition to the nonglycosylated peptide, two glycosylated forms of somatostatin-22 were accessed by incorporating as building blocks, respectively, Nalpha-Fmoc-Thr(Ac3-alpha-D-GalNAc)-OH and Nalpha-Fmoc-Thr(Ac4-beta-D-Gal-(1-->3)-Ac2-alpha-D-GalNAc)-O H. Acidolytic deprotection/cleavage of these peptidyl-resins with trifluoroacetic acid/scavenger cocktails gave the corresponding acetyl-protected glycopeptides with free sulfhydryl functions. Deacetylation, by methanolysis in the presence of catalytic sodium methoxide, was followed by mild oxidation at pH 7, mediated by Nalpha-dithiasuccinoyl (Dts)-glycine, to provide the desired monomeric cyclic disulfides. The purified peptides were tested for binding affinities to a panel of cloned human somatostatin receptor subtypes; in several cases, presence of the disaccharide moiety resulted in 2-fold tighter binding.

Molecular cloning and pharmacological characterization of a somatostatin receptor subtype in the gymnotiform fish Apteronotus albifrons

The actions of the various forms of somatostatin (SRIF), including those of the tetradecapeptide SRIF(14), are mediated by specific receptors. In mammals, five subtypes of SRIF receptors, termed sst(1-5), have been cloned. Using a combination of reverse transcriptase-polymerase chain reaction and genomic library screening in the gymnotiform fish Apteronotus albifrons, a gene encoding the first-known nonmammalian SRIF receptor has been isolated. The deduced amino acid sequence displays 59% identity with the human sst(3) receptor protein; hence, the gene is termed "Apteronotus sst(3)." The predicted protein consists of 494 amino acid residues exhibiting a putative seven-transmembrane domain topology typical of G protein-coupled receptors. A signal corresponding to the Apteronotus sst(3) receptor was detected in brain after amplification of poly(A)(+)-RNA by reverse transcriptase-polymerase chain reaction, but not by Northern blot analysis or in situ hybridization, suggesting a low level of expression. Membranes prepared from CCL39 cells stably expressing the Apteronotus sst(3) receptor gene bound [(125)I][Leu(8),d-Trp(22), (125) I-Tyr(25)]SRIF(28) with high affinity and in a saturable manner (B(max) = 4470 fmol/mg protein; pK(D) = 10.5). SRIF(14) and various synthetic SRIF receptor agonists produced a dose-dependent inhibition of radioligand binding, with the following rank order of potency: SRIF(14) approximately SRIF(28) > BIM 23052 > octreotide > BIM 23056. Under low stringency conditions, an Apteronotus sst(3) probe hybridized to multiple DNA fragments in HindIII or EcoRI digests of A. albifrons DNA, indicating that the Apteronotus sst(3) receptor is a member of a larger family of Apteronotus SRIF receptors.

Purification and characterization of insulin and peptides derived from proglucagon and prosomatostatin from the fruit-eating fish, the pacu Piaractus mesopotamicus

The fruit-eating teleost fish, the pacu Piaractus mesopotamicus (Characiformes, Characidae) is classified along with the carp and the catfish in the superorder Ostariophysi. The pacu is able to survive and grow in captive conditions feeding exclusively on carbohydrates. Hormonal polypeptides in an extract of pacu Brockmann bodies were purified to homogeneity by reversed phase HPLC and their primary structures determined by automated Edman degradation. Pacu insulin contains only two substitutions, Glu-->Asp at A15 and Thr-->Ser at B24 (corresponding to B22 in mammalian insulins) compared with carp insulin. The B-chains of both insulins contain a dipeptide extension to the N-terminus and a deletion of the C-terminal residue compared with human insulin. Pacu glucagon differs from catfish glucagon by a single substitution at position 17 (Arg-->Gln. The primary structure of the 34 amino acid residue glucagon-like peptide (GLP) differs from catfish GLP only at positions 12 (Ser-->Ala) and 33 (Pro-->Gln). In common with other teleost species, the pacu expresses two somatostatin genes. Somatostatin-14, derived from preprosomatostatin-I (PSS-I), is identical to mammalian/catfish somatostatin-14. Although pacu somatostatin-II was not identified in this study, a peptide was purified that shows 67% sequence identity with residues (1-58) of catfish preprosomatostatin-II (PSS-II). This relatively high degree of sequence similarity contrasts with the fact that catfish PSS-II shows virtually no sequence identity with the corresponding PSS-II from anglerfish (Acanthopterygii) and trout (Protoacanthopterygii). A comparison of the primary structures of the islet hormones suggest that amino acid sequences may have been better conserved within the Ostariophysi than in other groups of the taxon Euteleostei that have been studied.

Evolution of neuroendocrine peptide systems: gonadotropin-releasing hormone and somatostatin

Nine vertebrate and two protochordate gonadotropin-releasing hormone (GnRH) decapeptides have been identified and sequenced. Multiple molecular forms of GnRH peptide were present in the brain of most species examined, and cGnRH-II generally coexists with one or more GnRH forms in all the major vertebrate groups. The presence of multiple GnRH forms has been further confirmed by the deduced GnRH peptide structure from cDNA and/or gene sequences in several teleost species and tree shrew. High conservation of the primary structure of GnRH decapeptides and the overall structure of GnRH genes and precursors suggests that they are derived from a common ancestor. Somatostatin (SRIF) is a phylogenetically ancient, multigene family of peptides. A tetradecapeptide, SRIF (SRIF14) has been conserved, with the same amino acid sequence, in representative species of all classes of vertebrate. Four molecular variants of SRIF14 have been identified. SRIF14 is processed from preprosomatostatin-I, which contains SRIF14 at its C-terminus; preprosomatostatin-I is also processed to SRIF28 in mammals and SRIF26 in bowfin. Teleost fish possess a second somatostatin precursor, preprosomatostatin-II, containing [Tyr7, Gly10]-SRIF14 at the C-terminus, that is mainly processed into large forms of SRIF.

Possible relationship between coding recognition amino acid sequence motif or residue(s) and post-translational chemical modification of proteins

1. The "code-sequence" of N-glycosylation site(s), the amino acids located around O-glycosylation site(s), the sequence motifs of several kinases, the sequence motifs of--sulfation, amidation, isoprenylation, myristoylation, palmitoylation and N-acetylation, Aspartic and Asparagine hydroxylation-site, gamma-carboxyglutamate domain, phosphopantetheine attachment site etc. are extensively listed, compared to those reported by "PROSITE" Computer Screen Center and discussed. 2. The structural aspects of protein-DNA recognition are quoted as discussion and conclusion.

Radioimmunoassay for salmon pancreatic somatostatin-25

A specific and sensitive radioimmunoassay (RIA) for the measurement of plasma levels of somatostatin-25 (SS-25) in salmon was developed using antisera raised against coho salmon (Oncorhynchus kisutch) SS-25. Somatostatin-25 was iodinated by the chloramine-T method and repurified on Sephadex G-25. The RIA was performed using a double antibody (goat anti-rabbit gammaglobulin as second antibody) method under disequilibrium conditions. Plasma from several salmonids (coho, chinook, rainbow trout, brook trout, arctic char, lake trout, and whitefish) as well as plasma from some nonsalmonids (sucker, bluegill) cross-reacted with the antisera; serial dilutions of plasma from rainbow trout, brook trout, chinook salmon, and coho salmon were parallel to the SS-25 standard curve. Plasma from catfish showed negligible cross-reactivity. None of the mammalian somatostatins (somatostatin-14, somatostatin-28). U II, or other pancreatic hormones (insulin, glucagon) tested showed significant cross-reactivity with the antibody in the assay system. The lowest detectable level of SS-25 was 5 pg/tube; especially reproducible results were obtained in the range of 0.15-1.20 ng/ml, which appears to be the normal range of SS-25 circulating in the plasma of salmonids. Intra- and interassay coefficients of variation were 5.7 and 12.6%, respectively. Injection of glucose into chinook salmon resulted in an elevation of plasma SS-25 titers within 30 min and was coincident with hyperglycemia.

Pancreatic endocrine cells in sea bass (Dicentrarchus labrax L.) II. Immunocytochemical study of insulin and somatostatin peptides

Insulin (INS)- and somatostatin (SST)-immunoreactive cells were demonstrated by light immunocytochemistry in the endocrine pancreas of sea bass (Dicentrarchus labrax). INS-immunoreactive cells were identified using bovine/porcine, bonito, and salmon (s) INS antisera; the immunostaining was abolished when each antiserum was preabsorbed with its respective peptide but not with unrelated peptides. These cells also reacted with mammal (m) SST-28 (4-14) antiserum. The immunoreaction did not change when this antiserum was preabsorbed by bovine INS. INS-immunoreactive cells were located in the central part of the endocrine areas of the principal, intermediate, and small islets. Two SST-immunoreactive cell types (D1 and D2) were revealed. D1 cells, immunoreactive to SST 14 (562) and sSST-25 antisera, were located next to the glucagon-immunoreactive cells in the peripheral part of the endocrine areas. D2 cells, immunoreactive to SST-14 (562), SST-14 (566), and mSST-28 (4-14) antisera, were found in apposition to the INS-immunoreactive cells. The specificity controls showed that D1 cells expressed sSST-25-like peptides, while D2 cells might contain SST-14 and/or mSST-28-like peptides. The close topographic association between the different SST-immunoreactive cells and both glucagon- and insulin-immunoreactive cells might indicate the existence of a specific paracrine regulation of each endocrine cell type in the sea bass endocrine pancreas.

In vitro responses of rainbow trout (Oncorhynchus mykiss) somatotrophs to carp growth hormone-releasing factor (GRF) and somatostatin

To study the hypothalamic control of growth hormone (GH) release in lower vertebrates, we employed an in vitro technique using a monolayer cell culture system of rainbow trout pituitary glands. Two newly purified carp brain growth hormone-releasing factors, carp GRF(1-45) and carp GRF(1-29), and synthetic somatostatin-14 (SST-14) were applied to the cultured pituitary cells. The results indicate that: (1) The carp GRFs had a dose-related potency in stimulating growth hormone release. The dose of half maximum effect (ED50) for carp GRF(1-45) was 0.107 nM, and an equal potency for carp GRF(1-29) was 0.388 nM. (2) SST-14 inhibited GH release having a dose-dependent potency with an ED50 of 0.186 nM. (3) Osmotic pressure did not influence SST-14 inhibited GH secretion but did affect spontaneous GH release. (4) The response of cultured cells was not affected by length of incubation period with SST-14 or carp GRF but was affected by cell density.

Distribution of two forms of somatostatin in the brain, anterior intestine, and pancreas of adult lampreys (Petromyzon marinus)

The distribution of two major immunoreactive forms of somatostatin, somatostatin-14 and somatostatin-34, within the brain, pancreas and intestine of adult lampreys, Petromyzon marinus, was identified using antisera raised against these peptides. Immunostaining of the brain is similar in juveniles and upstream migrants, and somatostatin-14 is the major somatostatin form demonstrated. A few somatostatin-34-containing cells are localized within the olfactory bulbs, thalamus and hypothalamus, but cells immunoreactive to anti-somatostatin-34 in the hypothalamus and thalamus do not co-localize somatostatin-14. Immunostaining of pinealocytes within the pineal pellucida with anti-somatostatin-14 may infer a novel function for this structure. Somatostatin-14 and somatostatin-34 are co-localized within D-cells of the cranial pancreas and caudal pancreas of juveniles and upstream migrants. Numerous somatostatin-34-immunoreactive cells are distributed within the epithelial mucosa of the anterior intestine but not all of these cells cross-react with anti-somatostatin-14. It appears that somatostatin-34 is the major somatostatin in the pancreo-gastrointestinal system of adult lampreys.

Identification of disulfide-containing peptides in endocrine tissue extracts by HPLC-electrochemical detection and mass spectrometry

A new procedure to selectively identify disulfide-containing peptides in extracts of biological tissues is described. Disulfide-containing peptides are detected by their UV absorbance and electrochemical (EC) activity after chromatographic separation, and subsequently identified by fast atom bombardment mass spectrometry (FABMS). This combination of fractionation by HPLC and selective detection is attractive because it is rapid, highly specific for disulfide-containing peptides, and applicable to all disulfide-containing peptides that may be present in complex biological mixtures. Useful procedures for applying the method are demonstrated with tissue extracts from bovine pituitary and catfish pancreas. In addition to finding the expected disulfide-containing peptides, evidence for two forms of catfish insulin are presented. The merits of this and other methods used to detect peptides in similar tissue extracts are discussed.

Identification of two somatostatin-immunoreactive cell types in the principal islet of Sparus auratus L. (Teleostei) by immunogold staining

Two types of somatostatin (SST 14)-immunoreactive cells are identified by immunogold staining in the Lowicryl-embedded principal islet of Sparus auratus: D1 cells, having large moderate to low electron dense granules, located between A cells in the islet periphery and D2 cells, containing smaller electron-dense granules, present between B cells in the central region of the islet. Although SST 28-like immunoreactivity was not observed in D cells of S. auratus, the presence of SST 14 and a SST 22-,25-, or 28-like sequence in D2 and D1 cells, respectively, is discussed. A third SST 14-immunoreactive cell, found in the islet periphery, showed immunoreactive D1- and unreactive A-like granules. This cell type, which has a pyknotic-like nucleus and a dark appearance in osmicated Epon-embedded tissue, is supposed to be the product of fusion of D1 and A cells.

Neuropeptide families: an evolutionary perspective

Changes in the structure and function of five neuropeptide families during evolution are considered. The families of gonadotropin-releasing hormone (GnRH), corticotropin-releasing factor (CRF), growth hormone-releasing hormone (GH-RH), somatostatin (SS), and vasopressin/oxytocin (VP/Oxy) are used as models to illustrate the importance of a phylogenetic approach in understanding neuropeptide structure/activity relationships, precursors, processing, gene duplication, novel locations and functions, and gene-associated peptides.

Physiology of fish endocrine pancreas

From the very beginning of physiological studies on the endocine pancreas, fish have been used as experimental subjects. Fish insulin was one of the first vertebrate insulins isolated and one of the first insulins whose primary and then tertiary structures were reported. Before a second pancreatic hormone, glucagon, was characterized, a physiologically active 'impurity', similar to that in mammalian insulin preparations, was found in fish insulins.Fish have become the most widely used model for studies of biosynthesis and processing of the pancreatic hormones. It seems inconceivable, therefore, that until the recent past cod and tuna insulins have been the only purified piscine islet hormones available for physiological experiments. The situation has changed remarkably during the last decade.In this review the contemporary status of physiological studies on the fish pancreas is outlined with an emphasis on the following topics: 1) contents of pancreatic peptides in plasma and in islet tissue; 2) actions of piscine pancreatic hormones in fish; 3) specific metabolic consequences of an acute insufficiency of pancreatic peptides; 4) functional interrelations among pancreatic peptides which differ from those of mammals. The pitfalls, lacunae and the perspectives of contemporary physiological studies on fish endocrine pancreas are outlined.

Application of fast atom bombardment mass spectrometry to posttranslational modifications of neuropeptides

FABMS is a powerful and sensitive analytical technique capable of providing structural information unattainable by standard methods of peptide analysis. Many posttranslational modifications are undetectable by other routine analytical methods. In addition, FABMS is capable of providing information regarding posttranslational modifications at levels of peptide comparable to those required for other methods of analysis (10-1000 pmol). FABMS has had the effect on protein structure analysis that structure determination of any neuropeptide might now be considered incomplete without some form of mass spectrometric analysis. Much of the recent explosive increase in the use of mass spectrometry for solving problems in peptide structure analysis can be traced to improvements in methods capable of producing molecular ions from nonvolatile species. With the development of these methods, it can be expected that refinements of existing methods and new ionization methods will continue to increase the mass range and sensitivity available for peptide structure determination. For a brief review of other mass spectrometric methods applicable to peptides, see Delgass and Cooks.

The use of plasma desorption time-of-flight mass spectrometry to screen for products of prohormone processing in crude tissue extracts

Californium-252 plasma desorption mass spectrometry (252Cf PDMS) of a crude, desalted, extract of piscine endocrine pancreas provided mass information for the major biologically active peptide hormones present in this tissue. An extraction procedure compatible with 252Cf PDMS analysis was developed. In extracts of catfish pancreas, strong molecular ions were identified in the positive mode for somatostatin-14 (1638 amu), O-glycosylated somatostatin-22 (2944 amu), glucagon (3512 amu), glucagon-like peptide (3785 amu), insulin (ca. 5550 amu), and other prohormone-derived peptides. Both protonated species and sodium adducts were apparent in the mass spectrum. A number of other molecular ions were observed including somatostatin-26, 1-10 (1014 amu) and the entire portion of prosomatostatin-22 remaining after removal of somatostatin-22 (6465 amu). The data obtained by this method also resulted in the identification of the third major product of proglucagon processing in catfish pancreas, glicentin-related polypeptide. Subtractive Edman degradation analyzed by 252Cf PDMS was also used to confirm a mass assignment.

Different cellular distributions of two somatostatins in brain and pancreas of salmonids, and their associations with insulin- and glucagon-secreting cells

Invariant somatostatin-14 (SST-14) and somatostatin-25 (SST-25), isolated from coho salmon pancreas (Plisetskaya et al., 1986a) are likely coded by two distinct somatostatin genes. The present study was undertaken to investigate whether these genes are expressed in the same or in different cell types in the pancreatic islets and in the brain of two salmonids: rainbow trout and coho salmon. Antibodies generated against SST-14, mammalian (m) SST-28(1-14), salmon (s) SST-25, salmon insulin, and salmon glucagon were used as immunocytochemical probes. Two distinct cell types containing SSTs were revealed in the pancreas of both salmonid species: one cell type immunoreactive to both SST-14 and mSST-28(1-14) and the other cell type immunoreactive only to sSST-25. The SST-14/mSST-28(1-14)-positive cells were limited to the more central parts of the islets, in apposition to the insulin-positive cells: sSST-25-positive cells were located more peripherally and were associated topographically with the glucagon-positive cells. In contrast to the pancreas, neurons in the neurohypophysis and hypothalamus of the rainbow trout and coho salmon contained only SST-14-like and mSST-28(1-14)-like immunoreactivities, while immunoreactivity to sSST-25 was completely absent. These results suggest that differentiation in the pancreas and brain of salmonid fishes results in cell types in which SST genes are separately expressed. The close topographical association of sSST-25 with glucagon cells, and of SST-14 with insulin cells, in the pancreatic islets implies yet unknown functional regulatory relationships that require detailed study.

Structural characterization of peptides derived from prosomatostatins I and II isolated from the pancreatic islets of two species of teleostean fish: the daddy sculpin and the flounder

The primary structures of three peptides from extracts from the pancreatic islets of the daddy sculpin (Cottus scorpius) and three analogous peptides from the islets of the flounder (Platichthys flesus), two species of teleostean fish, have been determined by automated Edman degradation. The structures of the flounder peptides were confirmed by fast-atom bombardment mass spectrometry. The peptides show strong homology to residues (49-60), (63-96) and (98-125) of the predicted sequence of preprosomatostatin II from the anglerfish (Lophius americanus). The amino acid sequences of the peptides suggest that, in the sculpin, prosomatostatin II is cleaved at a dibasic amino acid residue processing site (corresponding to Lys61-Arg62 in anglerfish preprosomatostatin II). The resulting fragments are further cleaved at monobasic residue processing sites (corresponding to Arg48 and Arg97 in anglerfish preprosomatostatin II). In the flounder the same dibasic residue processing site is utilised but cleavage at different monobasic sites takes place (corresponding to Arg50 and Arg97 in anglerfish preprosomatostatin II). A peptide identical to mammalian somatostatin-14 was also isolated from the islets of both species and is presumed to represent a cleavage product of prosomatostatin I.

The amino-acid sequences of sculpin islet somatostatin-28 and peptide YY

Two pancreatic peptides, somatostatin-28 and peptide YY, have been isolated from the Brockmann bodies of the teleost fish Cottus scorpius (daddy sculpin). Following purification by reverse-phase HPLC, each peptide was sequenced completely through to the carboxyl-terminus by gas-phase Edman degradation. Somatostatin-28 was the major form of somatostatin detected and is similar to the gene II product from anglerfish. Peptide YY (36 amino acids) more closely resembles porcine neuropeptide YY and intestinal peptide YY than it does the pancreatic polypeptides.

The influence of mammalian and teleost somatostatins on the secretion of growth hormone from goldfish (Carassius auratus L.) pituitary fragments in vitro

The effect of various vertebrate somatostatins (SRIF) on basal growth hormone (GH) secretion from goldfish pituitary fragments was studied using an in vitro perifusion system. SRIF-14 caused a rapid and dose-dependent decrease in the rate of GH release from goldfish pituitary fragments. The half-maximal effective dose (ED50) of SRIF-14 was calculated as 1.3 nM following exposure to two minute pulses of increasing concentrations of SRIF-14, whereas the ED50 of SRIF-14 calculated after continuous exposure to sequentially increasing doses of SRIF-14 was 65 nM. This difference suggests that the pituitary fragments were less responsive to SRIF-14 in the latter experiment, possibly as a result of previous exposure to SRIF-14. SRIF-28 was found to be equipotent with SRIF-14 in decreasing basal GH secretion from the goldfish pituitary. In contrast, catfish SRIF-22, a uniquely teleost SRIF isolated from catfish pancreatic islets, did not alter GH secretion. These results provide further support for the hypothesis that SRIF-14 or a very similar molecule functions as a GH release-inhibiting factor in teleosts, indicating that this action of SRIF-14 has been fully conserved throughout vertebrate evolution.

Characterization of coho salmon (Oncorhynchus kisutch) islet somatostatins

Three different somatostatins have been isolated from the pancreatic islet tissue of the coho salmon (Oncorhynchus kisutch) by gel filtration and HPLC. Two of these peptides contain 14 amino acids and the larger third peptide consists of 25 amino acids. The sequence of the salmon SST-25 is Ser-Val-Asp-Asn-Leu-Pro-Pro-Arg-Glu-Arg-Lys-Ala-Gly -Cys-Lys-Asn-Phe-Tyr-Trp-Lys-Gly-Phe-Thr-Ser-Cys. The sequence of the salmon SST-14-I is Ala-Gly-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys. The other small somatostatin (SST-14-II) which was not sequenced has an amino acid composition identical to the C-terminal 14 amino acids of the SST-25 and it is probably derived from this larger form. Evidence for low levels of a somatostatin containing 28 amino acids is also presented. This SST-28 appears to be an N-terminal extended precursor of SST-25 or a peptide derived via alternative processing of a common preprosomatostatin. Injected into juvenile salmon, SST-25 caused a decline in circulating levels of plasma insulin, depletion of liver glycogen, and activation of lipolytic pathways. Juvenile salmon treated with anti-SST-25 serum revealed elevated levels of plasma insulin as well as an increase of the glycogen content of the liver.

Biosynthesis and processing of the somatostatin family of peptide hormones

Understanding of the biosynthesis of the somatostatin family of peptide hormones has greatly increased in recent years. Isolation and sequencing of the rat somatostatin gene indicates that it contains a single intron located between the codons for Gn(-57) and Glu(-56) of pre-prosomatostatin. The gene contains three repetitive sequences, one at the 5' end of the gene and two of them 3' to the coding portion. Two of the sequences consist of alternating purine-pyrimidine bases and have been shown to adopt Z-DNA structures in vitro. The cDNA for rat somatostatin codes for a 116-residue peptide structurally similar to the anglerfish and catfish precursors to the 14-residue somatostatin (SST-14). In addition to SST-14, the catfish and the anglerfish both contain an additional pancreatic somatostatin, each derived from a different gene. The catfish contains a 22-residue somatostatin, which is O-glycosylated at Thr-5. The second somatostatin gene from anglerfish encodes a prosomatostatin that is processed to a 28-residue peptide. The mature peptide contains a hydroxylated lysine at position 23.

Somatostatin: historical aspects

Somatostatin, in essence an almost universal chalone, initially described as a 14 amino-acid-long peptide that inhibits growth hormone (GH) release, has been shown to be one of a family of related peptides, ubiquitous in distribution and versatile as a paracrine factor with a potentially important role in the regulation of gut, pancreatic, and nervous system function, in addition to its well-recognized influence on the pituitary secretion of GH and thyroid-stimulating hormone. With the development of new super agonists, it has become possible to manipulate the endocrine milieu, to modify gut, pancreatic, and pituitary function, and, in the case of several diseases such as acromegaly and intractable diarrhoea, to make a significant advance in therapy.

Interaction of synthetic N- and C-terminal fragments of helodermin with rat liver VIP receptors

Synthetic helodermin was more potent than natural helodermin (purified from the venom of Gila monster) in activating adenylate cyclase in rat liver membranes. Two possible reasons for this discrepancy were discussed. Comparing adenylate cyclase activation to the binding of 125I-natural helodermin and 125I-VIP in the presence of six synthetic helodermin fragments (1-27, 7-35, 13-35, 17-35, 18-35 and 22-35), we conclude that the effects of both synthetic and natural helodermin were mediated through an interaction with VIP receptors. The C-terminal (28-35) extension of these peptides favored VIP receptor recognition. Their N-terminal extremity was not necessary for binding and for ensuing adenylate cyclase activation, illustrating again the atypical coupling of VIP receptors with the effector enzymatic system, in rat liver membranes.

An elasmobranchian somatostatin: primary structure and tissue distribution in Torpedo marmorata

Extracts of brain, stomach, pancreas, and intestine from Torpedo marmorata, an elasmobranchian cartilaginous fish, contained somatostatin-like immunoreactivity. Gel filtration studies demonstrated that material with the elution volume of somatostatin-14 was the only component detected in all tissue extracts. This result contrasts with the situation in mammals where prosomatostatin is processed to multiple molecular forms in a tissue-specific manner. Somatostatin from pancreas and gut was purified to homogeneity and amino acid sequence analysis indicated that T. marmorata somatostatin from both tissues has the same structure as somatostatin-14 isolated from the higher vertebrates. Further examination of other lower vertebrate species is required in order to test the hypothesis that the ability to regulate the production of multiple forms of a regulatory peptide from a single precursor molecule developed only relatively late in evolution.

Somatostatins of the channel catfish

Pancreatic tissue of the channel catfish (Ictalurus punctatus) contains two somatostatins. The amino acid sequence of a 14-residue containing peptide (SS-14) is identical in sequence and has the same mass ion by fast atom bombardment mass spectrometry as mammalian SS-14. A 22-residue somatostatin (SS-22) has also been isolated and its amino acid sequence determined. The amino acid sequence differs from that reported by Oyama et al. (J Biol Chem 255:2251, 1980) in two of the 22-residues (positions 5 and 19). Somatostatin-22 is a glycoprotein, with carbohydrate attached at Thr-5. Several 22-residue peptides have been purified and all of them have identical amino acid compositions and different carbohydrate structure and/or composition. SS-22 has 7 amino acid residues which are identical to those observed in SS-14. The nucleotide sequence of the two cDNAs coding for SS-14 and SS-22 have been determined. The mRNAs encode precursors to SS-14 and SS-22 of 114 and 105 residues, respectively. The two precursors also have different amino acid sequence at the site of prohormone proteolytic processing. Analysis of genomic DNA reveals that SS-14 and SS-22 sequences are present on different restriction fragments and thus are encoded by separate genes.