Isolation and structure of the principal products of preproglucagon processing, including an amidated glucagon-like peptide

Evolution of the glucagon-like system across fish

In fishes, including the jawless lampreys, the most ancient lineage of extant vertebrates, plasma glucose levels are highly variable and regulation is more relaxed than in mammals. The regulation of glucose and lipid in fishes in common with mammals involves members of the glucagon (GCG)-like family of gastrointestinal peptides. In mammals, four peptides GCG, glucagon-like peptide 1 and 2 (GLP1 and GLP2) and glucose-dependent insulinotropic peptide (GIP) that activate four specific receptors exist. However, in lamprey and other fishes the glucagon-like family evolved differently and they retained additional gene family members (glucagon-related peptide, gcrp and its receptor, gcrpr) that are absent from mammals. In the present study, we analysed the evolution of the glucagon-like system in fish and characterized gene expression of the family members in the European sea bass (Dicentrarchus labrax) a teleost fish. Phylogenetic analysis revealed that multiple receptors and peptides of the glucagon-like family emerged early during the vertebrate radiation and evolved via lineage specific events. Synteny analysis suggested that family member gene loss is likely to be the result of a single gene deletion event. Lamprey was the only fish where a putative glp1r persisted and the presence of the receptor gene in the genomes of the elephant shark and coelacanth remains unresolved. In the coelacanth and elephant shark, unique proglucagon genes were acquired which in the former only encoded Gcg and Glp2 and in the latter, shared a similar structure to the teleost proglucagon gene but possessed an extra exon coding for Glp-like peptide that was most similar to Glp2. The variable tissue distribution of the gene transcripts encoding the ligands and receptors of the glucagon-like system in an advanced teleost, the European sea bass, suggested that, as occurs in mammals, they have acquired distinct functions. Statistically significant (p < .05) down-regulation of teleost proglucagon a in sea bass with modified plasma glucose levels confirmed the link between these peptides and metabolism. The tissue distribution of members of the glucagon-like system in sea bass and human suggests that evolution of the brain-gut-peptide regulatory loop diverged between teleosts and mammals despite the overall conservation and similarity of glucagon-like family members.

Major contributions of comparative endocrinology to the development and exploitation of the incretin concept

An incretin is a factor released by the gut in response to nutrients that facilitates uptake of glucose by peripheral tissues. The incretin concept predates the discovery of insulin but it is now clear that incretins act by stimulating secretion of this hormone. As glucagon has insulin-releasing activity, it was speculated that intestinal glucagon-like immunoreactivity (enteroglucagon) was involved in the incretin effect but it was an achievement in the field of comparative endocrinology that led to the demonstration that the preproglucagon gene encodes the most potent incretin in the human. Characterization of cloned cDNAs encoding two preproglucagons from the Brockmann body of the anglerfish Lophius americanus demonstrated that the glucagon sequence is flanked by a 34 amino-acid-residue sequence with appreciable structural similarity to glucagon that was termed glucagon-like peptide (GLP). A 36 amino-acid-residue ortholog of anglerfish GLP was subsequently identified in human preproglucagon but this peptide had only weak insulin-releasing activity. However, alignment of GLP sequences from human and teleost fish showed that the human ortholog is extended from its N-terminus by a hexapeptide. Removal of this extension by an endogenous protease generates GLP-1-(7-36)amide, the potent and effective form of the incretin. More recently, comparative endocrinology has contributed to the exploitation of incretins as antidiabetic drugs. Exendin-4, a GLP-1 receptor agonist first isolated from the venom of the Gila monster Heloderma suspectum, is a clinically valuable, long-acting incretin and the skins of several species of frogs synthesize potent insulin-releasing peptides with therapeutic potential.

Glucagon-like peptide 1 (GLP-1) and metabolic diseases

Glucagon-like peptide-1 (GLP-1) is a gastrointestinal hormone, mainly secreted after meals, which enhances glucose-induced insulin secretion and induces satiety. It has been reported that GLP-1 levels after a mixed meal and after an oral glucose load are reduced in patients with Type 2 diabetes. The reduction of oral glucose-stimulated active GLP-1 levels in patients with Type 2 diabetes has also been observed during euglycemic iperinsulinemic clamp. The reduction of post-prandial circulating active GLP-1 in Type 2 diabetic subjects, as a consequence of chronic hyperglycemia, could contribute to the reduction of early post-prandial insulin secretion; in fact, the administration of GLP-1 receptor antagonists to healthy volunteers elicits both an impairment of meal-induced insulin secretion and an increase of post-prandial glycemia similar to that observed in Type 2 diabetes. GLP-1 is rapidly inactivated by dipeptidyl peptidase IV (DPP-IV), an enzyme produced by endothelial cells in different districts and that circulates in plasma. It is still not clear whether the reduction of mealor oral-glucose stimulated GLP-1 levels in Type 2 diabetic patients is due to impairment of secretion, increase of degradation, or both. The major limitation of using GLP-1 to treat diabetic patients is the short half-life of the native compound. There are now several compounds in various stages of pre-clinical or clinical development for the treatment of Type 2 diabetes that utilize the GLP-1 signaling pathway; these include GLP-1 receptor agonists with extended half-lives, and inhibitors of DPP-IV that increase circulating levels of endogenous, intact and bioactive GLP-1.

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.

Glucagon and glucagon-like peptides in fishes

Glucagon and glucagon-like peptides (GLPs) are coencoded in the vertebrate proglucagon gene. Large differences exist between fishes and other vertebrates in gene structure, peptide expression, peptide chemistry, and function of the hormones produced. Here we review selected aspects of glucagon and glucagon-like peptides in vertebrates with special focus on the contributions made by analysis of piscine systems. Our topics range from the history of discovery to gene structure and expression, through primary structures and regulation of plasma concentrations to physiological effects and message transduction. In fishes, the pancreas synthesizes glucagon and GLP-1, while the intestine may contribute oxyntomodulin, glucagon, GLP-1, and GLP-2. The pancreatic gene is short and lacks the sequence for GLP-2. GLP-1, which is produced exclusively in its biologically active form, is a potent metabolic hormone involved in regulation of liver glycogenolysis and gluconeogenesis. The responsiveness of isolated hepatocytes to glucagon is limited to high concentrations, while physiological concentrations of GLP-1 effectively regulate hepatic metabolism. Plasma concentrations of GLP-1 are higher than those of glucagon, and liver is identified as the major site of removal of both hormones from fish plasma. Ultimately, GLP-1 and glucagon exert effects on glucose metabolism that directly and indirectly oppose several key actions of insulin. Both glucagon and GLP-1 show very weak insulinotropic activity, if any, when tested on fish pancreas. Intracellular message transduction for glucagon, especially at slightly supraphysiological concentrations, involves cAMP and protein kinase A, while pathways for GLP are largely unknown and may involve a multitude of messengers, including cAMP. In spite of fundamental differences in GLP-1 function between fishes and mammals, fish GLP-1 is as powerful an insulinotropin for mammalian B-cells as mammalian GLP-1 is a metabolic hormone if tested on piscine liver.

Characterization of the pancreatic hormones from the Brockmann body of the tilapia: implications for islet xenograft studies

The Brockmann body of the teleost fish, the tilapia (Oreochromis nilotica) has been considered as a potential source of islet xenograft tissue for patients with insulin-dependent diabetes. This study describes the purification from an extract of tilapia Brockmann bodies of insulin and several peptides arising from different pathways of post-translational processing of two proglucagons, two prosomatostatins and proPYY. The primary structure of tilapia insulin is similar to insulins from other teleosts (particularly the anglerfish, Lophius americanus) except that the strongly conserved glutamine residue at position 5 in the A-chain, a residue that is important in the binding of insulin to its receptor, is replaced by glutamic acid. In common with other teleosts, the tilapia Brockmann body expresses two non-allelic glucagon genes. Alternative pathways of post-translational processing lead to glucagons with 29 and 36 amino acid residues derived from proglucagon I and glucagons with 29 and 32 residues derived from proglucagon II. Glucagon-like peptides with 30 and 34 residues derived from proglucagon II were also isolated. In each case, the longer peptide is a C-terminally extended form of the shorter. Tilapia peptide tyrosine-tyrosine (PYY) was isolated in a C-terminally alpha-amidated from with 36 amino acid residues that is structurally similar (89% sequence identity) to anglerfish PYY. A 30-amino acid peptide, representing the C-terminal flanking peptide of PYY, was also isolated that shows only 53% sequence identity with the corresponding anglerfish peptide. Tilapia somatostatin-14 is identical to mammalian somatostatin but the [Tyr7, Gly10] somatostatin-containing peptide derived from prosomatostatin II contains the additional substitution (Phe11-->Leu) compared with the corresponding peptide from other teleosts.

Distribution of peptidyl-glycine alpha-amidating monooxygenase immunoreactivity in the brain, pituitary and islet organ of the anglerfish (Lophius americanus)

Peptidyl-glycine alpha-amidating monooxygenase (PAM; EC 1.14.17.3) is an enzyme that catalyzes conversion of glycine-extended peptides to alpha-amidated bioactive peptides. Two peptides that are processed at their carboxyl-termini by this enzyme are neuropeptide Y and anglerfish peptide Y, both of which possess a C-terminal glycine that is used as a substrate for amidation. Results from previous reports have demonstrated that neuropeptide Y-like and anglerfish peptide Y-like immunoreactivities are present in the brain of anglerfish (Lophius americanus). Furthermore, neuropeptide Y-like peptides, namely anglerfish peptide Y and anglerfish peptide YG (the homologues of pancreatic polypeptide) are present in the islet organ of this species. Neuropeptide Y has also been localized in the anterior, intermediate and posterior lobes of the pituitary gland in a variety of species. In order to learn more about the distribution of the enzyme responsible for alpha amidation of these peptides in the brain and pituitary and to specifically investigate the relationship of this enzyme to peptide synthesizing endocrine cells of the anglerfish islet, we performed an immunohistochemical study using several antisera generated against different peptide sequences of the enzyme. PAM antisera labeled cells in the islet organ, pituitary and brain, and fibers in the brain and pituitary gland. The PAM staining pattern in the brain was remarkably similar to the distribution of neuropeptide Y immunoreactivity reported previously. Clusters of cells adjacent to vessels in the anterior pituitary displayed punctate PAM immunoreactivity while varicose fibers were observed in the pituitary stalk and neurohypophysis. Endocrine cells of the islet organ were differentially labeled with different PAM antisera. Comparison of the staining patterns of insulin, glucagon, and anglerfish peptide Y in the islet organ to PAM immunoreactivity suggests a distribution of forms of PAM enzyme in insulin and anglerfish peptide Y-containing cells, but no overlap with glucagon-producing cells. The results also indicate that PAM immunoreactivity is widely distributed in the brain, pituitary and islet organ of anglerfish in cells, that contain peptides that require presence of a C-terminal glycine for amidation.

Primary structures of peptides derived from proglucagon isolated from the pancreas of the elasmobranch fish, Scyliorhinus canicula

Three peptides derived from the posttranslational processing of proglucagon have been isolated from the pancreas of the elasmobranch fish, Scyliorhinus canicula (European common dogfish). The peptide HSEGT FTSDY SKYMD NRRAK DFVQW LMST represents the 29 amino acid residue form of glucagon previously identified in dogfish intestine. A second component with 33 amino acid residues represents glucagon extended from its COOH-terminal region by -KRNG. The peptide HAEGT YTSDV DSLSD YFKAK RFVDS LKSY represents glucagon-like peptide (GLP). The primary structures of the GLPs from mammals have strongly conserved but a comparison of the amino acid sequences of known GLPs from different classes of fish shows that the structure of the peptide has been very poorly conserved in lower vertebrates. Only three residues (Ala2, Asp9, and Leu26) are found in the same position in all fish GLPs. A similar comparison of the primary structures of glucagons from the same species shows 13 amino acid residues in common.

Structural requirements for biological activity of glucagon-like peptide-I

Glucagon-like peptide-I (GLP-I) is encoded together with glucagon by the glucagon gene and is related in its structure to the glucagon-secretin family of peptides. Three of the predicted forms of the peptide, a 37-residue long GLP-I(1-37), a 31-residue GLP-I(7-37) and a 30-residue GLP-I(7-36)amide as well as three analogs des [Gly37, Arg36] GLP-I(7-37), des [Gly37, Arg36, Gly35] GLP-I(7-37) and des [His7] GLP-I(7-37) were synthesized by the stepwise solid phase method. These synthetic peptides were used to define the structural domains required for the binding of GLP-I to the pancreatic beta cell. The competitive binding experiments showed that both the amino and carboxyl terminal domains of the molecule contribute to GLP-I binding. In these experiments glucagon, another peptide that stimulates insulin secretion, was a weak full agonist of GLP-I binding. Results from these studies provide further characterization of the physiological role of this new peptide.

Isolation and primary structure of glucagon from the endocrine pancreas of Thunnus obesus

Glucagon has been isolated from the endocrine pancreas of a tunid, Thunnus obesus. The primary structure of the glucagon molecule was established as H S E G T F S N D Y S K Y L E T R R A Q D F V Q W L K N S. The sequence is identical to those of sculpin and flounder glucagon and glucagon II from anglerfish. It also shows high homology to the mammalian hormone (76%). The mass determined by fast-atom bombardment (3508) was consistent with the proposed structure. Immunological properties of the tuna glucagon were analyzed by radioimmunoassay, showing a high degree of cross-reactivity with the 30K antibody.

The primary structure of glucagon-like peptide but not insulin has been conserved between the American eel, Anguilla rostrata and the European eel, Anguilla anguilla

Insulin was isolated from the pancreas of the American eel, Anguilla rostrata, and its primary structure was established as (Formula: see text). Eel insulin contains unusual substitutions at B-21, B-22, and B-26 in the putative receptor-binding region of the molecule compared with other mammalian and fish insulins. The A-chain of insulin from the European eel contains an asparagine rather than a serine residue at position A-12. Similarly, amino acid composition data indicate the B-chain of insulin from the European eel is appreciably different from that from the American eel. The primary structure of glucagon-like peptide (GLP) from the American eel is identical to that from the European eel, Anguilla anguilla. The primary structure of the peptide was established as (Formula: see text). Fast-atom bombardment mass spectrometry demonstrated that the COOH-terminal arginyl residue is alpha-amidated. The strong evolutionary pressure to conserve the structure of GLP provides further support for the assertion that the peptide plays an important regulatory role in teleost fish.

Recent developments in the chemistry of gastrointestinal peptides

Work carried out in different laboratories has shown that the peptide pattern of the intestinal tissue is very complex and that some of the peptides are identical to those found in the central nervous system. The best studied of the peptides are of a hormonal nature, but recently evidence has been obtained that others may primarily act as antibiotics. In addition, peptides have been isolated that are fragments of some well-known proteins that have not been viewed as being prohormones. Whether the latter peptides only represent transient degradation products of the proteins or whether, at least some of them, have a physiologically meaningful selective function of their own is not yet clear.

Glycine-extended anglerfish peptide YG (aPY) a neuropeptide Y (NPY) homologue may be a precursor of a biologically active peptide

The 37 residue peptide YG (aPY), isolated from anglerfish endocrine pancreas, bears distinct sequence homology to the pancreatic polypeptide family of hormones. However, instead of a carboxyl-terminal tyrosine-amide, aPY has a free carboxyl-terminus ending with glycine. Towards studying the structure-activity relationship of this hormone, we have synthesized aPY by solid phase methodology using Boc-amino acid derivatives and phenylacetamidomethyl resin. The crude peptide was purified to homogeneity in 20% yield by reversed phase chromatography. The purified peptide had the expected amino acid composition and sequence, and was found to be identical with the natural aPY by analytical HPLC and peptide mapping of proteolytic digests. Neither the snythetic nor the natural aPY exhibited the characteristic vasoconstrictor activity of the related pancreatic polypeptide family of hormones. However, [Des37-Gly]-aPY, isolated from the anglerfish pancreas, caused vasoconstriction in rats. Based on these results and by analogy to the glycine-extended gastrin peptides, it may be suggested that aPY is a precursor of a biologically active peptide, namely [Des37-Gly]-aPY-amide.

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.

Multiple molecular forms of insulin and glucagon-like peptide from the Pacific ratfish (Hydrolagus colliei)

The primary structure of an insulin isolated from the pancreas of the holocephalan fish, Hydrolagus colliei (Pacific ratfish), has been established as A-chain: GIVEQCCHNTCSLANLEGYCN B-chain: VPTQRLCGSHLVDALYFVCGERGFFYSPKPIRELEPLL. Three further molecular forms of insulin were also isolated and shown to have the same A-chain but truncated B-chains of 31-, 36-, and 37-amino acid residues. It is proposed that all four insulins arise from a single proinsulin by proteolytic cleavages at different sites within the C-peptide region. The insulin with 38 amino acids in the B-chain was equipotent with human insulin in inhibiting the binding of radiolabelled human insulin to rat fat cells but the maximum effect of ratfish insulin upon the transport of 3-O-methylglucose into the cells was only 65% of the maximum effect of human insulin. Two molecular forms of glucagon-like peptide were isolated from the ratfish pancreas. The primary structure of the more abundant peptide was established as HADGIYTSDVASLTDYLKSKRFVESLSNYNRKQND. The primary structure of the second peptide was the same except that it was extended from the C-terminus by the sequence RRM. It is probable, therefore, that both glucagon-like peptides also arise from a single proglucagon by different pathways of post-translational processing.

Isolation and characterisation of GLP-1 7-36 amide from rat intestine. Elevated levels in diabetic rats

Glucagon-like peptide-1 (GLP-1) was purified to homogeneity by HPLC and anion-exchange chromatography. A molecular mass of 3297.4 Da was obtained by FAB mass spectrometry which corresponded exactly to GLP-1 7-36 NH2, providing evidence that amidation occurs at an arginine residue during the post-translational processing of GLP-1. The distribution of GLP-1 7-36 NH2-like immunoreactivity (GLP-1 7-36 NH2 IR) was determined in the rat gastrointestinal tract. Highest concentrations were found in terminal ileum and colon. Streptozocin-induced diabetic rats, who showed a significant increase in food intake, had a significant increase of GLP-1 7-36 NH2 IR in the colon.

Somatostatin-related and glucagon-related peptides with unusual structural features from the European eel (Anguilla anguilla)

Peptides derived from prosomatostatins I and II and from two distinct proglucagons have been isolated from the pancreas of a teleost fish, the European eel (Anguilla anguilla). The product of prosomatostatin I processing, somatostatin-14, is identical to mammalian somatostatin-14. A 25-amino-acid-residue peptide (Ser-Val-Asp-Asn-Gln5-Gln-Gly-Arg-Glu-Arg10-Lys-Ala-Gly-Cys- Lys15-Asn-Phe-Tyr- Trp-Lys20-Gly-Pro-Thr-Ser-Cys25) is derived from prosomatostatin II. Compared with the corresponding peptides from other teleost fish, the eel somatostatin-25 contains the unusual substitution Pro for Phe at position 22. This peptide was also isolated in a form containing a hydroxylsyl residue at position 20. A 29-amino-acid-residue eel glucagon contains four substitutions relative to human glucagon Asn for Ser8, Glu for Asp15, Thr for Ser16, and Ser for Thr29). In common with mammalian and avian glucagons but unlike most other fish glucagons, the eel peptide possesses a glutamine residue at position 3. A peptide derived from a second proglucagon comprises 36 amino acid residues. A 7-residue C-terminal extension to the glucagon sequence shows structural similarity to the corresponding extension in ratfish (Hydrolagus colliei) glucagon and mammalian oxyntomodulin.

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.

Isolation of alligator gar (Lepisosteus spatula) glucagon, oxyntomodulin, and glucagon-like peptide: amino acid sequences of oxyntomodulin and glucagon-like peptide

Oxyntomodulin, glucagon, and a glucagon-like peptide (GLP) have been isolated from the endocrine pancreas of the alligator gar (Lepisosteus spatula), a ganoid fish. The three peptides were isolated by gel filtration and HPLC and were identified by size, composition, and glucagon-like immunoreactivity. The amino acid sequences of the oxyntomodulin and GLP were determined. The oxyntomodulin contains 36 amino acid residues and its sequence is H S Q G T F T N D Y S K Y L D T R R A Q D F V Q W L M S T K R S G G I T. The composition of the glucagon is identical to the N-terminal 29 residues of the gar oxyntomodulin. The single form of GLP found contains 34 amino acid residues in the following sequence: H A D G T Y T S D V S S Y L Q D Q A A K K F V T W L K Q G Q D R R E. These findings suggest that all three peptides are derived from a common precursor.

Peptidyl-glycine alpha-amidating monooxygenase is present in islet secretory granules of the anglerfish, Lophius americanus

Anglerfish islet secretory granules have been examined for the presence of an enzyme which could perform C-terminal amidation of glucagon-like peptide II and possibly anglerfish peptide Y. Using [125I]D-Tyr-Val-Gly as substrate, a peptidyl-glycine alpha-amidating monooxygenase (PAM) was detected in islet secretory granule lysates. The enzyme is active between pH 6.0 and 8.5 and exhibits maximal activity near pH 7.0. The islet PAM requires Cu2+, ascorbate, and molecular oxygen for activity. Other divalent metal ions and redox cofactors were tested and found to be inactive in the assay. Even though added Cu2+ and ascorbate are required for detecting islet PAM activity, when these factors were incubated with substrate in the absence of secretory granule lysate, no activity was observed. It was also found that the addition of higher than optimal concentrations of either Cu2+ or ascorbate inhibited amidating activity. The results demonstrate that a PAM is present in secretory granules of anglerfish islet tissue. The characteristics of the islet PAM are similar to those of PAMs identified and characterized in other tissues which produce bioactive C-terminally amidated peptides.

Primary structure of glucagon from the gut of the common dogfish (Scyliorhinus canicula)

The primary structure of glucagon isolated from the intestine of the common dogfish, Scyliorhinus canicula, was established as H S E G T F T S D Y S K Y M D N R R A K D F V Q W L M N T. The peptide shows four substitutions compared with human glucagon: Glu-3 for Gln, Met-14 for Leu, Asn-16 for Ser and Lys-20 for Gln. Glucagon represented the predominant molecular form of the glucagon-like immunoreactivity in the dogfish gut extracts demonstrating that the pathway of posttranslational processing of proglucagon in the gut of this fish differs markedly from the pathway in the mammalian gut.

Primary structures of three fragments of proglucagon from the pancreatic islets of the daddy Sculpin (Cottus scorpius)

Three peptides isolated from the Brockmann bodies of the daddy sculpin, a teleostean fish, have been identified as fragments of one or more proglucagons. The peptide L Q D A E D S S R F D A D D T L A G E A R E L S T P K represents the NH2 terminus of proglucagon (residues 1-27), H S E G T F S N D Y S K Y L E T R R A Q D F V Q W L K N S represents glucagon and H A D G T F T S D V S S Y L N D Q A I K D F V A K L K S G K V represents the glucagon-like peptide at the COOH terminus of the precursor. The fast-atom bombardment mass spectra of the three peptides were consistent with the proposed structures and demonstrated that further posttranslational modifications of the peptides had not taken place. Sculpin glucagon is identical to anglerfish glucagon II but sculpin proglucagon(1-27) and glucagon-like peptide show stronger homology to the corresponding regions of anglerfish proglucagon I than to proglucagon II. The structures of the peptides are suggestive of the action of trypsin-like and carboxypeptidase-B-like enzymes at the site of pairs of basic amino acid residues in proglucagon. The presence of a COOH-terminal lysyl group in proglucagon(1-27) may indicate, however, that the penultimate prolyl residue partially inhibits the action of the carboxypeptidase-B-like activity.

Biological activities of catfish glucagon and glucagon-like peptide

The ability of catfish glucagon and glucagon-like peptide to bind and activate mammalian glucagon receptors was investigated. Neither catfish peptide binds to glucagon receptors of rat liver, hypothalamus or pituitary. Neither stimulates adenylate cyclase activity in liver membranes. Catfish glucagon fails to activate adenylate cyclase in hypothalamic or pituitary membranes in contrast to mammalian glucagon. However, catfish glucagon-like peptide does stimulate hypothalamic and pituitary adenylate cyclase (EC50 approximately 1 pM) possibly through mammalian glucagon-like peptide receptors.

Specific glucagon-related peptides isolated from anglerfish islets are metabolic cleavage products of (pre)proglucagon-II

Sequence analyses of cDNAs prepared from anglerfish islet mRNA have demonstrated the presence of mRNAs coding for two different preproglucagons, aPPG-I and aPPG-II. Each of these precursors was predicted to contain 29 residue and 34 residue glucagon-related peptides as potential cleavage products. Recently, several glucagon-related peptides found in extracts of anglerfish islets have been isolated and characterized. In order to determine whether any of these peptides could be identified as metabolic cleavage products in anglerfish islets, differentially radiolabeled Mr 2,500-8,000 peptides from islet extracts were subjected to reverse phase HPLC under varying conditions. The potential cleavage products aPPG-II[52-80] and aPPG-II[89-122] could be readily identified among the extract peptides. Both peptides became labeled appropriately (as predicted from their sequences) with 13 different amino acids and demonstrated glucagon-like immunoreactivity in a radioimmunoassay. Conversely, a third peptide (aPPG-II[89-119]) could be found among the labeled products in small amounts only. These results demonstrate that glucagon-II[52-80] and aGLP-II[89-112] are primary cleavage products of aPPG-II and suggest that aGLP-IIc[89-119] may be a peptide generated more slowly by post-translational modification of aGLP-II.