Glucagon carboxyl-terminal derivatives: preparation, purification, and characterization

Stabilization of soluble active rat liver glucagon receptor

Active glucagon receptor was solubilized with 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate (Chaps) from rat liver plasma membranes but rapidly (less than 8 h) lost activity. Either inclusion of 1X Hanks' balanced salt solution in the 3 mM Chaps solubilization buffer or its addition after solubilization increased the percentage of total binding attributable to specific glucagon binding from approximately 10 to greater than 80%; of great importance, it increased the stability from near zero binding at 8 h to 50% binding at 48 h (4 degrees C). Of the Hanks' solution components, either NaCl (137 mM) or CaCl2 (1.26 mM) was effective in increasing specific binding to approximately 70 and 60% respectively: Mg salts were ineffective. Soluble receptor binding activity was assayed by dextran-coated charcoal adsorption of free hormone. The assay is rapid, simple, and reproducible. It is suitable for monitoring receptor activity during purification and molecular characterization. Competition binding studies gave an IC50 value of 10-20 nM (slope factor approximately 1), with or without GTP. Dissociation assays revealed GTP sensitivity when receptors were solubilized either as glucagon-receptor complexes or free receptor. Active glucagon-receptor complexes could be eluted from wheat germ lectin-agarose: neither concanavalin A-agarose nor soybean agglutinin-agarose bind receptor. A glucagon degrading activity which co-solubilized with the receptor but did not require detergent for extraction was distinguishable from the soluble receptor not only by solubility but also by its heat stability (30 degrees C), its inhibition by bacitracin, its affinity for glucagon, its retention of activity for at least 1 week at 4 degrees C, and its size.

Quantitative analysis of internalization of glucagon by isolated hepatocytes

Biochemical methods have been used to quantitate total, acid-stable and acid-labile association of (mono[125I]iodoTyr10) glucagon with rat hepatocytes in suspension to evaluate internalization of glucagon and its receptors. Internalization is inhibited by low temperature, phenylarsine oxide, and by blocking receptor binding, consistent with receptor-mediated endocytosis. Approximately 30% of the total cell-associated hormone is internalized at 30 min of incubation. The rate declines until 90 min when the internalization of glucagon ceases, although the cells remain competent to internalize asialofetuin. From 90 min to 4 h, 27% of the maximum label internalized at 30 min remains within cells. The number of cell surface receptors decreases but the affinity of those remaining is unchanged. However, 1.7-2.7 surface receptors are lost to binding for each molecule of radiolabeled glucagon internalized. Uptake occurs according to a rate constant of 0.183 min-1 (t1/2 = 3.8 min). We conclude that (i) hepatocytes internalize a finite quantity of glucagon, implying the existence of undefined regulatory mechanisms; (ii) hormone is retained for greater than 2 h within cells and may play a physiological role within cells; and (iii) both occupied and unoccupied receptors become inaccessible to extracellular hormone as internalization proceeds; rapid recycling of receptors does not occur.

Guanine nucleotide regulation of the interconversion of the two-state hepatic glucagon receptor system of rat

To investigate whether guanine nucleotides regulate interconversion of the two-state hepatic glucagon receptor we have utilized kinetic assays of glucagon binding to partially purified rat liver plasma membranes. Dissociation of glucagon at 30 degrees C exhibited biexponential character in either the absence or presence of GTP, indicating that the system previously seen in intact hepatocytes is independent of intracellular modulators. In each case the receptors underwent a time-dependent conversion from a low affinity to a high affinity state. However, GTP decreased the fraction of receptors in the high affinity state. The rank order for stabilizing the low affinity state was Gpp(NH)p greater than GTP greater than GDP much greater than GMP = no nucleotides. Data from competition binding assays with increasing concentrations of GTP allow calculation of equilibrium constants which are 3.32 nM for glucagon and receptor in the absence of GTP, 18.6 nM for glucagon and receptor in the presence of GTP, 1.55 microM for the association of receptor and GTP presumably linked to an N protein, and 8.86 microM for the association of the glucagon-receptor complex and GTP again presumably linked to an N protein, Glucagon binding to receptor is noncooperative in both the absence and presence of GTP, distinguishing this system from the beta-adrenergic system. With GTP, binding to the low affinity state is favored because of the relative affinities reported. Therefore, GTP regulates the activation by slowing the conversion of the receptor from a low affinity to high affinity form.

Semisynthetic D-His1,N epsilon-acetimidoglucagon: structure-function relationships

The histidine residue at the amino terminus of lysine-12 protected glucagon was replaced by its D-isomer by an established semisynthetic strategy to extend a stepwise series of replacements at this position. The product was examined for its secondary structure and its function. Circular dichroism spectra obtained at concentrations from 0.25 to 1.09 mg/ml at pH 10.2 in 0.2 M phosphate buffer were similar to those obtained with native hormone. Competitive binding assays and adenylate cyclase activation assays with partially purified rat liver plasma membranes show this D-His1 analog of glucagon to be a full agonist, causing the same maximum activation of adenylate cyclase as native hormone; but both binding and activation assays show the binding affinity to be diminished about 10-fold. The data suggest that the adjustment of the bonding of the imidazole group to the receptor to bring about transduction results in constraints on the conformation along the peptide sequence which interfere with the peptide adopting the same binding conformation achieved by the native hormone.

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.

Receptor binding and adenylate cyclase activities of glucagon analogues modified in the N-terminal region

In this study, we determined the ability of four N-terminally modified derivatives of glucagon, [3-Me-His1,Arg12]-, [Phe1,Arg12]-, [D-Ala4,Arg12]-, and [D-Phe4]glucagon, to compete with 125I-glucagon for binding sites specific for glucagon in hepatic plasma membranes and to activate the hepatic adenylate cyclase system, the second step involved in producing many of the physiological effects of glucagon. Relative to the native hormone, [3-Me-His1,Arg12]glucagon binds approximately twofold greater to hepatic plasma membranes but is fivefold less potent in the adenylate cyclase assay. [Phe1,Arg12]glucagon binds threefold weaker and is also approximately fivefold less potent in adenylate cyclase activity. In addition, both analogues are partial agonists with respect to adenylate cyclase. These results support the critical role of the N-terminal histidine residue in eliciting maximal transduction of the hormonal message. [D-Ala4,Arg12]glucagon and [D-Phe4]glucagon, analogues designed to examine the possible importance of a beta-bend conformation in the N-terminal region of glucagon for binding and biological activities, have binding potencies relative to glucagon of 31% and 69%, respectively. [D-Ala4,Arg12]glucagon is a partial agonist in the adenylate cyclase assay system having a fourfold reduction in potency, while the [D-Phe4] derivative is a full agonist essentially equipotent with the native hormone. These results do not necessarily support the role of an N-terminal beta-bend in glucagon receptor recognition. With respect to in vivo glycogenolysis activities, all of the analogues have previously been reported to be full agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-function relationships of S-carboxymethyl methionine27 glucagon

Carboxymethylation of glucagon and subsequent purification of the hormone has provided a derivative modified by the addition of bulk to the methionine at position 27 without a net charge alteration in the side chain. Unreacted glucagon was removed after methylation of the methionine which provides a positively charged chromatographic handle. The derivative has a half-maximum concentration for binding of 5.3 nM and is a full agonist. These findings along with those provided by methylation of the methionine indicate that a positive charge rather than bulk on the methionine side chain disrupts the binding of hormone to its receptor. The S-carboxymethyl derivative lacks the concentration-dependent aggregation characteristic of glucagon at pH 10.2 as does the S-methyl derivative but increases its helical content in 30% 2-chloroethanol to the same extent as native and S-methyl hormone. Full activity of the S-carboxymethyl methionine27 glucagon does not favor the existence of the globular structure proposed by Korn and Ottensmeyer [(1983) J. Theor. Biol. 105, 403] as the binding species whereas multiple considerations do favor a flexible hormone with nucleation followed by conformational changes for complete binding and activation. Isotopic enrichment using labeled iodoacetate is feasible and can provide more definitive structural information.

Human glucagon-like peptides 1 and 2 activate rat brain adenylate cyclase

Two human glucagon-like peptides, GLP-1 and GLP-2, which are coencoded with pancreatic glucagon in the preproglucagon gene, do not significantly inhibit [125I]monoiodoglucagon binding to rat liver and brain membranes and do not activate adenylate cyclase in liver plasma membranes. Nevertheless, GLP-1 and GLP-2 were each found to be potent stimulators of both rat hypothalamic and pituitary adenylate cyclase. Only 30-50 pM concentrations of each peptide elicited half-maximal adenylate cyclase stimulation. Our data suggest that GLP-1 and GLP-2 may be neurotransmitters and/or neuroendocrine effectors, which would account for their high degree of sequence conservation through vertebrate evolution.

N alpha-Malto-glucagon and N alpha-malto, S-methyl methionine27-glucagon: preparation and characterization of two partial agonists

N alpha-Maltoglucagon was prepared by demethylation of N alpha-malto, S-methyl methionine27 glucagon, and the two derivatives were purified to greater than 99% and 99.7%, respectively. S-Methylation of glucagon lowers the reactivity of Lys-12 and provides an alternative strategy to epsilon-amino protection for directing glycosylation of glucagon to the alpha-amino group. Both derivatives are partial agonists, with their adenylate cyclase activation and binding reduced in parallel. N alpha-Maltoglucagon produces 70% and N alpha-malto, S-methyl methionine27 glucagon 40% of the maximum activity of native hormone. N alpha-Maltoglucagon binds equivalently to N alpha-biotinyl, N epsilon-acetimidoglucagon whose maximum activity is near 35%, but a pK shift of the imidazole moiety cannot account for the difference in their abilities to produce transduction. Both glycosylated derivatives bind noncooperatively and both inhibit adenylate cyclase at high concentrations. The presence of a maltose residue on the amino terminal of glucagon may be required but, alone provides insufficient structural complementarity for concanavalin A binding to occur. The glycosylated derivatives are resistant to aminopeptidase degradation, are more soluble, and the maltose residue is unlikely to cause toxicity with in vivo use. Such attributes may be advantageous in the development of other analogs.

Identification of glucagon receptors in rat brain

The binding of radiolabeled glucagon to rat brain membranes was investigated. Regional distribution studies indicate higher specific binding of 125I-labeled monoiodoglucagon to olfactory tubercule, hippocampus, anterior pituitary, and amygdala membranes, with somewhat lower binding to membranes from septum, medulla, thalamus, olfactory bulb, and hypothalamus. 125I-labeled glucagon bound to rat brain synaptic plasma membrane fractions with high affinity (KD = 2.24 nM). Specific binding was greater to synaptosomal membrane fractions relative to myelin, mitochondrial nuclear, or microsomal fractions. Inclusion of 0.1 mM GTP in the binding assay reduced the glucagon binding affinity (KD = 44.5 nM). Several neuropeptides and other neuroactive substances tested did not affect binding of labeled glucagon to brain membranes. Three different glucagon analogs inhibited labeled glucagon binding. Synthetic human pancreatic growth hormone-releasing factor, hpGRF-44, also inhibited binding, although the concentration required for half-maximal displacement was 100-fold higher than for native glucagon. Addition of glucagon to brain membranes resulted in approximately equal to 3-fold maximal activation of adenylate cyclase over basal levels. Glucagon at a concentration of 4.74 nM was required for half-maximal activation of pituitary membrane adenylate cyclase. These findings provide evidence for rat brain binding sites that respond to the pancreatic form of glucagon and can transduce this binding into the activation of adenylate cyclase.

Noncooperative receptor interactions of glucagon and eleven analogues: inhibition of adenylate cyclase

Glucagon and 11 glucagon derivatives were characterized and compared with respect to the cooperativity of their receptor interactions and their ability to elicit a biphasic (activation-inhibition) response from the adenylate cyclase system of rat liver plasma membranes. Slope factors were evaluated from two sets of experimental data, binding to hepatocyte receptors and activation of adenylate cyclase. The results are consistent with noncooperative binding to a single affinity state of the glucagon receptor for all derivatives, irrespective of the modification and the agonist properties of the derivatives. High-dose inhibition of adenylate cyclase activity was observed for native glucagon and all of the derivatives which were examined at high concentrations (greater than 10(-5) M). Partial agonism of some low-affinity glucagon derivatives is not caused by high-dose inhibition. Several mechanisms which might give rise to high-dose inhibition such as receptor cross-linking or multivalent receptor binding are discussed in relationship to the glucagon-receptor interaction. These phenomena indicate that significant differences exist between the glucagon system and the beta-adrenergic system.

Semisynthetic derivatives of glucagon: (des-His1)N epsilon-acetimidoglucagon and N alpha-Biotinyl-N epsilon-acetimidoglucagon

N epsilon-Acetimidoglucagon to be used for semisynthesis was prepared by reacting glucagon with methyl acetimidate hydrochloride at pH 10.2, favoring acetimidation of the sole epsilon-amino group. N epsilon-Acetimidoglucagon was isolated from the crude acetimidoglucagon mixture by anion-exchange chromatography at pH 9.4, producing a derivative which was identical with native glucagon on isoelectric focusing and which by amino acid analysis had greater than 98% of the lysine blocked. The yield was greater than that obtained when tetrahydrophthalic anhydride was used as a chromatographic handle to remove peptides with unreacted amino groups. N epsilon-Acetimidoglucagon closely resembled native glucagon in its biological activity and binding affinity, eliminating the need for deprotection. Semisynthetic N alpha-biotinyl-N epsilon-acetimidoglucagon, prepared by reacting (N-hydroxysuccinimido)biotin with N epsilon-acetimidoglucagon and purified by cation-exchange chromatography, was homogeneous upon isoelectric focusing (pI = 5.2) and exhibited 1.2% of the binding affinity, 2.4% of the biological potency, and 30% of the maximum activity of the native hormone. Preliminary fluorescence microscopy demonstrated binding of N alpha-biotinyl-N epsilon-acetimidoglucagon to glucagon specific receptors following exposure to fluorescein-labeled avidin. Capping of labeled receptors could be visualized with time. (Des-His1)N epsilon-acetimidoglucagon, prepared via a manual Edman degradation of N epsilon-acetimidoglucagon and isolated by cation-exchange chromatography, was homogeneous upon isoelectric focusing (pI = 5.2). The second residue, serine, has also been removed. Semisynthetic coupling of alternative residues to such derivatives will provide insight into the role of the amino-terminal residues in mediating the biological actions of the hormone.