Methylation of glucagon, characterization of the sulfonium derivative, and regeneration of the native covalent structure

Synthesis of poly(meth)acrylates with thioether and tertiary sulfonium groups by ARGET ATRP and their use as siRNA delivery agents

The field of RNA interference depends on the development of safe and efficient carriers for short interfering ribonucleic acid (siRNA) delivery. Conventional cationic monomers for siRNA delivery have utilized the nitrogen heteroatom to produce cationic charges. Here, we polymerized cationic sulfonium (meth)acrylate by activators regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP) to form polymers with narrow molecular weight distributions for siRNA delivery. The tertiary sulfonium species was stable toward dealkylation in water but less stable in the polar aprotic solvent dimethyl sulfoxide. Block copolymers poly(ethylene oxide) with poly(meth)acrylate containing sulfonium moieties were prepared as an siRNA delivery platform. Results suggested block copolymers were biocompatible up to 50 μg/mL in vitro and formed polyplexes with siRNA. Additionally, block copolymers protected siRNAs against endonuclease digestion and facilitated knockdown of glyceraldehyde 3-phosphate dehydrogenase (Gapdh) mRNA expression in murine calvarial preosteoblasts. The versatility, biocompatibility, and cationic nature of these tertiary sulfonium groups are expected to find widespread biological applications.

Experimental allergic encephalomyelitis in rabbits. A major encephalitogenic determinant within residues 1-44 of myelin basic protein

Experimental allergic encephalomyelitis could be induced in rabbits by injection in Freund's complete adjuvant of either peptide 1-44 or peptide 45-87 of rabbit myelin basic protein. In order to localize the encephalitogenic determinant present in peptide 1-44, several smaller derivative peptides were prepared and examined. Peptic peptide 15-44 and thrombic peptide 1-31 were as active as peptide 1-44, whereas peptic peptides 1-14 and 18-38 and BrCN peptide 22-44 were virtually inactive. Weak activity was shown by BrCN peptide 1-21. These results provide evidence that a major encephalitogenic determinant present in peptide 1-44 lies within sequence 15-31. The encephalitogenic activity of peptide 15-44 was essentially destroyed by oxidation of methionine-21 to methionine sulfoxide; methylation of Met-21, on the other hand, appeared to be relatively ineffective in eliminating the encephalitogenicity of peptide 1-44.

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.

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.

o-Phthaldialdehyde precolumn derivatization and reversed-phase high-performance liquid chromatography of polypeptide hydrolysates and physiological fluids

A rapid and ultrasensitive fluorescence amino acid analysis system has been developed which utilizes omicron-phthaldialdehyde as a precolumn derivatizing agent. omicron-Phthaldialdehyde in the presence of mercaptan reacts rapidly with primary amino acids to form intensely fluorescent derivatives. These derivatives are analyzed with good selectivity by high-performance liquid chromatography employing 3-microns particle size reversed-phase columns. Resolution of the amino acid derivatives is accomplished with a methanol gradient in 0.1 M aqueous sodium acetate, pH 7.2. The quantitation of the individual amino acid derivatives is reproducible within an average relative deviation of +/- 1.5% and has a detection limit of less than 100 fmoles. Amino acid mixtures obtained by either enzymatic or acid hydrolysis of polypeptides are efficiently resolved with an analysis time of less than 18 min. Methods for the amino acid analysis of physiological fluids such as serum, urine and cerebrospinal fluid were also developed which employ the above separation procedure for the identification and quantitation of amino acids and other biological amines. Mixtures which contained as many as 48 components were resolved with an analysis time of less than 50 min.

Experiments on the protection of the thioether in methionine

The decrease in the solubility of peptides when their methionine residues are replaced by methionine sulfoxides prompted the exploration of an alternative approach to the protection of the thioether in methionine side chains. Alkylation of tert.-butyloxycarbonyl methionine p-nitrophenyl ester with methyl p-toluenesulfonate yielded the crystalline derivative of methionine S-methyl p-toluenesulfonate which could be incorporated into peptide chains. Alternatively, methionine S-methyl p-toluenesulfonate (Mmt) residues could be generated by the action of methyl p-toulensulfonate on methionine containing peptides. The protecting group remained intact under the conditions of aminolysis and ammonolysis commonly used in peptide synthesis, and it was unchanged after the removal of other blocking groups with trifluoroacetic acid or diethylamine. On treatment with hydrobromic acid in acetic acid the toluenesulfonate anion was replaced by bromide ion, while hydrogenation resulted in the decomposition of the modified methionine side chain. In the process of deprotection Mmt residues could be smoothly converted to methionine residues by thiolysis. Thus, the protecting group functioned well in several respects but an increase in solubility (in dimethylformamide) on alkylation was observed only in a part of the peptide derivatives tested. Therefore, the value of the new approach for the protection of the methionine side chain in peptide synthesis remains to be established.

Structure-conformation-activity studies of glucagon and semi-synthetic glucagon analogs

Examination of glucagon structure-activity relationships and their use for the development of glucagon antagonists (inhibitors) have been hampered until recently by the lack of high purity of semisynthetic glucagon analogs and inadequate study of full dose-response curves for these analogs in sensitive bioassay systems. Recently a number of highly purified glucagon fragments and semi-synthetic analogs have been prepared and their full dose-response activities examined over a wide concentration range using the hepatic membrane adenylate cyclase assay, the hepatic membrane receptor binding assay, and glycogenolytic activity in isolated rat hepatocytes. The results of these studies have enabled us to identify and dissociate the structural (and in some cases conformational) features of glucagon important for binding from those most responsible for biological activity (transduction). Key findings in these studies were the observation that: (1) the C-terminal region of glucagon is primarily of importance for hormone binding to receptors; (2) glucagon 1-21 and glucagon 1-6 have low potency, but are essentially fully active glucagon derivatives; and (3) highly purified glucagon 2-29 ([1-des-histidine]-glucagon), [1-N alpha-carbamoylhistidine]-glucagon and [1-N alpha-carbamoylhistidine, 12-N alpha-carbamoyllysine]-glucagon are all partial agonists. These and other findings led us to synthesize several semisynthetic analogs of glucagon which were found to possess no intrinsic biological activity in the hepatic adenylate cyclase assay system, but which could block the effect of glucagon (competitive inhibitors) in activating adenylate cyclase in this system. Two of these highly purified analogs [1-des-histidine][2-N alpha-trinitrophenylserine, 12-homoarginine]-glucagon and [1-N alpha-trinitrophenylhistidine, 12-homoarginine]-glucagon were quite potent glucagon antagonists (inhibitors) with pA2 values of 7.41 and 8.16 respectively. The latter compound has also been demonstrated to decrease dramatically blood glucose levels of diabetic animals in vivo. These results demonstrate that glucagon is a major contributor to the hyperglycemia of diabetic animals. Examination of the known and calculated conformational properties of glucagon provide insight into the structural and conformational properties of glucagon and its analogs most responsible for its biological activity. Consideration of these features and the mechanism of glucagon action at the membrane receptor level provide a framework for further developing glucagon analogs for theoretical and therapeutic applications.

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

Chemical and enzymatic methods have been used to prepare the following series of seven glucagon derivatives modified in the carboxyl-terminal region important for hormone-receptor binding: [des-Asn28,Thr29](homoserine lactone27)glucagon, [des-Asn28,Thr29](homoserine27)glucagon, (S-methyl-Met27)glucagon, [des-Thr29](S-methyl-Met27)-glucagon, [des-Thr29]glucagon,[des-Asn28,Thr29](S-methyl-Met27)glucagon, and [des-Asn28,Thr29]glucagon. The derivatives were isolated in high yield, extensively purified, and chemically characterized. All were found to be full agonists of native glucagon. Binding affinity was evaluated by displacement of mono[125I]iodoglucagon prepared by new methods. Binding and biological activities closely correlated, indicating that most modifications affected the relative binding affinity and relative biological potency of glucagon to a comparable extent. Circular dichroism measured in dilute acid solution resembled that of native glucagon except for [des-Asn28,Thr29]glucagon which displayed increased alpha helicity (25%). All derivatives formed helical structures in 2-chloro-ethanol, although the amount of helicity induced was not closely correlated with biological activity. Binding and biological activities were not affected by removal of Thr-29, though both were reduced 20-fold when Asn-28 was also removed, irrespective of whether homoserine or native methionine remained at the carboxyl terminus. Lactone formation was associated with a further 5-fold reduction in binding affinity but not in activity. Methylation of Met-27 had essentially the same effect as removing the two carboxyl-terminal residues, although the combined effect of both modifications was greater than 100-fold reduction in binding and activity. These findings provide additional insight concerning glucagon structure-function relationships.

Preparation and properties of glucagon analogs prepared by semi-synthesis from CNBr-glucagon

The semi-synthetic approach has been used to obtain new analogs of the peptide hormone glucagon. Using the highly purified 27 amino acid fragment of cyanogen bromide-treated glucagon, we have prepared, by nucleophilic addition to the lactone ring, the following derivatives: CNBr-Gly28-glucagon, CNBr-glucagon hydrazide, CNBr-glucagon n-butylamide and CNBr-glucagon biotinamide. Direct aminolysis of the lactone was successful only with sterically unhindered primary amines. Addition of an amino acid could be accomplished by formation of the peptide hydrazide followed by azide coupling. All these analogs were full agonists with decreased potency relative to the native hormone. Examination of the structure-function relationships of these new C-terminal glucagon derivatives suggests that the hydrophobic side-chain of methionine is important to the binding of glucagon to its receptor and that the C-terminal portion of glucagon is only involved in the binding of the hormone to the receptor and not in the transduction process.