Synthesis and properties of a photoaffinity probe for the glucagon receptor in hepatocyte plasma membranes

Importance of the 10-13 region of glucagon for its receptor interactions and activation of adenylate cyclase

The role of the Tyr10-Ser11-Lys12-Tyr13 region of glucagon in the binding interaction and activation of the glucagon receptor was investigated by means of the synthetic glucagon analogues [Phe13]glucagonamide, [Phe10]glucagonamide, [Phe10]glucagon, [Phe10,13]glucagon, [Pro11]glucagon, [Pro11,Gly12]glucagonamide, [Ala11]glucagon, and [Oac11-13]glucagonamide. These analogues were synthesized by solid-phase peptide synthesis on p-methylbenzhydrylamine or Merrifield resins with protected N alpha-tert-butyloxycarbonyl amino acids. Purification by dialysis, cation-exchange chromatography, gel filtration, and preparative reverse-phase high-performance liquid chromatography (HPLC) gave products that proved homogeneous by thin-layer chromatography and HPLC and on analysis by amino acid analysis, by sequencing, and by alpha-chymotryptic peptide mapping with HPLC. Biological activities were examined by measurement of the stimulation of liver plasma membrane adenylate cyclase and by specific displacement of [125I]glucagon from glucagon receptors. The results of these studies indicate that while the biological "message" region of glucagon is located elsewhere, the 10-13 region has multiple roles in the glucagon-glucagon receptor interaction: this region provides functional groups for direct binding interaction with the receptor, and this region interacts with the receptor in such a way as to allow the "transduction message" portion of glucagon to interact and activate the receptor.

Effect of specific trinitrophenylation of the lysine epsilon amino group of glucagon on receptor binding and adenylate cyclase activation

The trinitrophenyl group was specifically introduced into the epsilon-amino group of glucagon by reaction of N alpha-citraconyl glucagon with trinitrobenzenesulfonic acid. The N alpha-citraconyl blocking group was subsequently removed by acid treatment yielding N epsilon-trinitrophenyl glucagon which was purified by anion-exchange chromatography. The derivative showed less secondary structure as measured by circular dichroism than the native hormone at pH 8.0 and at pH 2.0 in the presence of sodium dodecyl sulfate. The analog possessed 4-5% the potency of glucagon in stimulating adenylate cyclase with 90% maximal stimulation and possessed 30% the potency of glucagon in competing for glucagon-specific receptor sites in hepatic plasma membranes. Although the structure of N epsilon-trinitrophenyl glucagon is very similar to N epsilon-4-azido-2-nitrophenyl glucagon, the photoaffinity antagonist synthesized by M.D. Bregman and D. Levy [(1977) Biochem. Biophys. Res. Commun. 78, 584-590.], the biological activities of the two are different. Possible explanations for these differences are discussed.

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: structure-function relationships investigated by sequence deletions

A series of glucagon analogues, des-(1-4)-glucagon, des-(5-9)-glucagon, des-(10-15)-glucagon, des-(16-21)-glucagon, des-(22-26)-glucagon and des-(27-29)-glucagon, were prepared by condensation of synthetic fragments and characterized biologically and immunologically. Fully synthetic glucagon was also characterized. The potencies with regard to glucagon receptor binding in purified rat liver plasma membranes were, in decreasing order: synthetic glucagon 108%, des-(1-4)-glucagon 5.7%, des-(27-29)-glucagon 0.92%, des-(5-9)-glucagon 0.47%, des-(10-15)-glucagon 0.0028%, des-(16-21)-glucagon 0.0017% and des-(22-26)-glucagon 0.00060% relative to that of natural porcine glucagon. Des-(27-29)-glucagon was the only analogue that activated the adenylate cyclase in rat liver plasma membranes or stimulated the lipolysis in isolated free fat cells from rat epididymal fat pad. The potencies were 0.16% and 0.20% of that of glucagon, respectively. Des-(1-4)-glucagon was a glucagon antagonist in the adenylate cyclase assay. The immunoreactivities of the glucagon analogues were determined with two commonly used anti-glucagon sera, K 5563 and K 4023, directed towards the C-terminus and some segment in the sequence 2-23, respectively. In the K 5563 assay, des-(27-29)-glucagon and des-(22-26)-glucagon had potencies of 0.0009% and less than 0.09% of that of glucagon, respectively. The remaining analogues had potencies varying from 45% to 141% of that of glucagon. In the K 4023 assay, the analogues showed a non-linear dilution effect. The combined results indicate a partition within the glucagon molecule with regard to receptor binding and adenylate cyclase activation. The region 10-26 appears to be the most important for receptor binding, whereas 1-4 is essential for adenylate cyclase activation. The C-terminal segment 27-29 is important for the maintenance of full receptor binding but non-essential for adenylate cyclase activation.

[12-Homoarginine]glucagon: synthesis and observations on conformation, biological activity, and copper-mediated peptide cleavage

Specific modification of the single lysine residue (Lys-12) in glucagon with O-methylisourea has been effected by blocking the reactivity of the amino terminal histidine with copper, providing a method for obtaining [12-homoarginine]glucagon. It was found that as a side reaction, under the conditions of the modification reaction, Cu(II) catalyzed cleavage of the polypeptide chain between Asp-9 and Tyr-10, and between Lys-12 and Tyr-13. This observation may be of value for development of a sequence-specific peptide cleavage procedure. The dilute solution conformations of glucagon and [12-homoarginine]-glucagon were compared by circular dichroism, fluorescence, phosphorescence, energy transfer, and optical detection of magnetic resonance. The results indicate that conversion of Lys-12 to homoarginine does not alter the helix content the side chain conformation in the vicinity of the tyrosine and tryptophan residues, or the relative distances and orientations between these residues. However, the modification reduces the hormone potency towards activation of lipolysis in isolated rat epididymal fat cells by a factor of seven. We attribute the loss of potency to an interference with a specific interaction between the lysine residue and the fat cell hormone receptor, and not to a change in the solution conformation of the hormone.