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

Time-resolved phosphorescence of tyrosine, tyrosine analogs, and tyrosyl residues in oxytocin and small peptides

We present the time-resolved phosphorescence of oxytocin, two oxytocin derivatives, vasopressin and a series of compounds that serve as models for free tyrosine. One of the oxytocin derivatives, desaminodicarbaoxytocin, has the disulfide bridge replaced by an ethylene bridge, and lacks the N-terminus. Similar to the reported fluorescence decays of tyrosine in these peptides, the phosphorescence decays generally are not single exponentials, but can be fit as biexponentials. The decay times for the oxytocin peptides are shorter than for desaminodicarbaoxytocin or the model compounds, and this we attribute to enhanced spin-orbit coupling due to the presence of sulfur. We measured the phosphorescence decay of the model cyclic pentapeptide that contains tyrosine and compared it to that observed for the same cyclic pentapeptide in which tyrosine is replaced by tryptophan. We also report the phosphorescence of 2-tryptophan-oxytocin, and deamino-2-tryptophan-oxytocin in which biexponential phosphorescence decay is also observed.

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.

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.

Glucagon structure-function relationships using isolated rat hepatocytes

The ability of glucagon and several of its semi-synthetic analogues to stimulate glucose production in isolated rat hepatocytes was measured and compared for relative potencies. The order of decreasing biological activities of glucagon in this assay was as follows: glucagon greater than [HArg12]-glucagon greater than [des-Asn28, Thr29][homoserinehydrazide27]-glucagon approx. equal to [des-His1]-glucagon greater than [des-Asn28, Thr29][homoserinelactone27]-glucagon greater than [des-Asn28, Thr29]-[n-butylhomoserineamide27]-glucagon greater than glucagon1-21. Qualitatively, these results are similar to those obtained previously in the hepatic plasma membrane adenylate cyclase assay. Minor exceptions were noted for the hydrazide derivative and the partial agonist [des-His1]-glucagon, both of which were slightly more potent relative to glucagon in the glycogenolytic assay than in the adenylate cyclase assay. The assay provides important insight into glucagon structure-function relationships.

Properties of amidinated glucagons

Porcine glucagon has been reacted with a series of alkyl imidates. The epsilon-amino group and both the alpha and epsilon-amino groups were modified and the subsequent glucagon derivatives were purified by ion-exchange chromatography and characterized. The modified glucagons were compared with native glucagon in their ability to activate hepatic adenylate cyclase and to compete with 125I-glucagon for binding to sites specific for glucagon in hepatic plasma membranes. N epsilon-acetamidino-glucagon was as biologically potent, in both activity and binding, as native glucagon, whereas N epsilon-4-hydroxyphenylamidinoglucagon required a twofold higher concentration to obtain similar levels. These findings suggest that modification through the epsilon-amino group with alkyl imidates possessing reporter groups should result in glucagon derivatives with significant biological potency, thus providing a new approach to the study of this peptide hormone. Amidination of both epsilon and alpha-amino groups resulted in glucagon derivatives which were agonists with respect to adenylate cyclase activation and which displayed unexpected anomylous behavior on chromatography.