The mammalian precursor gene proglucagon, which contains the glucagon sequence together with two structurally related glucagon-like peptides, glucagonlike peptides-1 and -2 (GLP-1 and GLP-2), is found in both gut and pancreas. In the pancreas, glucagon is the major biologically active hormone to be cleaved from proglucagon, but in the gut the cleavage site differs and glucagon-like peptide-1 (7-36) amide (GLP-1 (7-36) amide), a truncated form of GLP-1, is produced as a major active peptide. GLP-1 (7-36) amide is also the predominant form found circulating post-prandially in man.
Degradation of GLP-1
There are four forms of GLP-1 secreted in vivo: the inactive form of GLP-1 (1-37) and GLP-1 (1-36)-NH2, and the biologically active form of GLP-1 (7-37) and GLP-1 (7-36)-NH2. Both GLP-1 (7-37) and GLP-1 (7-36)-NH2 are produced from their full-length precursors by the action of PC enzyme and show the equipotent ability to stimulate glucose-dependently insulin secretion. It was found that GLP-1 (7-36)-NH2 is the major form of GLP-1 in the circulation in humans. It was also reported that addition of the amide group (NH3) to GLP-1 (1-36) is associated with prolonged survival of GLP-1 (7-36) in plasma. The concentration of total GLP-1 (including GLP-1(7-36)-NH2 and GLP-1(9-36)-NH2) levels in fasting plasma is 10-20 pM in healthy Caucasians, while it peaks at 30-60 pM 30 min after 75-g glucose or mixed meals loading.
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In vivo, biologically active forms of GLP-1 [GLP-1 (7-37) and GLP-1 (7-36)-NH2] are rapidly degraded to the respective inactive forms [GLP-1 (9-37) and GLP-1 (9-36)-NH2] through dipeptidyl peptidase-4 (DPP-4)-mediated cleavage of the alanine residue at position 2, with a half life of less than 2 minutes. Hansen et al. examined the GLP-1 newly secreted from the porcine ileum and found that about 75% of the GLP-1 leaving the gastrointestinal tract has already been degraded by DPP-4. Study in the anesthetized pig showed that GLP-1 is further degraded in liver, eventually resulting in only 10% to 15% of newly secreted GLP-1 in circulation system. It was also reported that less than 5% of intact GLP-1 reaches systemic circulation in Japanese.
Introduction of GLP-1 (7-36) amide
GLP-1 (7-36) amide is a 30 amino acid peptide with the sequence HAEGTFTSDVSSYLEGQAAKEFIAWLVKGR-NH2. It is an incretin hormone that can potentiate glucose-induced insulin secretion, stimulate insulin biosynthesis and inhibit glucagon secretion. Accordingly, it is a potential drug for the treatment of non-insulin-dependent diabetes mellitus (NIDDM or type II diabetes). However, GLP-1 suffers from physical instability. Thus it has been found that the conformation, aggregation and solubility of GLP-1 depend on the purification procedure and the in-process storage and handling. Efforts have been made to characterize the structural properties of GLP-1 in greater detail, including its aggregation behaviour in solution, but so far only limited structural information about GLP-1 has been reported. However, the α-helix seems to be an important structural motif of GLP-1 in solution. Thus it was found that GLP-1 can form oligomers with a high helical content, and that it is mainly helical in membrane-like environments (dodecylphosphocholine micelle). However, β-sheet formation also seems significant and can lead to less soluble GLP-1 aggregates.
GLP-1 (7-36) amide is a potent stimulator of insulin secretion. Together with the gastrointestinal hormone glucosedependent insulinotropic polypeptide (GIP), it is capable of acting as an 'incretin’, making an important contribution to insulin secretion via the entero-insular axis. In addition to stimulating insulin secretion, GLP-1 (7-36) amide has, in common with GIP, a direct anabolic insulin-like action on adipose tissue, stimulating de novo lipogenesis, and can also act as an enterogastrone, inhibiting pentagastrin-stimulated gastric acid secretion.
The nutritional stimuli for GIP secretion have been well documented. In contrast, comparatively little is known about the circulating levels of GLP-1 (7-36) amide and the effects of different nutrients on its secretion. There are discrepancies between the reported results from the small number of groups able to measure GLP-1 (7-36) amide by immunoassay, with mean fasting levels of immunoreactive GLP-1 ranging between 15 pmol/1 and 236 pmol/1. Several groups have found a small rise in circulating plasma immunoreactive GLP-1 following oral glucose or a mixed meal. In contrast, Hirota et al. observed a reduction in total immunoreactive GLP-1 in normal subjects following an oral glucose load. These discrepancies are largely due to varying cross-reactivity with extended forms of GLP-1 secreted from the pancreas and problems associated with different plasma extraction techniques. It is therefore difficult to assign a physiological endocrine role to GLP-1 (7-36) amide with any confidence.
The present study describes a radioimmunoassay to measure circulating levels of GLP-1 (7-36) amide in man. It compares the acute effects of ingestion of different macronutrients on GIP and GLP-1 (7-36) amide secretion, with particular emphasis on carbohydrate. In addition, circulating levels of GIP and GLP-1 (7-36 )amide have been investigated over a 24-h period in healthy subjects consuming a typical Western diet, to assess secretion of the hormone under more normal dietary circumstances.
References:
Kang, Z. (2012). Impaired Incretin Effects in Type 2 Diabetes: Mechanism and Therapeutic Implication (Doctoral dissertation, Chinese University of Hong Kong).
Elliott, R. M., Morgan, L. M., Tredger, J. A., Deacon, S., Wright, J., & Marks, V. (1993). Glucagon-like peptide-1 (7-36) amide and glucose-dependent insulinotropic polypeptide secretion in response to nutrient ingestion in man: acute post-prandial and 24-h secretion patterns. Journal of Endocrinology, 138(1), 159-166.
Chang, X., Keller, D., Bjørn, S., & Led, J. J. (2001). Structure and folding of glucagon-like peptide-1-(7-36)-amide in aqueous trifluoroethanol studied by NMR spectroscopy. Magnetic Resonance in Chemistry, 39(8), 477-483.
