Molecular Forms of Glucagon-Like Peptide-1 in Human Pancreas and Glucagonomas

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Abstract
The structure of human preproglucagon, as deduced from nucleotide sequencing of the preproglucagon gene, contains two glucagon-like peptides (GLP-1 and GLP-2) in the portion C-terminal to glucagon. A rabbit antiserum was raised against synthetic GLP-1-(1–19) which had 20% cross-reactivity with synthetic GLP-1 and des-Gly37-GLP-l amide, two possible forms of the GLP-1 whole molecule, but no significant crossreactivity with glucagon or other pancreatic peptides. Immunocytochemistry revealed that the distribution of GLP-1-(1–19) immunoreactivity followed that of glucagon-like immunoreactivity in the normal human pancreas and in two human glucagonsecreting pancreatic tumors. Chromatography of human pancreas extracts on Sephadex G-50 gave peaks of cross-reactivity at Kav values of 0.06–0.16, 0.34–0.39, 0.54–0.58 (the elution position of synthetic GLP-1), and 0.64–0.70. The concentration of immunoreactivity in the Kav 0.54–0.58 peak measured by RIA using GLP-1 or des-Gly37-GLP-l amide as standard was 94 ± 7 pmol/g (mean ± SEM), while the total pancreatic glucagon content was 4.8 ± 0.8 nmol/g. One extract of a human glucagonsecreting pancreatic tumor contained a prominent peak of GLP-1–(1–19) peptide cross-reactivity with properties identical to those of GLP-1 or des-Gly37-GLP-l amide on gel filtration and reverse phase high pressure liquid chromatography, but another tumor contained a preponderance of cross-reactive forms of greater molecular size. Pretreatment plasma from three patients with radiological and biochemical evidence of glucagon-secreting tumors contained a peak of cross-reactivity with the chromatographic properties of intact GLP-1. The low concentrations of intact GLP-1 in normal pancreas compared with pancreatic glucagon concentrations suggest that the majority of the proglucagon is cleaved in a manner that does not produce GLP-1, as defined by its delimiting pairs of basic amino acid residues.