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-1
amide, two possible forms of the
GLP-1 whole molecule, but no significant cross-reactivity 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
glucagon-secreting 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-1
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
glucagon-secreting 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-1
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.