Tissue and Plasma Concentrations of Amidated and Glycine-Extended Glucagon-Like Peptide I in Humans

Using specific radioimmunoassays, we studied the occurrence of amidated and glycine-extended glucagon-like peptide I (GLP-I) molecules in the human small intestine and pancreas and in the circulation system in response to a breakfast meal. Through gel permeation chromatography of extracts of the human pancreas (n = 5), we found that 71% of the GLP-I immunoreactivity eluted as a large molecule corresponding to the major proglucagon fragment, 24% corresponded to GLP-I 1–36 amide, and 5% to GLP-I 1–37. By gel permeation chromatography of extracts of human small intestine (n = 6), we found that all immunoreactivity eluted in one peak at the common elution position of the two insulin-releasing peptides, GLP-I 7–36 amide and GLP-I 7–37. Of the GLP-I immunoreactivity, 80% corresponded to GLP-I 7–36 amide and 20% to GLP-I 7–37. The mean concentrations of amidated GLP-I and glycine-extended GLP-I in fasting plasma were 7 ± 1 and 6 ± 1 pM, respectively (n = 6). In response to a breakfast meal, the concentration of amidated GLP-I rose significantly amounting to 41 ± 5 pM 90 min after the meal ingestion, whereas the concentration of glycine-extended GLP-I only rose slightly to a maximum of 10 ± 1 pM. Thus, both amidated and glycine-extended GLP-I molecules are produced in the small intestine and in the pancreas in humans. Both amidated and glycine-extended GLP-I are measurable in fasting plasma. The higher meal response of amidated GLP-I compared with glycine-extended GLP-I probably reflects the larger amount of amidated GLP-I produced in the tissues compared with glycine-extended GLP-I.