PROGLUCAGON PROCESSING IN PORCINE AND HUMAN PANCREAS

In the pancreas proglucagon (PG), a peptide precursor of 160 amino aci ds is cleaved to produce glucagon and a 30-amino acid N-terminal flank ing peptide, but the fate of the C-terminal flanking peptide (99 amino acids) is incompletely known. We subjected acid ethanol extracts of h uman and porcine pancreases to gel filtration and analyzed the fractio ns with specific radioimmunoassays for the following regions of proglu cagon: PG 62-69, PG 72-81, PG 78-87, PG 98-107 amide, PG 126-134, and PG 149-158. Based on these assays and successive purifications by high performance liquid chromatography we isolated and purified to homogen eity three porcine peptides which were subjected to mass spectrometry and sequencing. One peptide was PG 64-69. The second was PG 72-108, as determined by mass spectrometry, N-terminal amino acid sequencing, an d specific radioimmunoassays. The third had a molecular size of approx imately 10,000, an N-terminal sequence corresponding to PG 72-81, and a C terminal sequence terminating at PG 158 (specific radioimmunoassay ). A similar peptide with an identical N-terminal sequence, a C-termin al sequence corresponding to PG 146-158, and a molecular mass of 9969 (theoretical mass for human PG 72-158 = 9971) was isolated from human pancreas together with small amounts of a peptide corresponding to PG 72-107 amide. Thus, the pancreatic processing of the C-terminal flanki ng peptide in proglucagon includes the formation of equimolar (to gluc agon) amounts of PG 64-69 and PG 72-158 (major proglucagon fragment) a nd smaller amounts of N-terminally extended glucagon-like peptide-1 (G LP-1) (PG 72-108 in pigs and PG 72-107 amide in humans).