Endopeptidase from rat liver membranes, which generates miniglucagon from glucagon

An endopeptidase activity that cleaves glucagon, producing miniglucagon or glucagon (19-29), a Ca2+ pump inhibitory peptide, was isolated from rat liver membranes. The purified enzyme has a molecular mass of approximately 100 kDa and a pH optimum of approximately 8. It is inhibited by both sulfhydryl-blocking reagents and metal-chelating reagents and activated by thiol compounds. The partial N-terminal amino acid sequence of the 100-kDa protein does not correspond to any known protein. An antiserum was raised against a synthetic octapeptide corresponding to the N-terminal sequence. Immunoblot analysis of crude liver membranes revealed a single band at 100 kDa. Immunoreactivity was found in liver, pancreas, and heart, which are glucagon and miniglucagon target tissues, and in gastric mucosa and kidney. Low levels were detected in spleen, whereas immunoreactivity was undetectable in skeletal muscle and intestinal mucosa. The endopeptidase activity was inhibited by insulin, glucagon-like peptide-1, and glucagon-like peptide-1 (7-36) amide, whereas other peptides that contain dibasic sites had no effect on its activity, indicating that the endopeptidase does not display strict selectivity toward basic doublets.