title: Modification of the isoinhibitors of human serum alpha 1-proteinase inhibitor (alpha 1-antitrypsin) by pancreatic proteases.
abstract: Due to the action of a serum protease, the two most cathodal isoinhibitors of the alpha 1-proteinase inhibitor (alpha 1-PI) are cleaved at the Gly5-Asp6 bond and lack two negative charges. In spite of this, these can bind trypsin and chymotrypsin, showing that the N-terminal pentapeptide is not indispensable for inhibition function. Pancreatic proteases also cleave a bond near the N-terminus in alpha 1-PI, resulting in a loss of two negative charges and a corresponding cathodal shift in the electrofocusing behavior of the isoinhibitors. Trypsin cleaves isoinhibitors near the N-terminus at a large inhibitor excess and unless an additional cleavage takes place, at least two of the new isoinhibitors remain active. An additional cleavage(s), most likely at a distance of 30-40 residues from the C-terminus results in a corresponding decrease of the molecular mass and a loss of inhibition function. Although the C-terminal cleavage peptide does separate from the protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, it remains associated with it under conditions of polyacrylamide gel isoelectric focusing. Chymotrypsin also cleaved alpha 1-PI near the N-terminus but this could be observed only at protease excess and the modified isoinhibitors did not form complexes with chymotrypsin. The molecular polymorphism of alpha 1-PI is partly explained by the absence of the N-terminal pentapeptide from some of the isoinhibitors.
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D004791:Enzyme Inhibitors / Q000302:isolation & purification; D006801:Humans; D007525:Isoelectric Focusing; D010179:Pancreas / Q000201:enzymology*; D010447:Peptide Hydrolases / Q000378:metabolism*; D000515:alpha 1-Antitrypsin / Q000378:metabolism*
chemical_list: D004791:Enzyme Inhibitors; D000515:alpha 1-Antitrypsin; D010447:Peptide Hydrolases