Kunitz-Type Proteinase Inhibitors Derived by Limited Proteolysis of the Inter-α-Trypsin Inhibitor, IV. The Amino Acid Sequence of the Human Urinary Trypsin Inhibitor Isolated by Affinity Chromatography

Abstract

An acid-resistant trypsin inhibitor from human urine and serum is released in vivo by limited proteolysis from the high molecular acid-labile inter-.alpha.-trypsin inhibitor. The inhibitor shows an apparent molecular mass of 30,000 Da (daltons) and is composed of 2 Kunitz-type domains. The domains are released in vitro by prolonged tryptic hydrolysis. The C-terminal domain is responsible for antitryptic activity. For the other domain no inhibitory activity towards proteinases, i.e., chymotrypsin (EC 3.4.2.1.1), trypsin (EC 3.4.21.4), pancreatic and leukocytic elastase has been demonstrated so far. The polypeptide chain comprising both domains consists of 122 residues and has a molecular mass of only 13,400 Da. Both the N-terminal extension peptide with 21 residues and the inactive domain are linked O-glycosidically and N-glycosidically, respectively, with large carbohydrate moieties. The N-terminal amino acid sequence of the human urinary trypsin inhibitor was determined by solid-phase Edman degradation of a single peptide. The molecular mass calculated for the total polypeptide chain of 143 residues should be 15,340 Da; from the difference to the measured value (30,000 Da) the glycopeptide apparently contains a considerable carbohydrate moiety.