Protease inhibitors in tracheobronchial secretions

The isolation and characterization of three protease inhibitors from tracheobronchial secretions are described. As starting material, pooled secretions from patients with tracheostomies was used. The isolation procedure consisted of precipitation with 3% perchloric acid (the major acid-stable inhibitors remain in solution), affinity chromatography on trypsin-Sepharose, and preparative zone electrophoresis. We found three distinct inhibitors. One was a basic protein with evidence of size and charge heterogeneity but immunologic homogeneity, Mr = 15,850 +/- 1200 daltons, 12,600 +/- 700, 6500 +/- 500. Two inhibitors were acidic proteins, Mr = 63,400 +/- 3200 (AI) and 19,960 +/- 1500 (AII) daltons. The basic inhibitor had tyrosine as the sole aminoterminal amino acid. For the two acidic inhibitors, an aminoterminus was not found. All three inhibitors contained neutral sugars and amino sugars but no sialic acid. They inhibit trypsin, chymotrypsin, and human granulocytic elastase. The two acidic inhibitors are immunologically related; they are apparently derived from serum ITI. Inhibitor AII originates from inhibitor AI, probably by limited proteolysis by several proteases. The concentration of the basic inhibitor in bronchial secretions of 42 patients with obstructive lung disease was 0.206 +/- 0.15 mg/ml.