Fibrinogenolytic activity of a novel trypsin-like enzyme found in human airways

We previously reported finding a novel trypsin-like enzyme in sputum samples from patients with chronic airway diseases, and named it human airway tryptase (HAT). We also obtained data suggesting that HAT is secreted from submucosal serous glands onto the mucous membrane of the airway, and that fibrinogen is cleaved by HAT. We studied whether HAT can act as an anticoagulant in the airway by breaking down fibrinogen. The concentration of fibrinogen in sputum samples was measured by enzyme-linked immunosorbent assay. The concentration was higher in mucoid sputum from patients with bronchial asthma (6.3 +/- 5.5) than in mucoid sputum from patients with chronic bronchitis (1.9 +/- 1.1), and it was higher in purulent sputum from patients with bronchiectasis (18.8 +/- 8.8) than in mucoid sputum from patients with asthma. The trypsin-like activity in sputum (milliunit/ml) was 23.4 +/- 18.0 in mucoid sputum from patients with chronic bronchitis, 46.9 +/- 43.9 in mucoid sputum from patients with asthma, and 14.9 +/- 8.23 in purulent sputum from patients with bronchiectasis. The effect of HAT on human fibrinogen at concentrations from 4 to 2000 micrograms/ml was examined at pH 7.4 and 8.6, with purified HAT at concentrations from 0.5 to 10 milliunit/ml. SDS-polyacrylamide gel electrophoresis showed that HAT can cleave fibrinogen. especially the alpha-chain, at low concentrations (4 to 16 micrograms/ml) and at a high concentration (2000 micrograms/ml) of fibrinogen. Exposure of fibrinogen to HAT resulted in the loss of thrombin-induced clotting capacity; the strength of that effect depended on the duration of exposure to HAT and on the concentration of HAT. From these results we postulate that HAT acts at an anticoagulant in the airways, especially on the mucous membrane, by cleaving fibrinogen transported from blood.