Comparison of biochemical properties of human airway tryptase isolated from mucoid sputum with those of lung mast cell tryptase

: We found a novel trypsin-like enzyme (tryptase) in sputum from patients with chronic airway diseases, and named this enzyme human airway tryptase (HAT). To clarify its physiological significance in the airway, we compared biochemical properties of purified HAT with those of purified lung mast cell tryptase (MCT). Studies with model peptide substrates showed that both the HAT and MCT preferentially cleaved the COOH-terminal side of arginine residues of certain peptides, but substrate specificities to nine synthetic model substrates of HAT differed from those of MCT. Effects of protease inhibitors on the two enzymes were examined at a concentration of 10 microM. Both the HAT and MCT were strongly inhibited by the trypsin inhibitors leupeptin, antipain, and aprotinin. An alpha-1-protease inhibitor inhibited HAT by 50%, but it did not inhibit MCT. In contrast, a secretory leukocyte protease inhibitor strongly inhibited MCT, but not HAT. Mucoid sputum from patients with chronic bronchitis contained much more HAT than MCT. These differences in biochemical properties between HAT and MCT indicate that they play different physiological roles in the airways.