Inactivation of human bronchial mucosal proteinase inhibitor by Pseudomonas aeruginosa elastase.

Human bronchial mucus contains an acid-stable proteinase inhibitor called bronchial mucous inhibitor (BMI) that apparently protects the upper airways from proteolysis by polymorphonuclear leukocyte (PMN) elastase and cathepsin G, and infections of Pseudomonas aeruginosa, which also produces an elastase, can result in considerable lung damage. The possible role of BMI in protecting the lung from proteolytic attack in the presence of P. aeruginosa elastase (a zinc metalloprotease) was investigated. The BMI not only failed to inhibit P. aeruginosa elastase but the ability of BMI to inhibit PMN elastase and cathepsin G was rapidly lost in the presence of the bacterial elastase. The P. aeruginosa elastase also freed active PMN elastase from complexes with BMI, resulting in elastin digestion by both enzymes. The proteolysis of lung tissue observed with P. aeruginosa infections may be caused by a combination of bacterial and PMN proteases. Elastase and cathepsin G released from leukocytes, attracted to P. aeruginosa infection sites, would be free to attack the bronchial tissues after BMI inactivation by P. aeruginosa elastase, adding to the damage caused by the bacterial protease(s).