The rôle of proteases and antiproteases in bronchial secretions

A number of proteolytic enzymes (proteases) and their inhibitors (anti-proteases) have been demonstrated in the bronchial secretions. The most important of these proteases is probably the potent elastase derived from polymorphonuclear leukocytes (PMN) which are recruited into the bronchial tree during inflammation. This enzyme can degrade elastin, collagen and proteoglycan and can damage the bronchial epithelium; it is inhibited by antileukoprotease and by alpha protease inhibitor (API). Pulmonary macrophages also secrete an elastase, but Pseudomonas aeruginosa is the only microorganism known to produce elastase. Antileukoprotease is probably the most important of the antiproteases to be found in bronchial secretions. Other antiproteases such as low molecular weight inhibitors, API and alpha 2 macroglobulin have been demonstrated in bronchial secretions but they are present in low concentration. A balance between proteolytic enzymes and their inhibitors is thought to be of great importance in the prevention of proteolytic damage to local tissues, not only in emphysema but also in chronic bronchitis, bronchiectasis and cystic fibrosis. Oxidants derived from PMN or from tobacco smoke can inactivate protease inhibitors and in both the bronchial tree and the lung parenchyma, a balance between oxidants and anti-oxidants must presumably also be maintained to prevent local tissue damage.