Alveolar fluid neutrophil elastase activity in the adult respiratory distress syndrome is coinplexed to alpha-2-macroglobulin

We characterized the elastase and antielastase activity of the alveolar fluid of seven patients with the adult respiratory distress syndrome (ARDS) and thirteen normal volunteers. Alpha-l-antitrypsin (AlAT) concentrations were 60-fold higher in ARDS as compared to normal lavage fluid (2,140±498 nM; 36.1±4.2 nM, respectively). ARDS fluid antineutrophil elastase activity was also considerably higher than that of normals (979±204 nM; 31.3±2.9 nM, respectively). Despite the antineutrophil elastase excess, 5 of 7 ARDS lavage samples contained elastase activity (mean, 6.1±2.4 pM) as assayed using low-molecular-mass substrate, while only 1 of 13 normal subjects had detectable elastase activity (0.2 pM) (P < 0.01, compared with ARDS). That this activity was due to alpha-2-macroglobulin (A2MG)-complexed neutrophil elastase was evidenced by (a) the Sephadex G-75 elution profile; (b) the inactivity against insoluble VHielastin; (c) the inhibitory profile with phenylmethylsulfonyl fluoride, methoxy-succinyl-alanyl-alanyl-prolyl-valyl-chloromethylketone, ethylene diamine tetraacetic acid, and AlAT; and (d) the immobilization by A2MG antibody bound to polystyrene plates. Furthermore, in agreement with the predicted affinity of AMAT and A2MG for neutrophil elastase, the ratio of A2MG to AMAT in the fluid (0.57%) coincided with the ratio of the A2MG- to AlAT-complexed elastase (0.36%). These findings suggest that the net lung protease-antiprotease balance in ARDS is shifted largely in favor of the antiproteases (chiefly AMAT), and that the antiproteases, AMAT and A2MG, have similar affinities for neutrophil elastase in vivo