ALGINATE, THE SLIME EXOPOLYSACCHARIDE OF PSEUDOMONAS-AERUGINOSA, BINDS HUMAN-LEUKOCYTE ELASTASE, RETARDS INHIBITION BY ALPHA(1)-PROTEINASE INHIBITOR, AND ACCELERATES INHIBITION BY SECRETORY LEUKOPROTEASE INHIBITOR

The interaction of alginate from Pseudomonas aeruginosa ATCC 39324 wit h human leukocyte elastase was studied by determining the effects of t he polysaccharide on the amidolytic activity of the enzyme toward a ra nge of synthetic peptide substrates of different length. The data supp ort a model in which each elastase molecule interacts with a total of 19 uronic acid units on the alginate, primarily through electrostatic forces. Binding of alginate results in occlusion of distal subsites, m ost likely S-4 and S-5, of the enzyme's extended substrate-binding dom ain. As a result, alginate alone appears to be a weak inhibitor of the hydrolysis of long synthetic peptide substrates and [C-14]elastin by elastase. Alginate also has effects on the antielastase function of na turally occurring protease inhibitors in the lung: It reduces the asso ciation rate of elastase and alpha(1)-proteinase inhibitor, whereas it increases the association rate of elastase and secretary leukoproteas e inhibitor. In the presence of 36 mu g/ml alginate, the median concen tration found in sputum from cystic fibrosis patients infected with mu coid strains of P. aeruginosa, the second-order rate constant for inhi bition of elastase by secretory leukoprotease inhibitor is 2.6-fold gr eater than that for alpha(1)-proteinase inhibitor. Alginate has only a minor effect or; the antielastase activities of elafin and a recombin ant form of the isolated C-terminal domain of secretory leukoprotease inhibitor. Based on these findings, alginate may be an important facto r in determining the local distribution of leukocyte elastase and pert urbing the overall protease-antiprotease balance in the infected lungs of cystic fibrosis patients.