ACCELERATED BINDING OF SECRETORY LEUKOPROTEASE INHIBITOR TO HUMAN-LEUKOCYTE ELASTASE MEDIATED BY SINGLE-STRANDED SITES IN DNA FROM TRACHEOBRONCHIAL MUCUS

We have found that preparations of DNA isolated from purulent sputum p ossess a novel activity which accelerates and stabilizes the binding o f human leukocyte elastase to secretory leukoprotease inhibitor, a maj or endogenous antielastase in the respiratory tract. DNA in sputum is derived from the nuclear debris of disintegrated inflammatory leukocyt es, and can attain concentrations ranging from 10(2) to 10(4) mu g/ml, depending on the severity of pulmonary infection and inflammation. In the presence of 23 mu/ml DNA, a concentration lower than those found in most purulent sputa, the rate constant for association of secretory leukoprotease inhibitor with elastase is increased to 1.1 x 10(8) M(- 1)s(-1), 44-fold greater than that in the absence of DNA. The equilibr ium dissociation constant for the enzyme-inhibitor complex drops to 0. 7 pM, two orders of magnitude lower than that in the absence of DNA. T he accelerating effect of DNA is further increased by thermal denatura tion or by modification with exonuclease III, while it is significantl y reduced by digestion with S1 nuclease or by binding of Escherichia c oli single-stranded DNA binding protein. The results from these experi ments indicate that the structural elements in sputum DNA that are res ponsible for the accelerating effect have the characteristics of singl e-stranded sites. Similar kinetic effects on elastase inhibition were also observed with human placental DNA and genomic DNAs from a variety of other species, These findings suggest that DNA in pulmonary secret ions may participate in antielastase defense by promoting the binding of secretory leukoprotease inhibitor to leukocyte elastase. The result s may have important implications for use of nuclease preparations in mucolytic therapy for cystic fibrosis.