Pseudomonas aeruginosa may cause severe lung infections in humans. This bacteria secretes an elastase that might degrade lung elastin. We have studied the solubilization of human lung elastin by P. aeruginosa elastase in an attempt to delineate the pathogenic role of this proteinase in P. aeruginosa lung infections. We also used bovine ligamentum nuchae elastin and human leukocyte elastase for comparative purposes.With an elastin concentration of 5 mg-ml -1 and at physiologic ionic strength, P. aeruginosa elastase is about 50 times more active on human lung elastin than on bovine elastin. In contrast, human leukocyte elastase has similar specific activities on the 2 substrates. In addition, the bacterial enzyme is about 10 times more active on human elastin than the neutrophil elastase but the latter is about 5 times more active on bovine elastin than the former.In order to better quantitate these enzyme-substrate interactions, we have measured initial rates of elastolysis, derived from product versus time curves, as a function of elastin concentration. The substrate-velocity curves, analyzed using an equation similar to the classic Michaelis-Menten one, yielded 2 empirical kinetic parameters: [S50]~1, the apparent elastase-elastin affinity and Vm, the apparent catalytic efficiency of elastase. This analysis shows that human leukocyte elastase exhibits similar [S50]" 1 and Vm values for the 2 elastins. The low activity of P. aeruginosa elastase on bovine elastin is due to the combined effects of low S50~1 and Vm values, which could not be measured separately. Human leukocyte elastase has a 14-fold higher affinity for human elastin than does P. aeruginosa elastase but the latter enzyme has a 27-fold higher Vm than the former. Ionic strength inhibits the solubilization of human elastin by P. aeruginosa elastase by decreasing Vm. By contrast, the leukocyte elastase activity is enhanced by salt. At low ionic strength the latter enzyme-substrate system exhibits the phenomenon of inhibition by excess substrate.Our data demonstrate that solution kinetics may be applied to the elastase-elastin reaction and that bovine elastin is not necessarily a useful substrate for investigating pathologically important elastases. The fast solubilization of human lung elastin by P. aeruginosa elastase suggests that this enzyme may degrade lung elastin during bacterial infections. AM REV RESPIR DIS 1987; 135:860-863Introduction Pseudomonas aeruginosa may cause severe lung infections in children with cystic fibrosis or in postoperative patients using respirators. Most P aeruginosa strains secrete 2 proteolytic enzymes: alkaline proteinase and elastase (PsE) (1). The latter was isolated and extensively studied by Morihara and his group (2-5). It has a molecular mass of about 40 kDa and a pi of 5.9. It is a metalloenzyme with 1 Zn 2+ atom per molecule.Animal experiments strongly suggest that PsE plays a pathogenic role in P aeruginosa infections (4-6). In addition, in vitro experiments have documented a PsE-catalyzed inactivation of (Xi-pr...

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