Growth of Pseudomonas aeruginosa and secretion of protease in the presence of the protease inhibitors

The compounds benzyloxycarbonyl-L-leucyl-hydroxamate (Z-Leu-NHOH), benzyloxycarbonyl-glycyl-hydroxamate (Z-Gly-NHOH) and 2-mercaptoacetyl-L-phenylalanyl-L-leucine (HSAc-Phe-Leu) are potent inhibitors of Pseudomonas aeruginosa elastase. The effect of these inhibitors on growth and protease secretion by the bacteria was studied under conditions where the organisms secrete elastase as the major proteolytic constituent. Z-Gly-NHOH and Z-Leu-NHOH inhibited bacterial growth and enzyme secretion by 40 and 30%, respectively, although the inhibition by Z-Leu-NHOH was expressed only when growth was in dialysed medium. Inhibition of growth by both compounds was first observed at the end of the logarithmic phase of growth, suggesting that these compounds are not toxic to the bacteria. HSAc-Phe-Leu did not inhibit growth or enzyme secretion. The level of HSAc-Phe-Leu in the medium decreased constantly during incubation, probably because of gradual oxidation of the -SH group. The rate of this decrease was markedly enhanced in presence of the bacteria, suggesting that HSAc-Phe-Leu is either consumed or destroyed by the organisms. We propose that the partial reduction of growth exerted by the hydroxamate derivatives is the result of inhibition of the extracellular proteases by these compounds. HSAc-Phe-Leu failed to inhibit growth because of its rapid loss during incubation with the bacteria. Since the three inhibitors have no antibacterial effects, their therapeutic potential should be examined in combination with antibiotics. Experimental treatment with HSAc-Phe-Leu should be frequent in order to overcome its loss in presence of the organisms.