Oxidation of 2-mercaptoacetyl-phenylalanyl-leucine, a specific inhibitor of Pseudomonas aeruginosa elastase, in the presence of the bacteria and its prevention

The compound 2-mercaptoacetyl-l-phenylalanyl-l-leucine (HSAc-Phe-Leu) is a specific and potent inhibitor of Pseudomonas aeruginosa elastase with demonstrated potential as a drug for the treatment of Pseudomonas eye infections. The thiol moiety of this compound is lost in the presence of the bacterial at a rate greater than that seen in the absence of the organisms. Tritium-labeled inhibitor was prepared and used to examine whether this accelerated loss is due to the uptake of the inhibitor by the bacteria or due to its modification by some bacterial products. Our results excluded by possibility of uptake and indicated that the loss of the thiol resulted from its oxidation to the inactive disulfide form. The oxidation reaction is probably catalyzed by a low molecular weight extracellular bacterial product, and is effectively prevented in the presence of sodium bisulfite. It is suggested that HSAc-Phe-Leu preparations used for further investigation of the inhibitor's therapeutic potential should include antoxidants such as sodium bisulfite to provide maximal inhibitory capacity.