The Effect of Reducing Agents on Proteolytic Enzymes and Oxidation of α1-Proteinase Inhibitor

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Abstract
We have studied the effect of the mucolytic agent N-acetylcysteine and dithiothreitol on the oxidation of .alpha.1-PI by hydrogen peroxide, and their effect on porcine pancreatic elastase and leukocyte elastase. In addition, the effect of S-(carboxymethyl)cysteine (= carbocisteine, a mucolytic agent which does not have reducing properties) was studied in vitro and in patients with chronic obstructive bronchitis. Following addition of 59.6 mM N-acetylcysteine, the amidolytic activity of leukocyte elastase was decreased by 55.3% and that of porcine pancreatic elastase by 57.0%. Dithiothreitol (5.7 mM) caused the loss of 97.4% and 67.6% of amidolytic activity of leukocyte elastase and porcine pancreatic elastase respectively whereas S-(carboxymethyl)cysteine had no effect. Similar results were found for the effect on elastolytic activity. Oxidation of .alpha.1-PI by 8.6 mM H2O2 resulted in partial loss of inhibitory function (mean 68.7% activity of native .alpha.1-PI). N-Acetylcysteine and dithiothreitol prevented oxidation of .alpha.1-PI when pre-incubated with H2O2 or incubated with .alpha.1-PI and H2O2 simultaneously (94.5% and 94.4% activity of native .alpha.1-PI for N-acetylcysteine; 78.3% and 87.6% activity for dithiothreitol - p < 0.025). S-(Carboxymethyl)cysteine, when pre-incubated with H2O2 or incubated concurrently with .alpha.1-PI and H2O2, caused a further decrease in the porcine pancreatic elastase inhibitory capacity of .alpha.1-PI (53.1% and 63.0% respectively - p < 0.025). None of the agents reversed oxidative inactivation once it had occurred. S-(Carboxymethyl)cysteine had no effect on .alpha.1-PI function in sputum at the dose used.

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