1. We investigated the effect of stimulated human polymorphonuclear leucocytes (PMN's) on both the antitrypsin and anti-elastase activity of bronchial antileucoprotease (ALP).

2. Incubation of ALP with stimulated human PMN's resulted in a rapid loss of anti-elastase activity which paralleled that of the antitrypsin activity, suggesting that both inhibitor activities are represented by the same active site.

3. The myeloperoxidase-oxidizing system was found to be responsible for the inactivation of ALP.

4. The oxidized inhibitor was unable to form stable complexes with PMN elastase but was resistant to breakdown by proteolytic enzymes from stimulated PMN's.

5. It was observed that stimulated cells are capable of releasing elastase which shows full activity in the presence of a large molar excess of ALP.

6. We conclude from this study that stimulated PMN‘s are able to inactivate ALP by which released elastase is able to express enzymatic activity in spite of the presence of this low-molecular-weight inhibitor. Thus, inactivation of ALP by triggered PMN's may contribute to destructive processes in which elastase is thought to be a mediator.

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