The antielastase screen of the lower respiratory tract.

The protease-antiprotease theory of pulmonary emphysema holds that alveolar structures may be destroyed by neutrophil elastase but are normally protected from destruction by elastase inhibitors. Bronchoalveolar lavage allows to collect three types of antielastases: alpha 1-proteinase inhibitor (alpha 1 PI), bronchial mucus inhibitor and "unidentified inhibitors". Alpha 1 PI acts as an irreversible inhibitor of serine-proteinases. It reacts much faster with neutrophil elastase than with other enzymes and is therefore considered as a physiological inhibitor of this leukoproteinase. Oxidation of Met into methionine sulfoxide leads to a dramatic reduction of the inhibitory capacity of alpha 1 PI. This oxidative inactivation may be brought about by oxidants excreted by phagocytes and cigarette smoke condensate. A back-up control is provided by methionine sulfoxide reductase, an enzyme present in phagocytes and capable of reducing oxidized alpha 1 PI. The bronchial inhibitor is an acid-stable protein secreted by the mucus cells of the respiratory tract. It is a reversible tight-binding inhibitor which reacts with neutrophil elastase. Although it occurs in low amounts in the lower respiratory tract it may play an active physiological role because of its low molecular weight which allows its easy diffusion within the interstitial tissue.