The immunological assessment of alpha 1-antitrypsin with reference to its function in bronchial secretions

The quantification of alpha 1-antitrypsin (alpha 1AT) by standard immunological techniques is altered by interaction of the protein with leucocyte elastase. The results obtained for alpha 1-antitrypsin-leucocyte elastase mixtures in the presence of a functional excess of the inhibitor were relatively accurate for the first 6 h. However, continued incubation for more than 24 h led to a major overestimation of the alpha 1AT as the result of breakdown of the enzyme-inhibitor complex releasing a partially proteolysed form of the inhibitor. In the presence of excess enzyme up to a twofold overestimation of alpha 1AT occurred within 1 h and the degree of overestimation increased with time (up to threefold at 24 h). This was eventually associated with the presence of only a partially proteolysed form of alpha 1AT (mol wt. approximately equal to 50 000). Different results for each sample were obtained when different polyclonal antisera were used to quantify the alpha 1AT. Complete inactivation of alpha 1AT by oxidation resulted in little change in the immunological quantification of the protein. However, further addition of H2O2 led to a progressive underestimation of the alpha 1AT. The effect of physiochemical alteration on the immunological quantification of alpha 1AT by different antisera should be borne in mind for all studies assessing this protein in lung secretions.