Development and evaluation of an ELISA for quantification of human alpha-1-proteinase inhibitor in complex biological mixtures

Human alpha-1-proteinase inhibitor1 (α1-PI) is the most abundant serine protease inhibitor in plasma. Its major function is inhibition of neutrophil elastase in lungs. α1-PI deficiency may result in severe, ultimately fatal emphysema. Three plasma-derived (pd-) α1-PI products are licensed in the US for replacement therapy of deficient patients. The recombinant versions (r-α1-PI), proposed as alternatives to pd-α1-PI products, have been under intensive investigation. For accurate determination of α1-PI from different sources and in various forms, there is an obvious need for reliable standardized assays for α1-PI quantification and potency measurements. As a part of our multi-step research focused on α1-PI structure-function investigation, we have established a simple and reproducible double-sandwich ELISA based on commercially available polyclonal antibodies. The developed ELISA allows the quantification of both pd-α1-PI and r-α1-PI in various complex matrices. A validation of the ELISA was performed with the working range of the assay (3.1-50 ng/ml) established on the bases of the following parameters: linearity (3-100 ng/ml, r2 = 0.995); accuracy (87.3-114.6% recovery); intra-assay precision (%CV, 2.8%); inter-assay plate-to-plate precision (3.9% per day and 4.1% day-to-day); detection limit (1.10 ng/ml); and quantification limit (3.34 ng/ml). The analytical performance of the α1-PI ELISA indicates that this assay can be used for monitoring concentration levels of α1-PI in multi-component biological matrices, based on the following: (a) quantification of r-α1-PI in various fermentation mixtures (E. coli and A. niger); (b) investigation of α1-PI enzymatically digested in the conditions of harsh fungal proteolysis; (c) evaluation of thermally polymerized α1-PI; (d) quantification of α1-PI in human serum; and (e) comparative quantification of α1-PI in commercially available products.