The effect of assay conditions on the measurement of anti-elastase function in lung secretions

Abstract

We have determined the effect of altering assay conditions on the observed neutrophil elastase inhibitory capacity in lung secretions from emphysematous patients with normal serum α 1 PI. α 1 PI, ALP and α 2-macroglobulin were detected in all samples. Measurement at low enzyme concentration (< 33.6 nmol/l) caused a 43% reduction in observed neutrophil elastase inhibitory capacity of sputum sol-phase, while inhibition by secretions in buffer without added NaC1 was 20% greater than in the presence of 0.2 mol/l NaCl. Increasing the concentration of the synthetic substrate Suc-[Ala] 3-pNA in the assay from 0.45 mmol/l to 7.2 mmol/l reduced the observed inhibitory capacity by 53% and the use of elastin-fluorescein gave lower results for inhibitory capacity than the Suc-[Ala] 3-pNA (median 0.26 mol neutrophil elastase/mol measured inhibitors (range 0–0.72) with elastin; 1.40 mol neutrophil elastase/mol measured inhibitors (0.80–3.21) with Suc-[Ala] 3-pNA). Assay conditions therefore greatly influence the results. In addition these findings suggest the presence of an additional inhibitor of neutrophil elastase in these secretions.