Inhibition of neutrophil elastase by the α1‐proteinase inhibitor‐immunoglobulin A complex

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Abstract
Neutrophil elastase is thought to be involved in cartilage destruction occurring in rheumatoid arthritis despite the local presence of α 1‐proteinase inhibitor. Part of synovial fluid α 1‐proteinase inhibitor forms a mixed disulfide with immunoglobulin A, which has been postulated to lack inhibitory activity. We show here that the immunoglobulin‐inhibitor complex tightly inhibits neutrophil elastase and cathepsin G, bovine pancreatic trypsin and chymotrypsin, and porcine pancreatic elastase. Although the rate constant of inhibition of neutrophil elastase by immunoglobulin A‐bound α 1‐proteinase inhibitor (k ass = 9.2 × 105 M−1 · s−1) is about 10‐fold lower than that measured with the free inhibitor, it is high enough to enable efficient inhibition of elastase in vivo.