Inhibition of Human Mast Cell Chymase by Secretory Leukocyte Proteinase Inhibitor: Enhancement of the Interaction by Heparin

The inhibition of human chymase, a chymotrypsin-like proteinase stored in mast cell granules, by secretory leukocyte proteinase inhibitor (SLPI) is investigated in this study. SLPI is a serine proteinase inhibitor present in human mucus secretions and tissues. It binds heparin, a highly sulfated glycosaminoglycan also found in mast cell secretory granules, and this interaction increases its effectiveness as an inhibitor of neutrophil elastase. Analysis of the chymase–SLPI interaction by equilibrium and kinetic methods indicates that the inhibition of chymase results from the reversible formation of a stable 1:1 enzyme–inhibitor complex. The dissociation equilibrium constant (Ki) determined in reactions containing 0.18Mor 1.0MNaCl (pH 8.0, 25°C) was 5 × 10−8and 2 × 10−8M, respectively. Addition of heparin to the low-salt reaction decreased theKiapproximately 10-fold to a value of 3 × 10−9M, making SLPI a more effective inhibitor of human chymase. The decrease was due primarily to an approximately 10-fold increase in the association rate constant (kass) from 2 × 104to 3 × 105M−1s−1. The magnitudes of the rate and dissociation equilibrium constants indicate that SLPI has the potential to be a good chymase inhibitorin vivo,especially if chymase and heparin are released from mast cell granules simultaneously. The enhanced interaction in the presence of heparin supports the importance of this glycosaminoglycan to the inhibitory function of SLPI.