Cross-competition for binding of α₁-antitrypsin (α₁ AT)-elastase complexes to the serpin-enzyme complex receptor by other serpin-enzyme complexes and by proteolytically modified α₁ AT

The serpin-enzyme complex (SEC) receptor recognizes a pentapeptide neo-domain of α₁-antitrypsin (α₁ AT)-elastase complexes and, in so doing, mediates internalization and intracellular catabolism of the macromolecular complex, mediates an increase in synthesis of α₁ AT, and elicits neutrophil chemotactic activity. In previous studies we have shown that this pentapeptide domain is highly conserved among members of the serpin family and that binding of a synthetic peptide corresponding to this region (¹²⁵I-peptide 105Y, SIPPEVKFNKPFVYLI, based on α₁ AT sequence 359-374) to HepG2 cells is blocked by several serpin-enzyme complexes. To determine whether the SEC receptor is the primary HepG2 cell surface binding site for these serpin-enzyme complexes, we examined the capacity for serpin-enzyme complexes to compete with each other for binding to the SEC receptor. The results indicate that binding of ¹²⁵I-elastase-α₁ AT complexes is blocked by thrombin-antithrombin III (ATIII), thrombin-heparin cofactor II, and cathepsin G-α₁-antichymotrypsin (α₁ ACT) complexes. Moreover, unlabeled elastase-α₁ AT complexes compete for binding of ¹²⁵I-thrombin-ATIII, ¹²⁵I-thrombin-heparin cofactor II, and ¹²⁵I-cathepsin G-α₁ ACT complexes. Preformed soluble tissue plasminogen activator-plasminogen activator inhibitor 1 complexes also compete for binding of elastase-α₁ AT complexes to the SEC receptor but do so to a less effective extent, probably because of a less favorable pentapeptide sequence for binding to the SEC receptor. Under conditions in which these serpin-enzyme complexes would be expected to bind to the SEC receptor there is an increase in synthesis of on AT but not in synthesis of ATIII or α₁ ACT. Proteolytically modified α₁ AT also competes for binding of ¹²⁵I-elastase-α₁ AT complexes to the SEC receptor and vice versa. The purified 51-kDa amino-terminal fragment of α₁ AT does not compete for binding of ¹²⁵I-elastase-α₁ AT complexes, indicating that the pentapeptide neodomain in the 4-kDa carboxyl-terminal fragment is sufficient for binding to the SEC receptor.