Antichymotrypsin interaction with chymotrypsin. Partitioning of the complex

The interactions of bovine pancreatic chymotrypsin (Chtr) and recombinant alpha 1-antichymotrypsin (rACT) and rACT variants were studied by kinetic and gel electrophoretic analyses, leading to the formulation of a general kinetic scheme that accounts for all known results concerning this serpin-protease pair, as well as for results obtained with other such pairs. Incubation of rACT and Chtr leads rapidly to the formation of an inhibited complex, Chtr.rACT*, that is stable toward sodium dodecyl sulfate denaturation and boiling. The extent of release of active Chtr from this complex increases markedly as ionic strength, mu, is raised. The kinetic scheme quantitatively accounts for this effect on the basis of a partitioning of Chtr.rACT* between dissociation of the complex to yield active enzyme and cleaved rACT, and Chtr-catalyzed conversion of the complex to a form that is much more resistant to release of active enzyme. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses of reaction mixtures of rACT and Chtr are consistent with the scheme. Also consistent are the results of experiments measuring the effects of 1) Chtr.rACT* concentration, 2) uncomplexed Chtr, and 3) added alpha 2-macroglobulin on active Chtr release from Chtr.rACT*. Proteolysis of Chtr.rACT* to give a resistant complex is also catalyzed by human neutrophil elastase, a process with potential physiological relevance. Comparison of the rates of Chtr dissociation from the complexes formed with rACT and with rACT variants mutated at the P1 site suggests that such rates are more sensitive to P1 substitution at low mu than at high mu. Several equivalents of the L358R-rACT variant are required for full inhibition of Chtr. This observation is also quantitatively accounted for by the proposed kinetic scheme, on the basis of another partitioning step between L358R-rACT acting as a substrate or as an inhibitor toward Chtr.