The effect of alpha 2-macroglobulin on the interaction of alpha 1-proteinase inhibitor with porcine trypsin.

The rate of dissociation of the alpha 1-proteinase inhibitor:porcine trypsin complex was compared with that in the presence of alpha 2-macroglobulin. In the presence of the latter inhibitor the dissociation was more rapid and active alpha 1-proteinase inhibitor could be recovered in the mixture. However, no active inhibitor could be detected after dissociation in the absence of alpha 2-macroglobulin. This recovery of active alpha 1-proteinase inhibitor from complexes with porcine trypsin is the first demonstration of a thermodynamic equilibrium between this inhibitor and proteinase. Consequently, the transfer of trypsin from complexes with alpha 1-proteinase inhibitor to alpha 2-macroglobulin may be explained as a passive phenomenon which does not require a physical collision between alpha 2-macroglobulin and the alpha 1-proteinase inhibitor:porcine trypsin complex. The dissociation of the complex occurs more rapidly in the presence of alpha 2-macroglobulin because this inhibitor complexes trypsin leaving the alpha 1-proteinase inhibitor:porcine trypsin complex by both the irreversible breakdown step and by reversible dissociation of the complex.