The interaction between rat plasma alpha 1 inhibitor 3 and chymotrypsin. A protease-protease inhibitor system which...

Complexation of chymotrypsin with rat alpha 1 inhibitor 3 (alpha 1I3) leads to a significant modification of circular dichroism (CD) spectra in the near ultraviolet. These spectra variations are due to structural changes of the inhibitor resulting from partial proteolysis. Taking advantage of this change in CD spectral properties a 1:1 stoichiometric ratio was determined for the interaction. However, when an excess of inhibitor was present in the mixture, a slow change of the CD spectrum was observed. This was shown to result from a partial proteolysis of native inhibitor identical to that occurring during complex formation. This hydrolysis was due to the remaining activity of the bound enzyme since no significant release of free enzyme from the complex was demonstrated. Addition of human alpha 1 proteinase inhibitor (alpha 1 Pi) to dissociate the complex resulted in the formation of an enzymatically inactive ternary complex which gave evidence for the stability of the alpha 1I3 chymotrypsin binding.