Reactions of α2-macroglobulin (α2M) with primary amines (ammonium chloride, methylammonium chloride and ethylammonium chloride) or proteolytic enzymes (trypsin, chymotrypsin and thrombin) resulted in changes of the absorption, fluorescence and circular-dichroism spectra and of the sedimentation coefficient of the inhibitor. All physico-chemical changes caused by the inactivation of α2M by the amines were identical with, or highly similar to, those induced by the formation of the enzyme–inhibitor complexes. This suggests that similar conformational changes of the inhibitor occur in the two types of reactions. The frictional ratio, calculated from the increase in sedimentation coefficient, decreased from 1.67 for untreated α2M to 1.57 for the amine- or proteinase-treated inhibitor. This change is due to a decrease in either asymmetry or hydration of the protein, resulting in a slightly smaller hydrodynamic volume. The circular-dichroism analyses indicated that the reaction of α2M with either amines or proteinases is accompanied by a loss of the small amount (about 5%) of α-helix of the untreated protein. The changes of u.v. absorption and fluorescence suggested that about one out of the eight to ten tryptophan residues of each α2M subunit is buried as a result of the conformational change. All spectroscopic and hydrodynamic changes that were observed are compatible with a spatial rearrangement of the subunits of α2M, as implicated by the ‘trap’ hypothesis for the mechanism of inhibition of proteinases. However, a conformational change involving a decrease in the hydrodynamic volume of each subunit cannot be excluded.
Activated Human Plasma Carboxypeptidase B Is Retained in the Blood by Binding to α2-Macroglobulin and Pregnancy Zone Protein