Circular dichroism spectroscopy, sedimentation velocity and ultraviolet difference spectroscopy were used to compare alpha 2-macroglobulin, alpha 2-macroglobulin-trypsin complex and alpha 2-macroglobulin-methylamine complex. The circular dichroic spectrum of native alpha 2-macroglobulin is significantly changed in shape and magnitude following reaction with either trypsin or methylamine. The spectra of alpha 2-macroglobulin-trypsin and alpha 2-macroglobulin-methylamine are, however, indistinguishable. The ultraviolet difference spectrum between alpha 2-macroglobulin-methylamine and native alpha 2-macroglobulin displays a tyrosine blue shift consistent with the exposure of several tyrosine residues to solvent. The conformational change which occurs in alpha 2-macroglobulin during reaction with methylamine follows pseudo-first-order kinetics. T 1/2 was 10.5 min for the reaction with 200 mM methylamine at pH 8.0 and 45 min for the reaction with 50 mM methylamine, also at pH 8.0. Reaction of methylamine with alpha 2-macroglobulin results in loss of trypsin-binding activity which appears to be a direct consequence of the conformational change induced by methylamine. A sedimentation coefficient (S0(20),W) of 20.5 was determined for alpha 2-macroglobulin-methylamine compared to a value of 18.5 for unreacted alpha 2-macroglobulin. This increase in sedimentation velocity is attributed to a 10% decrease in alpha 2-macroglobulin Stokes radius. alpha 2-Macroglobulin-trypsin complex prepared by reaction of the protease at a 2-fold molar excess with the inhibitor was a S0(20),W of 20.3. Although this sedimentation coefficient does reflect compacting of the alpha 2-macroglobulin structure compared to native alpha 2-macroglobulin, it is not large enough to rule out significant protrusion of the proteases from pockets in the alpha 2-macroglobulin structure.