Dissociation of alpha 2 M-macroglobulin into functional half-molecules by mild acid treatment.

A slight decrease in pH below neutrality causes the dissociation of alpha 2-macroglobulin (alpha 2M) into dimers formed of two disulfide-bonded subunits. Half-dissociation occurs at pH 6.30 (50 mM NaCl), as determined by gel filtration analysis. The dissociation can be reversed either by increasing the pH or the ionic strength. The ability of alpha 2 M half-molecules at pH 5.75 to bind chymotrypsin is not too different from that of the whole molecule at pH 7.5. Furthermore, the steady-state kinetic parameters toward chromogenic substrate of chymotrypsin bound to alpha 2 M half and whole molecules are quite identical. Likewise, the accessibility of trypsin toward soybean trypsin inhibitor is also fairly similar when involved in half or whole alpha 2 M complexes. These results are consistent with the idea that alpha 2 M-half molecules on chymotrypsin binding undergo a conformational change. This change can be observed by electron microscopy.