Reversible dissociation of reduced subunits of human γM-globulins into 5 S component

Abstract

The subunits isolated from γM-globulin preparation reduced with 0·1 M mercaptoethanol have been shown to dissociate into the 5S components at pH lower than 6·0. A study of these subunits in an ultracentrifuge equipped with u.v.-optics (with scanner) has revealed that at the protein concentration lower than 2 mg/ml and pH 8·6 the sedimentation coefficient decreases to reach 5 S at 0·02 mg/ml. The pattern of sedimentation coefficient vs. concentration curve shows the existence of reversible equilibrium between the subunits and the 5 S component. This component is proved to be halves of the four chain subunits. Sedimentation coefficients of subunits with intact interchain disulphide bonds have a common linear dependence on concentration in the wide range of protein concentrations. The difference in sedimentation coefficient between the subunits with reduced and intact interchain disulphide bonds is ascribed to reversible dissociation of the reduced subunits into halves. A conclusion is made that no specific 7 S form of subunits exists.