Inactivation of the plasma protease inhibitor alpha 2-macroglobulin by the antitumor drug cis-dichlorodiamineplatinum(II)

Abstract

The plasma protease inhibitor alpha 2-macroglobulin (alpha 2M) was reacted in vitro with cis-dichlorodiamineplastinum(II) (cis-DDP). Following the reaction, alpha 2M demonstrated a significantly decreased ability to bind trypsin as determined by esterase activity assays in the presence of soybean trypsin inhibitor and studies with radiolabeled trypsin. Inactivation of alpha 2M by cis-DDP was not associated with a conversion to the "fast" electrophoretic form, as determined on nondenaturing gels, in contrast to the inactivation of alpha 2M by proteases and certain amine salts. The extent of reaction increased with the elevation of temperature within the thermal stability range of the protein; however, variation of pH within the range 6.82-8.55 had little effect. Binding of [14C]methylamine to alpha 2M was not affected by cis-DDP. The conformational change, however, which normally accompanies this reaction did not occur. It is concluded that the alpha 2M thiolesters are most likely not reactive sites for cis-DDP. cis-DDP-treated alpha 2M failed to dissociate into quarter subunits under denaturing and reducing conditions, suggesting cross-linking of subunits. This cross-linking may be responsible for locking the alpha 2M quarternary structure into the "slow conformation."