Differences in the proteinase inhibition mechanism of human α2‐macroglobulin and pregnancy zone protein

Abstract

Different conformational states of human α2‐macroglobulin (α2M) and pregnancy zone protein (PZP) were investigated following modifications of the functional sites, i. e. the ‘bait’ regions and the thiol esters, by use of chymotrypsin, methylamine and dinitrophenylthiocyanate. Gel electrophoresis, mAb (7H11D6 and α 1:1) and in vivo plasma clearance were used to describe different molecular states in the proteinase inhibitors. In α2M, in which the thiol ester is broken by binding of methylamine and the ‘trap’ is closed, cyanylation of the liberated thiol group from the thiol ester modulates reopening of the ‘trap’ and the ‘bait’ regions become available for cleavage again. The trapping of proteinases in the cyanylated derivative indicates that the trap functions as in native α2M. In contrast, cyanylation has no effect on proteinase‐treated α2M. As demonstrated by binding to mAb, the methylamine and dinitrophenylthiocyanate‐treated α2M exposes the receptor‐recognition site, but the derivative is not cleared from the circulation in mice. The trap is not functional in PZP. In native PZP and PZP treated with methylamine, the conformational states seem similar. The receptor‐recognition sites are not exposed and removal from the circulation in vivo is not seen for these as for the PZP‐chymotrypsin complex. Tetramers are only formed when proteinases can be covalently bound to the PZP. Conformational changes are not detected in PZP derivatives in which the thiol ester is treated with methylamine and dinitrophenylthio‐cyanate. The results suggest that the conformational changes in α2M are generated by mechanisms different to these in PZP. The key structure gearing the conformational changes in α2M is the thiol ester, by which the events ‘trapping’ and exposure of the receptor‐recognition site can be separated. In PZP, the crucial step for the conformational changes is the cleavage of the ‘bait’ region, since cleavage of the thiol ester does not lead to any detectable conformational changes by the methods used. Copyright © 1992, Wiley Blackwell. All rights reserved