Differential binding properties of human pregnancy zone protein- and α2-macroglobulin-proteinase complexes to low-density lipoprotein receptor-related protein

Human pregnancy zone protein (PZP) is a major pregnancy-associated plasma protein strongly related to alpha(2)-macroglobulin (alpha(2)-M). Both alpha-macroglobulins (alpha-Ms) covalently bind proteinases, which is accompanied by the exposure of carboxy terminal receptor recognition domains important for the rapid clearance from the circulation and tissues. It is accepted that the molecule responsible for the clearance of alpha(2)-M- and PZP-proteinase complexes is the low-density lipoprotein receptor-related protein (LRP). Although both alpha-M-proteinase complexes bind to the same receptor, differences in the binding properties have been reported. In addition, although it is known that the binding of alpha(2)-M-proteinase complexes to LRP can be blocked by Ni2+, the effect on PZP-proteinase has never been examined. In order to investigate differences in the binding properties of both alpha-Ms to the receptor, we purified LRP from human placenta by affinity chromatography and then analyzed the specificity and affinity of binding of alpha(2)-M-and PZP-proteinase complexes to the receptor by enzyme immunoassay. Our results clearly established that although both alpha-M-proteinase complexes specifically bind to LRP, PZP-chymotrypsin complexes bind to the receptor with lesser apparent affinity (K-d approximate to 320 nM) than alpha(2)-M-chymotrypsin complexes (K-d approximate to 40 nM). We also demonstrated that Ni2+ blocks the binding of alpha(2)-M-chymotrypsin complexes, but not PZP-chymotrypsin complexes, to LRP. These data suggest that the binding to LRP involves conformational differences between both alpha-Ms in a region immediately upstream of the carboxy terminal receptor recognition domain. The possibility that PZP-proteinase complexes interact with other receptors not available to alpha(2)-M-proteinase complexes could be considered. (C) 2002 Elsevier Science.