Differential Binding Properties of Human Pregnancy Zone Protein– and α2-Macroglobulin–Proteinase Complexes to Low-Density Lipoprotein Receptor-Related Protein

Abstract

Human pregnancy zone protein (PZP) is a major pregnancy-associated plasma protein strongly related to α2-macroglobulin (α2-M). Both α-macroglobulins (α-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 α2-M– and PZP–proteinase complexes is the low-density lipoprotein receptor-related protein (LRP). Although both α-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 α2-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 α-Ms to the receptor, we purified LRP from human placenta by affinity chromatography and then analyzed the specificity and affinity of binding of α2-M– and PZP–proteinase complexes to the receptor by enzyme immunoassay. Our results clearly established that although both α-M–proteinase complexes specifically bind to LRP, PZP–chymotrypsin complexes bind to the receptor with lesser apparent affinity (Kd ≈ 320 nM) than α2-M–chymotrypsin complexes (Kd ≈ 40 nM). We also demonstrated that Ni2+ blocks the binding of α2-M–chymotrypsin complexes, but not PZP–chymotrypsin complexes, to LRP. These data suggest that the binding to LRP involves conformational differences between both α-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 α2-M–proteinase complexes could be considered.