Pregnancy-associated plasma protein A: purification under mild conditions, peptide mapping and tests for possible interactions with trypsin, plasmin and complement

Isolation and characterization of circulating complex between human pregnancy-associated plasma protein-A and proform of eosinophil major basic protein

The plasma protein previously known as pregnancy associated plasma protein-A (PAPP-A) and believed to contain only one kind of polypeptide chain has recently been shown to be a complex containing two different chains in equimolar amounts. One of the chains is now defined as the PAPP-A subunit, and the other has been identified as the proform of eosinophil major basic protein (proMBP) (Oxvig et al. (1993) J. Biol. Chem. 268, 12243-12246). A procedure for large scale preparation of the circulating complex (PAPP-A/proMBP) from pooled pregnancy serum is described. The amino acid and carbohydrate compositions of the isolated reduced and carboxymethylated PAPP-A (199 kDa) and proMBP subunits (38 kDa), and of the intact PAPP-A/proMBP have been determined. The PAPP-A and proMBP subunits contain 13.4% (w/w) and 38.6% (w/w) carbohydrate, respectively, and the intact complex contains 17.4% (w/w) carbohydrate. The PAPP-A subunit contains N-bound carbohydrate groups. In contrast, the proMBP subunit contains both N- and O-bound groups as well as glycosaminoglycan, previously found among plasma proteins only in inter-alpha-trypsin inhibitor and pre-alpha-trypsin inhibitor. It is shown that PAPP-A/proMBP can competitively inhibit human leucocyte elastase (KI = (5-10) x 10(-9) M) at an ionic strength of 0.075, but the inhibition is negligible at ionic strengths greater than 0.15. Human cathepsin G is also competitively inhibited (KI approx. 1 x 10(-6) M). The inhibition of both enzymes is most likely due to interactions with the glycosaminoglycan moiety of PAPP-A/proMBP. It is concluded that PAPP-A/proMBP is neither a potent nor a specific inhibitor of human leucocyte elastase.

Molecular characterization of pregnancy-associated plasma protein-A by electrophoresis

Gradient polyacrylamide gel electrophoresis, isoelectric focusing and multidimensional immunoelectrophoretic techniques have been applied in order to physico-chemically characterize pregnancy-associated plasma protein-A (PAPP-A). By lectin affinity immunoelectrophoresis, PAPP-A contained sialic acid, glucose/mannose and N-acetyl-alpha-D-galactosamine. Immunoelectrophoretic analyses after incubation with various glycolases confirmed these findings and demonstrated that PAPP-A contained glucuronic acid, perhaps in chondroitin sulphate moities, thus indicating that PAPP-A may be a proteoglycan rather than a glycoprotein. Analysis by metal chelate and dye ligand affinity immunoelectrophoresis demonstrated many similarities between PAPP-A and alpha 2-macroglobulin (alpha 2M). However, unlike alpha 2M, PAPP-A did not form immunologically reactive complexes when incubated with proteases. Furthermore, as demonstrated by autoradiographic studies, PAPP-A did not contain internal thiolester groups, thus indicating that PAPP-A cannot inhibit proteases by molecular entrapment and, despite the homotetrameric molecular conformation, PAPP-A and alpha 2M may not have evolved from a common ancestral protein.

Binding of pregnancy-associated plasma protein-A (PAPP-A) to placental subfractions

Pregnancy-associated plasma protein-A (PAPP-A) has been shown to exert immunosuppressive effects both on complement and on lymphoblastogenesis. It was of interest to see whether this protein could bind to syncytiotrophoblast microvillous membranes since these represent the effective interface between fetal tissue and the immunocompetent mother. Placental subfractions were prepared according to established techniques. PAPP-A was purified from different sources (pregnancy serum and plasma, retroplacental serum, placental extracts) and labelled with radioactive iodine. It could be shown that radioactive PAPP-A, irrespective of its biological origin was primarily binding to brush border membrane preparations but that significant binding was also seen with plasma membrane preparations. The binding was specific since alpha 2-macroglobulin (a structurally related protein to PAPP-A) was unable to displace bound radioactive PAPP-A. Scatchard plot representation of the data indicated that the affinity of PAPP-A for its binding site was of the same order of magnitude as reported for the insulin or GnRH receptors of the trophoblast (Ka 2 10(7)-2.5 10(8) M-1). These results are in agreement with the immunohistochemical localization of PAPP-A on the trophoblast providing that the small binding to plasma membrane preparations is due to contamination.