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.