Thrombin receptor on the placental syncytiotrophoblastic plasma membrane

Purified placental syncytiotrophoblastic membrane has been used in a radioreceptor assay to study the binding of tritium radiolabeled human thrombin. Binding was found to be saturable at higher membrane concentrations when using a fixed amount of ligand and showed a hyperbola analogous to enzyme-substrate binding. A Scatchard plot was linear and revealed homogeneous binding sites with a high-affinity constant Ka = 3 X 10(10) M-1 and capacity of 3.05 X 10(11) sites/mg of membrane protein. This high-affinity compares well with chick and embryo cell thrombin receptor which has homogeneous sites and high-affinity in contrast to platelet thrombin receptor which exhibits multiple binding sites and cooperative effects as previously noted. A thrombin receptor on the placenta might serve to mobilize thrombin into placental tissue leading to conversion of fibrinogen into fibrin, fibrinoid necrosis being so common in certain placentae. A receptor-mediated transplacental passage of thrombin into the fetal circulation is also proposed.