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.
Lubega
J
J
Department of Chemical Pathology, Leicester Royal Infirmary.
D001777:Blood Coagulation; D005260:Female; D005337:Fibrin / Q000032:analysis; D006801:Humans; D008431:Maternal-Fetal Exchange; D008565:Membrane Proteins / Q000032:analysis*; D008938:Mitosis; D010920:Placenta / Q000502:physiology; D011247:Pregnancy; D011956:Receptors, Cell Surface / Q000032:analysis*; D018179:Receptors, Thrombin; D013917:Thrombin / Q000378:metabolism*; D014327:Trophoblasts / Q000737:chemistry*
D008565:Membrane Proteins; D011956:Receptors, Cell Surface; D018179:Receptors, Thrombin; D005337:Fibrin; D013917:Thrombin