Unmasking by neuraminidase of specific chorionic gonadotrophin binding activity of human placental syncytiotrophoblast

Summary. Purified human placental syncytiotrophoblast consistently failed to bind specifically to 125I-labelled hCG. Treatment of the syncytiotrophoblast with neuraminidase resulted in the ability to bind 125I-labelled hCG that was displaceable by excess of unlabelled hCG. Neuraminidase treatment removed 73\m=.\8%of the total neuraminic acid of syncytiotrophoblast. The specific binding of 125I-labelled hCG increased linearly with increasing amount of neuraminidase-treated syncytiotrophoblast, was saturable and had a Ka= 1\m=.\6\m=x\107 M\m=-\1. Excess of GH, prolactin, placental lactogen or insulin did not inhibit the binding, whereas LH did so completely and FSH partly.