Evidence for the involvement of the transcortin carbohydrate moiety in the glycoprotein interaction with the plasma membrane of human placental syncytiotrophoblast.

We have studied the interaction of human transcortin and the pregnancy-associated transcortin variant with the microvesicular membrane fraction derived from the human placental syncytiotrophoblast. Two classes of specific binding sites for these glycoproteins were found in this membrane preparation. One of these displays a relatively high binding capacity, Bmax = 140 +/- 60 fmol transcortin per mg membrane protein, and a significantly higher affinity for transcortin, Kd = (1.6 +/- 0.6).10(-10) mol/l, than for the pregnancy-associated variant, Kd = (4.5 +/- 1.2).10(-9) mol/l. On the contrary, another class of the binding sites, occurring in the membranes at a far lower concentration: Bmax = 3.0 +/- 2.2 fmol transcortin per mg membrane protein, shows a higher affinity for the pregnancy-associated transcortin variant, Kd = (3.3 +/- 2.0).10(-12) mol/l, than for normal transcortin, Kd = (2.5 +/- 0.7).10(-11) mol/l. Since the pregnancy-associated variant differs from normal transcortin with respect to its carbohydrate structures only (Avvakumov, G.V. and Strel'chyonok, O.A. (1987) Biochim. Biophys. Acta 925, 11-16), the results of the present work suggest that the transcortin carbohydrates are directly involved in the specific interaction of this serum hormone-binding globulin with the plasma membrane of the placental syncytiotrophoblast.