This report describes the J25I-IGF-II-binding sites in the trophoblast of the human full-term placenta, as well as their distribution over the brush-border microvillous and basal plasma membranes, which are the maternal-placental and the fetal-placental frontiers of the trophoblast. Both membranes possess type 1 and type 2 IGF receptors and insulin receptors, but the number and distributions of the receptors on the membranes differ. ventolin inhaler
The Kd of these binding sites for 125I-IGF-II are of an order of magnitude similar to those reported for human total placenta membranes , but the Kd for the microvillous membranes differ significantly from that of the basal plasma membranes. The 125I-IGF-II-binding sites in the microvillous membranes have no cooperative characteristics (Hill number value: 0.84), while the basal plasma membrane binding sites have a negative pattern (Hill number value: 0.33). This lack of cooperativity in the microvillous membranes is remarkable, since the PAGE of cross-linking experiments indicated that microvillous membranes are rich in insulin-receptor a subunits. This suggests that the affinity of this insulin receptor for 125I-IGF-II is close to that of the type 2 IGF receptor for 125I-IGF-II, since the a subunits of the insulin receptor account for about 50% of the 125I-IGF-II total binding activity. The negative cooperativity of IGF-II-binding sites in the basal plasma membranes may have two causes. First, the already bound ligand may prevent a second ligand’s binding to the same receptor or to a neighboring receptor. Second, there may be another type of receptor whose Kd for the same ligand is so close to that of the first type that only a single apparent Kd for a single binding site is obtained with a correlation coefficient of r = 0.92.