The high molecular mass band designated as В is always present with this F band on SDS-PAGE and has the same reactivity toward unlabeled competitors as the 130-kDa F band. This band is therefore an a-a dimer with two IGF-I-receptor a subunits. The third set of 125I-IGF-II-binding sites (E) migrates in the 135-kDa molecular mass region. levitra super active plus
The labeled ligand is displaced by high concentration of unlabeled insulin. The labeled complex formed with 125I-insulin that migrates in the same molecular mass position is also displaced by insulin. Wherever the 135-kDa band (E) is present, there is also a very high molecular mass labeled band (A) that has the same reactivity toward unlabeled competitors as the 135-kDa band. This band is likely to be an a-a dimer of two insulin-receptor a subunits. In the same way, band B, which is formed in 125I-IGF-I or 125I-insulin cross-linking experiments, is an ы-(3 association of a and (3 subunits of the IGF-I or insulin receptors. The varying affinities of each ligand for each of the three sets of receptors, and the different distributions of the three classes of receptors binding 125I-IGF-II—whether they are facing the maternal or the fetal side—can account for the differences in the their IC50 values for 125I-IGF-II binding to the two trophoblast membranes.