These values are similar to those reported for rat total placental membranes , ovine total placental membranes , and other tissues. There is one set of a subunits of the type 1 IGF receptor on basal plasma membranes that accounts for 30% of the total 125I-IGF-II bound. As reported by others for membranes of intact cells, the affinity of this type 1 IGF receptor for IGF-I is 0.2-1.0 nM, while its affinity for IGF-II is 2- to 15-fold lower. Consequently, the 125I-IGF-II bound by this receptor on the basal plasma membranes may account for the “pseudo-negative” cooperativity among 125-IGF-II-binding sites because there are in fact, two binding sites. These a subunits of the type 1 IGF receptor may have two origins, although our PAGE method does not distinguish between them: the real type 1 IGF receptor and a population of hybrid receptors. Populations of IGF-I/insulin hybrid receptors have been found in several human cell lines and total placental membranes. These chimeras consist of an a-(3 dimer of the IGF receptor linked to a-0 dimer of the insulin receptor, and their ligand properties resemble those of the type 1 IGF receptor. birth control pills
A population of atypical insulin receptors with a moderately high affinity for IGF-I and IGF-II (Kd 2-4 nM) and high affinity for insulin (0.2-0.6 nM) has also been purified from total human placenta. These receptors have the same structure as insulin and type 1 IGF receptors, and they make up 6-18% of the total insulin receptors. The a subunits of that receptor may include an appreciable fraction of the 125I-IGF-II bound to the basal plasma membranes in our system, although we cannot determine which of the two types of insulin-receptor a subunit is present.