The somatomedin-C receptor of human placenta

The binding and physical characteristics of the human placenta somatomedin-C (Sm-C) receptor were investigated and compared with those of the placental insulin receptor. These receptors were found to have many similar properties, but several distinctions exist between them. The two receptors have similar binding properties. Although distinct receptors, as evidenced by specificity studies, the Sm-C and insulin receptors possess similar affinities (10 X 10(9) L/mol and 3 X 10(9) L/mol, respectively) and receptor concentrations (0.4 pm/mg and 0.2 pm/mg) in placental membranes. There is also a low-affinity, high-capacity receptor for each hormone. The intact receptors are indistinguishable in size by three methods. The Stokes radius is 7.2 nm by gel filtration, the sedimentation coefficient is 11S, and the SDS polyacrylamide gel apparent molecular weight is approximately 350,000 daltons. The two receptors differ in their pH optimum for ligand binding, their isoelectric points, and their subunit size. The pH optimum for Sm-C binding is 8.2, and for insulin binding the optimum is 7.9. The isoelectric point for the Sm-C receptor is 4.6, whereas the isoelectric point for the insulin receptor is 4.2. Although the whole receptor and the half receptors for Sm-C and insulin cannot be distinguished by size on SDS gels, the binding subunit for the Sm-C receptor appears to be 8000 daltons smaller than that of the insulin receptor (134,000 v 142,000 daltons). We conclude from these studies that the insulin and Sm-C receptors of human placenta are similar in structure but are distinct entities.