The Binding of Insulin and Somatomedin A to Human Placental Membrane

A particulate membrane fraction from human placental membrane was shown to be rich in binding sites not only for insulin but also for somatomedin A. The binding of the 125I-labelled peptide was time and temperature dependent. Degrading activity present in the membrane fraction was negligible at +4 degrees C. The Scatchard plot for insulin binding revealed two types of binding sites with an apparent high affinity constant of 3.8 times 108 M(-1) and with 5.4 times 10(-9) moles of binding sites per mg of membrane protein. The Scatchard analysis of somatomedin A revealed two classes of binding sites with an apparent high affinity constant of 2.7 times 107 M(-1) and with 1.9 times 10(-8) moles of binding sites per mg of membrane protein. In high concentrations insulin interfered with the specific binding sites for somatomedin A and vice versa. In comparison with insulin the somatomedin A preparation was one million times more potent in displacing labelled somatomedin A than in displacing labelled insulin from their respective binding sites. A radioreceptor assay utilizing particulate placental membrane and labelled somatomedin A purified on the membrane enabled the determination of somatomedin in unextracted serum. The mean values of somatomedin A in sera from patients with pituitary dwarfism and acromegaly were 0.57 and 3.2 U/ml, respectively by radioreceptor assay and 0.41 and 1.61 U/ml, respectively by bioassay. Various causes of this discrepancy between the methods are discussed.