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\s=deg\C. The Scatchard plot for insulin binding revealed two types of binding sites with an apparent high affinity constant of 3.8\m=x\ 108 m\ m=-\ 1 and with 5.4 \ m=x\10\m=-\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 \m=x\107 m\ m=-\ 1 and with 1.9\m=x\10\m=-\8moles 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.
Somatomedin A was determined by radioreceptor assay in serum from patients with various disorders. The mean values of somatomedin A in serum from 20 adult subjects, 21 patients with acromegaly, and 17 patients with hypopituitary dwarfism were 1.08 ± 0.08, 3.02 ± 0.33 and 0.44 ± 0.03 U/ml, respectively. In children below 2 years of age, low levels were found (mean 0.40 ± 0.04 U/ml). Normal levels were found in patients with Turner's syndrome and primary hypothyroidism, increased levels in uraemic patients, and decreased levels in Laron dwarfs.
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