title: Somatomedin receptor of human placenta: solubilization, photolabeling, partial purification, and comparison with insulin receptor.
abstract: Using a recently isolated human basic somatomedin (basic SM) similar to insulin-like growth factor I (IGF-I), we studied both the photoaffinity-labeled and unlabeled basic-SM receptor solubilized from human placental cell membranes. Unlike the result with the insulin receptor, high yields of soluble basic-SM-binding activity are obtained with Triton X-100. The soluble basic-SM receptor retains high-affinity (Kd approximately 0.3 nM) peptide-specific binding of basic SM, similar to the binding present in particulate placenta membranes; the receptor exhibits a comparatively low affinity for insulin (Kd approximately 3 microM). On Sepharose 6B, like the crude soluble insulin receptor, the basic-SM receptor migrates as a species with an apparent Stokes radius of 7.2 nm; unlike the insulin receptor, the basic-SM receptor does not, under similar conditions, yield a smaller binding species (apparent Stokes radius 3.8 nm). Upon photoaffinity labeling with 125I-labeled basic SM, one principal specifically labeled constituent is detected. Upon gel electrophoresis in the presence of 2-mercaptoethanol, the photolabeled constituent, like the insulin receptor, migrates as a species with an apparent molecular weight of about 140,000; in the absence of reducing agent, a molecular weight greater than 240,000 is observed. Lectin-agarose affinity chromatography yields a 30-fold purification both of the basic-SM-binding activity and the photolabeled constituent. Anti-insulin receptor antibody does not appear to precipitate the basic-SM receptor. We conclude that the basic-SM receptor of human placenta is a glycoprotein, remarkably similar to (an isoreceptor) but distinct from the insulin receptor previously characterized in this tissue.
mesh_terms: D005260:Female; D006801:Humans; D007526:Isoelectric Point; D008970:Molecular Weight; D010777:Photochemistry; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D011972:Receptor, Insulin / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000302:isolation & purification* / Q000378:metabolism; D017451:Receptors, Somatomedin; D012995:Solubility; D013002:Somatomedins / Q000378:metabolism*
chemical_list: D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D013002:Somatomedins; D011972:Receptor, Insulin