Somatomedin receptor of human placenta: solubilization, photolabeling, partial purification, and comparison with insulin receptor.

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Abstract
Using a recently isolated human basic somatomedin (basic SM) similar to insulin-like growth factor I (IGF-I), the photoaffinity-labeled and unlabeled basic-SM receptor solubilized from human placental cell membranes were studied. 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 .apprxeq. 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 .apprxeq. 3 .mu.M). 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, 1 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 MW of .apprx. 140,000; in the absence of reducing agent, a MW 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. The basic-SM receptor of human placenta apparently is a glycoprotein, remarkably similar to (an isoreceptor) but distinct from the insulin receptor previously characterized in this tissue.