The purification and partial characterization of human somatomedin C

Somatomedin C was purified from a Cohn IV fraction of human plasma by acid release followed by dialysis, ultrafiltration, Sephadex G-50 chromatography, and isoelectric focusing between pH 7 to 11. The resulting preparation, which focused at pH 8.6, was essentially free of insulin contamination and appeared homogeneous by acid gel chromatography and sodium dodecyl sulfate gel electrophoresis. The somatomedin C was purified about one-half-million-fold with a yield of 7%. The purified preparation had an apparent molecular weight of 7,400 with no indication of intermolecular disulfide bonds. Its amino acid composition revealed a lysine-rich peptide containing significant amounts of histidine and methionine. No free sulfhydryl was found. It stimulated in vitro cartilage sulfation and DNA synthesis of hypophysectomized rats. In the presence of hypopituitary serum, it increased cartilage sulfation of embryonic chick. Somatomedin C competed potentially with [125I]iodoinsulin for insulin receptors on human placenta cell membranes. [125I]Iodosomatomedin C apparently binds preferentialy to a high affinity placental receptor which is different from the insulin receptor.