Purification of somatomedin-C from human plasma: chemical and biological properties, partial sequence analysis, and relationship to other somatomedins

Abstract

Somatomedin-C (SM-C) was purified from Cohn fraction IV1-4 of human plasma by a series of steps which included cation-exchange chromatography, gel filtration, isoelectric focusing and reverse-phase high-pressure liquid chromatography. The isolated peptide contained 10,142 units/mg, as judged by a radioimmunoassay [RIA], representing a 780,000-fold purification from native plasma. The isolated peptide was basic (pI [isoelectric point] 8.1-8.5) and was judged to be no less than 90% pure. The best fit for integral amino acids was obtained with 78 residues. Limited sequence data were obtained on the N terminus and on 5 fragments obtained by tryptic digestion after blocking the Lys residues. Three of these fragments and the N terminus could be aligned with portions of insulin-like growth factor I (IGF-I). Of the 25 residues so aligned, 22 were identical with IGF-I. In addition, 2 tryptic fragments were obtained which are not present in the sequence of IGF-I. SM-C and IGF-I produced identical curves of displacement in RIA and radioreceptor assays for SM-C and in an insulin radioreceptor assay. The potencies of SM-C and IGF-I in these assays differed significantly from IGF-II, somatomedin-A and several preparations of multiplication stimulating activity. At a concentration of 1 ng/ml, SM-C stimulated the progression of [mouse] Balb/c 3T3 cells into DNA synthesis and, when injected in vivo, restored mitosis in lens epithelium of hypophysectomized frogs.