Estimation of somatomedin-C levels in normals and patients with pituitary disease by radioimmunoassay

assay for somatomedin-C has for the first time made it possible to discriminate between serum concentrations of a single peptide or closely related group of peptides and the net somatomedin activity measured by less specific bioassay and radioreceptor techniques. Anti-bodies to human somatomedin-C were raised in rabbits using a somatomedin-C ovalbumin complex as the anti-gen. A variety of peptide hormones at concentrations up to 1,uM are not recognized by the antibody. Insulin at concentrations>0.1,uM cross reacts in a non-parallel fashion; purified somatomedin-A is only 3 % as active as somatomedin-C; and radiolabeled cloned rat liver multiplication stimulating activity does not bind to the antibody. Immunoreactive somatomedin-C can also be quantitated in the sera of a variety of subhuman species. Unusual assay kinetics, which are manifest when reactants are incubated under classic "equilibrium" assay conditions, appear to result from the failure of 1251-somatomedin-C to readily equilibrate with the somatomedin-C serum binding protein complex. It is, therefore, necessary to use nonequilibrium assay con-ditions to quantitate somatomedin-C in serum. With this assay it is possible to detect somatomedin-C in normal subjects using as little as 0.25,ul of unex-tracted serum. Serum somatomedin-C concentrations in normal subjects were lowest in cord blood and rose rapidly during the first 4 yr of life to near adult levels. In 23 normal adult volunteers, the mean seru