Demonstration of Specific Plasma Protein Binding Sites For Somatomedin

Abstract

Native somatomedin (SM) has an apparent mol wt of at least 60,000 daltons, while all the purified SM peptides have mol wt 5-8,000 daltsons. We have studied the behavior of somatomedin in plasma during gel filtration at pH 8.1 and pH 2.8, and by the recombination of fractions. Using porcine bioassay for SM, displacement of 125I-insulin by SM, and binding of 125I-SM to plasma proteins, we can demonstrate that the disparity in apparent mol wt of SM is due to reversible binding to plasma proteins which are themselves biologically inactive. This binding site is specific for the SM peptides, saturable, and has high affinity and relatively low capacity.

Measurement of an analog of insulin-like growth factor-I in blood plasma using a novel enzyme-linked immunosorbent assay

“…IGF-binding protein contamination is a major problem in all IGFs-I and -II RIAs (Hintz & Liu 1977, Powell et al 1986, Daughaday et al 1987, Breier et al 1991, Crawford et al 1992. In principle, IGF-binding proteins cause different problems in competitive tracer binding systems such as those used for RIA than those caused in noncompetitive ELISA systems.…”

“…Despite the low affinity of LR 3 IGF-I for IGF-binding proteins, these binding proteins interfere in the LR 3 IGF-I RIA in much the same way as they do in RIAs for IGFs-I and -II unless they are removed prior to assay (Daughaday et al 1987, Breier et al 1991. The preferred method for achieving this is chromatographic extraction under acidic, dissociating conditions before RIA (Hintz & Liu 1977, Powell et al 1986, Daughaday et al 1987, Breier et al 1991, Crawford et al 1992). Here we describe the development, performance and characteristics of a nonisotopic assay of LR 3 IGF-I in blood plasma that does not require sample extraction.…”

Triiodothyronine Regulates Insulin‐Like Growth Factor‐I Binding to Cultured Rat Pituitary Cells*

“…which have molecular weights of 30 to 150 kilodaltons (21). Since these binding proteins are also made by pituitary explants (22) and by cultured rat anterior pituitary cells (23), the somatomedins are also candidates for Hinkle and Kinsella's autocrine factor(s).…”

Radioimmunoassay for the Measurement of Insulin-Like Growth Factor I in Patients with Pituitary Disease in Comparison with Commercially Available Somatomedin-C Radioimmunoassays

“…About 80% of the IGF is bound to the larger form which has a molecular weight of 150000. The rest of the IGF is bound to a smaller form with a molecular weight of about 40000 (11,12).…”