Measurement of insulin-like growth factor-II in physiological fluids and tissues. I. An improved extraction procedure and radioimmunoassay for human and rat fluids.

The measurement of serum insulin-like growth factors (IGFs) in serum is complicated by the presence of high affinity IGF-binding proteins. The accurate measurement of IGFs by radioligand binding assays requires that the interference from binding proteins be eliminated. Acid-gel chromatography, the standard method for removing binding proteins, is laborious and time consuming. Alternative methods for extracting serum IGFs include the use of HCl-ethanol treatment and reverse phase minicolumns. However, these methods are unsuitable for use with serum for some species, such as rat and sheep, due to incomplete removal of binding proteins. We developed a fast protein liquid chromatography size-exclusion chromatographic method for characterizing the presence of IGF-binding proteins in physiological fluids and used this method to systematically investigate different combinations of acids and organic solvents as potential extraction methods for IGFs. We developed and validated an improved extraction procedure that uses formic acid, Tween-20, and acetone. The new extraction method was used in conjunction with purified biosynthetic human IGF-II and a commercially available anti-IGF-II monoclonal antibody in the development of an improved RIA for IGF-II. The new RIA is sensitive (5.0 pg/tube), specific (IGF-I cross-reactivity, less than 1%), and reproducible [interassay precision (coefficient of variation), less than 9.2%). We measured the serum concentrations of IGF-II in adults and found a significant difference between normal subjects and individuals with insulin-dependent diabetes mellitus.