Measurement of free form of insulin-like growth factor I in human plasma

A method to measure free form of insulin-like growth factor I (IGF-I) in human plasma using octadecylsilyl silica (Sep-Pak C18) cartridge has been developed. IGF-I was adsorbed by Sep-Pak C18 cartridge and eluted with 75% ethanol--0.01 M HCl. Labeled and non-labeled IGF-I were recovered in yields 92.5 +/- 2.1% (Mean +/- SEM) and 94.4 +/- 6.3% after adsorption to and elution from the Sep-Pak, respectively. When EDTA plasma was applied to the Sep-Pak, less than 5% of total IGF-I was recovered in the eluate. However, when acid-ethanol extracted plasma was applied to the Sep-Pak, IGF-I was recovered in yields greater than 75% of total IGF-I. When the Sep-Pak eluate was gel filtered, 88.4 +/- 4.0% of immunoreactive IGF-I eluted in the same fraction as synthetic IGF-I did, but the fraction passed through the Sep-Pak was observed as a high molecular weight form (bound form) of IGF-I. These data indicate that this Sep-Pak method does not extract all of the IGF-I in plasma, but extracts mainly the free form IGF-I. Using this method, IGF-I values of free form (fIGF-I) in EDTA plasma were measured. The fIGF-I values in normal adults, patients with acromegaly, and patients with growth hormone (GH)-deficiency were 2.4 +/- 0.1, 13.8 +/- 1.6, and 1.1 +/- 0.1 ng/ml, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)