Free insulin-like growth factors (IGF-I and IGF-II) in human serum

FEBS Lett. 1994 Jul 11;348(2):185-91. doi: 10.1016/0014-5793(94)00602-4.


Using ultrafiltration by centrifugation we have isolated the free, unbound fractions of insulin-like growth factor I and II (free IGF-I and IGF-II) in human serum. In this way near in vivo conditions could be maintained before and during isolation. The recovery was 80 to 100% in the ultrafiltrates, which contained no detectable amounts of IGF-binding proteins (IGFBPs) as measured by Western ligand blotting and IGFBP-1 and IGFBP-3 immunoassays. The concentration of free peptides was measured in two ultrasensitive non-competitive IGF-I and IGF-II time-resolved fluoroimmunoassays. We found that (i) equilibrium between free and protein-complexed IGF was strongly dependent on re-establishment of in vivo conditions (temperature, pH, ionic milieu and dilution); (ii) metabolic events (glucose load and fasting) caused significant changes in free IGF-I and IGF-II levels without concomitant changes in total circulating levels of IGFs; (iii) in 49 healthy adult subjects (20 to above 60 years) free IGF-I was inversely related to age and ranged from 950 +/- 150 ng/l (mean +/- S.E.M.) (20-30 years) to 410 +/- 70 ng/l (> 60 years). The relative percentage was, however, unchanged, being 0.38 +/- 0.02% of total IGF-I. In contrast, free IGF-II was independent of age, being 1,480 +/- 80 ng/l (approximately 0.20 +/- 0.01% of total IGF-II).

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acromegaly / blood
  • Adult
  • Centrifugation
  • Chromatography, High Pressure Liquid
  • Fasting
  • Fluorescent Antibody Technique
  • Glucose Tolerance Test
  • Growth Hormone / deficiency
  • Humans
  • Hydrogen-Ion Concentration
  • Insulin-Like Growth Factor I / isolation & purification*
  • Insulin-Like Growth Factor II / isolation & purification*
  • Middle Aged
  • Osmolar Concentration
  • Ultrafiltration


  • Insulin-Like Growth Factor I
  • Insulin-Like Growth Factor II
  • Growth Hormone