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

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Abstract
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 (∼0.20 ± 0.01% of total IGF‐II).

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