Free insulin-like growth factors (IGF-I and IGF-II) in human serum.
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).
Institute of Experimental Clinical Research, Aarhus University Hospital, Denmark.
J
J
Frystyk
C
C
Skjaerbaek
B
B
Dinesen
H
H
Orskov
D000172:Acromegaly / Q000097:blood; D000328:Adult; D002498:Centrifugation; D002851:Chromatography, High Pressure Liquid; D005215:Fasting; D005455:Fluorescent Antibody Technique; D005951:Glucose Tolerance Test; D013006:Growth Hormone / Q000172:deficiency; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007334:Insulin-Like Growth Factor I / Q000302:isolation & purification*; D007335:Insulin-Like Growth Factor II / Q000302:isolation & purification*; D008875:Middle Aged; D009994:Osmolar Concentration; D014462:Ultrafiltration
D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D013006:Growth Hormone