Development and clinical evaluation of a novel immunoassay for the binary complex of IGF-I and IGF-binding protein-1 in human serum.
Correlation studies have suggested that IGF-binding protein (IGFBP)-1 is a dynamic regulator of free IGF-I. To further study this, we developed a monoclonal immunofluorometric assay specific for the binary complex of IGF-I and IGFBP-1 in human serum. An IGFBP-1 antibody, which recognizes all phospho-forms of IGFBP-1, was used for coating. An europium-labeled IGF-I antibody served as tracer. Assay incubation was performed at conditions approaching those in vivo (i.e. pH 7.4, 37 C). The assay was highly specific: no signal was obtained unless both IGF-I and IGFBP-1 were present and neither IGFBP-2, -3, -4, nor IGF-II caused any cross-reaction. The linear standard curve covered 3 orders of magnitude, and within and in-between assay coefficients of variation were less than 5 and 15%, respectively. To study the dynamic relationship between free IGF-I and binary complex formation, seven healthy subjects were fasted for 72 h. Samples were collected every 3 h. During fasting, free IGF-I was reduced by two thirds (P < 0.0001). IGFBP-1 and the binary complex increased in parallel (P < 0.0001), and levels correlated positively in all subjects (0.89 < or = r < or = 0.98; P < 0.0001). Free IGF-I correlated inversely with IGFBP-1 (-0.81 < or = r < or = -0.48; 0.0001 < or = P < or = 0.05) and the binary complex (-0.79 < or = r < or = -0.41; 0.0001 < or = P < or = 0.05). To study overnight fasting levels, we compared healthy controls and patients with type 1 diabetes and chronic renal failure (n = 10), because these patients show profound alterations in their IGF-system. In both groups, the binary complex was increased about 2.5-fold (P < 0.0001), whereas IGFBP-1 was increased by 5- to 6-fold (P < 0.0001). Accordingly, free IGF-I was severely reduced (P < 0.0001). In conclusion, the assay enables us to study the role of IGFBP-1 as a dynamic regulator of free IGF-I. Our results clearly show that IGFBP-1 and free IGF-I are tightly associated peptides. Furthermore, it has now become possible to compare levels of IGF-I carried within the binary complex IGFBP-1:IGF-I in different (patho-) physiological conditions.
Frystyk
Jan
J
Medical Research Laboratories, Aarhus University Hospital, DK-8000 Aarhus C, Denmark. jan@frystyk.dk
Højlund
Kurt
K
Rasmussen
Kirsten Nyborg
KN
Jørgensen
Søren Peter
SP
Wildner-Christensen
Mette
M
Ørskov
Hans
H
D000328:Adult; D003920:Diabetes Mellitus / Q000097:blood; D015200:Fluoroimmunoassay / Q000379:methods*; D006801:Humans; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000097:blood* / Q000737:chemistry; D007334:Insulin-Like Growth Factor I / Q000032:analysis* / Q000737:chemistry; D007335:Insulin-Like Growth Factor II / Q000032:analysis; D007676:Kidney Failure, Chronic / Q000097:blood; D008297:Male; D008875:Middle Aged
D018970:Insulin-Like Growth Factor Binding Protein 1; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II