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Am J Physiol Endocrinol Metab 284: E1149-E1155, 2003. First published February 25, 2003; doi:10.1152/ajpendo.00410.2002
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Vol. 284, Issue 6, E1149-E1155, June 2003

A highly sensitive and specific assay for determination of IGF-I bioactivity in human serum

Jian-Wen Chen1, Thomas Ledet2, Hans Ørskov1, Niels Jessen1, Sten Lund1, Jonathan Whittaker3, Pierre De Meyts3, Maj Britt Larsen1,4, Jens Sandahl Christiansen1, and Jan Frystyk1

1 Medical Research Laboratories and Medical Department M, and 2 Laboratory of Biochemical Pathology, Aarhus University Hospital, DK-8000 Aarhus C; 3 Receptor Biology Laboratory, Hagedorn Research Institute, and 4 Department of Immunochemistry, Novo Nordisk, DK-2820 Gentofte, Denmark

At present, the circulating bioactivity of insulin-like growth factor I (IGF-I) is estimated by immunological measurements of IGF-I levels. However, immunoassays ignore the modifying effects of the IGF-binding proteins (IGFBPs) on the interaction between IGF-I and the IGF-I receptor (IGF-IR). Therefore, we developed an IGF-I kinase receptor activation assay (KIRA) based on cells transfected with the human IGF-IR gene. The bioassay was sensitive (detection limit 0.08 µg/l), specific (cross-reactivity of insulin, insulin analogs, and proinsulin was <1%; IGF-II cross-reactivity was 12%), and accurate (within- and between-assay coefficients of variation <7 and <15%). The operational range of the assay (0.25-10.0 µg/l) allowed for determination of IGF-I bioactivity in serum from patients with, for example, growth hormone deficiency, type 1 diabetes, and acromegaly. Addition of IGFBPs dose dependently reduced the KIRA signal, whereas addition of IGF-II to preformed complexes (1:1 molar ratio) of IGF-I and IGFBP dose dependently increased IGF-I bioactivity by displacement of bound IGF-I. In conclusion, the KIRA will enable us to compare IGF-I bioactivity with existing immunological measurements of IGF-I in serum and, hopefully, to elucidate the factors that determine IGF-I bioactivity in vivo.

kinase receptor activation assay; insulin-like growth factor I receptor


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