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1 Medical Research Lab and Medical Department M, Aarhus University Hospital, Aarhus, Denmark
2 Laboratory of Biochemical Pathology, Aarhus University Hospital, Aarhus, Denmark
3 Receptor Biology Laboratory, Hagedorn Research Institute, Gentofte, Denmark
4 Medical Research Lab and Medical Department M, Aarhus University Hospital, Aarhus, Denmark; Department of Immunochemistry, Novo Nordisk A/S, Gentofte, Denmark
* To whom correspondence should be addressed. E-mail: wen{at}iekf.au.dk.
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 analogues and pro-insulin was less than 1%; IGF-II cross reactivity was 12%) and accurate (with-in and in-between assay coefficients of variation less than 7 and 15%). The operational range of the assay (0.25 to 10.0 µg/l) allowed for determination of IGF-I bioactivity in serum from patients with e.g. growth hormone deficiency, type-1 diabetes and acromegaly. Addition of IGFBPs dose-dependently reduced the KIRA signal, while 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.
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