A highly sensitive and specific assay for determination of IGF-I bioactivity in human serum.
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 microg/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 microg/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.
Chen
Jian-Wen
JW
Medical Department M, Aarhus University Hospital, DK-8000 Aarhus C, Denmark. wen@iekf.au.dk
Ledet
Thomas
T
Orskov
Hans
H
Jessen
Niels
N
Lund
Sten
S
Whittaker
Jonathan
J
De Meyts
Pierre
P
Larsen
Maj Britt
MB
Christiansen
Jens Sandahl
JS
Frystyk
Jan
J
D000172:Acromegaly / Q000097:blood; D000328:Adult; D001681:Biological Assay; D002021:Buffers; D003429:Cross Reactions; D003920:Diabetes Mellitus / Q000097:blood; D004305:Dose-Response Relationship, Drug; D019382:Human Growth Hormone / Q000172:deficiency; D006801:Humans; D018969:Insulin-Like Growth Factor Binding Proteins / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000032:analysis / Q000378:metabolism*; D008297:Male; D010766:Phosphorylation; D012016:Reference Values; D015203:Reproducibility of Results; D014162:Transfection
D002021:Buffers; D018969:Insulin-Like Growth Factor Binding Proteins; D019382:Human Growth Hormone; D007334:Insulin-Like Growth Factor I