DIRECT-DETECTION OF INSULIN-LIKE GROWTH FACTOR-II (IGF-II) BY CHEMILUMINESCENCE WITHOUT INTERFERENCE BY IGF BINDING-PROTEINS

A dot blot method for the detection of picogram quantities of human and rat insulin-like growth factor II (IGF-II) in serum-free conditioned media is described. The crossreactivity of human recombinant IGF-I in the assay was <10%. None of the IGF binding proteins (IGFBP 1-6) diminished the IGF-II signal. In contrast, significant interference by the IGFBPs was observed when the same concentrations of IGFBPs and I-125-IGF-II were used in a radioimmunoassay which utilized the same antibody. Why IGF-II is detected in the dot blot assay without IGFBP interference is not understood. We speculate that the conformation of the IGF-II/binding protein complex may be altered by binding to the nitrocellulose, exposing the IGF-II epitope that is recognized by the antibody. IGF-II was detected in 1 mul of serum-free conditioned media from BRL 3A cells (which secrete IGF-II) while no signal was generated by 50 mul of BRL 3A2 conditioned media (which do not secrete IGF-II) . In summary, this method is ideal for screening cells in serum free-culture for production of IGF-II without the need for separation of IGF-II from cell derived IGFBPs.