Different forms of insulin-like growth factor-binding protein-3 detected in serum and seminal plasma by immunofluorometric assay with monoclonal antibodies.

Monoclonal antibodies against recombinant insulin-like growth factor-binding protein-3 (IGFBP-3) were produced to study the various forms of IGFBP-3 and to develop a specific immunofluorometric assay. We tested seven antibodies that showed no cross-reactions with the other five human IGFBPs. By immunoblotting, the main bands in normal serum were seen at > 90, 45, 41, 29, and 25 kDa. In pregnancy serum, the 29-kDa was stronger, and the double band at 41-45 kDa was weaker or totally absent. We characterized two immunofluorometric assays. IGF-I had no effect on either assay. Added IGF-II lowered the amount of recombinant IGFBP-3 measured by the 5C11/7F8 assay, but not by the 1B6/5C11 assay. In normal serum and follicular fluid, IGFBP-3 measurements were lower by the 5C11/7F8 assay, but in most pregnancy sera and amniotic fluids the assays gave similar results. The 5C11/7F8 assay also detected IGFBP-3 in seminal plasma, whereas the 1B6/5C11 assay did not. We conclude that the results of IGFBP-3 measurement depend on the antibodies used, and that different antibodies are suitable for the IGFBP-3 measurement in different body fluids.