Novel application of streptavidin-hapten derivatives as protein-tracer conjugate in competitive-type immunoassays involving biotinylated detection probes

Abstract

To investigate the use of streptavidin-hapten derivatives as potential protein-tracer conjugates for competitive-type immunoassays, we labeled streptavidin with cortisol and compared biotin-binding activity of the conjugates with that of unlabeled streptavidin. In this model system, streptavidin labeled with one to approximately 17 cortisol molecules retained its capability to cross-link a biotinylated protein on microtiter wells to a biotin-based general detection reagent developed for time-resolved fluorometry. Compared with unlabeled streptavidin, there was no reduction in the binding activity of the conjugate carrying as many as 2.6 cortisol molecules per molecule of streptavidin. Conjugation ratios greater than 4.4 showed a slight decrease in binding activity, presumably because of the aggregate formation evident at these labeling ratios. As expected, the conjugates were also capable of linking a solid-phase-bound anti-cortisol monoclonal antibody to the biotinylated detection reagent. The fluorescence signal generated increased almost linearly with increasing conjugation ratios from about three to nine cortisol molecules per molecule of streptavidin. At greater ratios, the assay response plateaued. The calibration curves obtained were typical for competitive-type immunoassays when the conjugates were incorporated in a cortisol assay based on a second-antibody immobilization approach.