Development of highly fluorescent detection reagents for the construction of ultrasensitive immunoassays

We developed two kinds of highly fluorescent streptavidin-based conjugates for use as universal detection reagents in ultrasensitive immunoassays. The direct conjugate was produced by covalently linking streptavidin to poly(Glu: Lys) which was labeled heavily with Eu chelates; the indirect conjugate was made by first conjugating bovine serum albumin (BSA) to poly(Glu:Lys) labeled heavily with Eu chelates and then further linking streptavidin to the conjugate of BSA-poly(Glu:Lys)-Eu chelate. Both direct and indirect conjugates were used to construct a highly sensitive time-resolved fluorometric assay for prostate-specific antigen (PSA). Of two monoclonal antibodies used in the assay, one was coated on the well surface of the microtitration strips, and the other was biotinylated. When 10 microL of sample volume was used, we found that the assay using the indirect conjugate had a detection limit of 0.006 microg/L, which was approximately 5.6-fold more sensitive than the one using Eu chelate directly labeled detection antibody and 6.8-fold more sensitive than the one using Eu chelate-labeled streptavidin. However, the assay that used the direct conjugate was 1.5-fold more sensitive than the one that utilized the indirect conjugate. When 45 microL of sample volume was used, a detection limit of 0.001 microg/L was achieved by using the direct conjugate. This improvement in sensitivity should be equally obtainable for the analytes other than PSA. We further demonstrated that the final immunoassay performance was affected not only by the quality of the streptavidin-based conjugate used but also by the quality of the biotinylated antibody reagent. The universal detection reagents described here are believed to be particularly useful for the construction of ultrasensitive time-resolved fluorometric immunoassays and are potentially applicable in other fields such as immunohistochemistry and nucleic acid detection.