Determination of the affinity constant of streptavidin-coupled magnetic particles and a biotinylated antibody for high performance of magnetic solid carrier in immunoassays

Streptavidin-coupled magnetic particles (SAMPs) usually serve as a separation tool in immunoassays to specifically recognize and capture biotinylated protein. Determining the affinity constant (K-A) of SAMPs and biotinylated proteins and further evaluating the performance of SAMPs as a solid-phase carrier are of great importance. Towards this goal, we present a simple method to detect the K-A of SAMPs and a biotinylated antibody (Ab). A combined equilibrium equation derived from fluorescence labeling experiments was employed to evaluate the binding capacity between SAMPs and a biotinylated antibody. The main experimental conditions were optimized, and the K-A of SAMPs and a biotinylated antibody was determined to be 3.64 +/- 0.31 x 10(7) M-1 with a coefficient of variation (CV) of 8.38%. The initial effective biotinylated antibody binding sites of as-synthesized SAMPs were found to be 132 pmol/mg SAMPs. In addition, the K-A of Ab towards SAMPs with four different streptavidin coating densities (16-83 mu g/mg MPs) was studied, and the results showed no significant difference for the K-A among these four SAMPs. This suggests that the binding activity of SAMPs with Ab would not be affected by the streptavidin coupling density at least below 83 mu g/mg MPs. However, when the four SAMPs were used as the capture reagent in sandwich immunoassays, the SAMPs with streptavidin coating density of 83 mu g/mg MPs had a lower affinity than the other three SAMPs with less streptavidin coating density. (C) 2013 Elsevier B.V. All rights reserved.