Pk (also referred to as Gb3 or CD77) shares the terminal Galα1–4Galβ1 motif with P1 trisaccharide, and the anti-Pk antibody may thus cross-react to some extent with P1. The secondary biotinylated anti-rat IgM antibody was used for
binding detection, followed by streptavidin-R-PE. The contribution of direct binding of the secondary biotinylated antibody to the beads was determined in the absence of the primary anti-Pk antibody. The results are shown in Fig. 4B. For the regular P1 beads the MFI values were comparable, irrespective selleckchem of the presence or absence of the anti-Pk antibody. This indicates that the secondary antibody binds directly to streptavidin on these beads. In contrast, the MFI values in the absence of anti-Pk antibodies were lower for both biot-PEGm (to a greater
extent with biot-PEG50). This demonstrates that direct binding of secondary biotinylated antibody to streptavidin was almost completely abolished (30-fold reduction) for biot-PEG50 and intermediately (2-fold) reduced for biot-PEG280, suggesting that the remaining streptavidin binding sites were almost completely saturated by biot-PEG50 and partially saturated by biot-PEG280. These results indicate that (i) not all biotin-binding sites on streptavidin were occupied by regular glycopolymers initially, (ii) unspecific binding due to these remaining free biotin-binding sites did not have any influence in our standard HTS assay experimental setup in the absence of secondary biotinylated antibodies, (iii) the use of secondary biotinylated antibodies is feasible and still allows for the correct detection of analyte binding in the case of end-point addition of biot-PEG50
(or to a lesser degree of biot-PEG280) to block the remaining free streptavidin binding sites, and (iv) we can minimize the risk of unspecific binding often caused by endogenous biotin in serum and cell and tissue lysate samples by using biot-PEG50. The heterobifunctional PEG23 and PEG60 (see PEGs used for glycopolymer Thymidylate synthase and microbead modifications and Fig. 2B for structure and details) were coupled to the beads prior to the anchoring of streptavidin and the immobilization of the glycopolymers. In this setup the two versions of biot-PEGs-NH2 were bifunctional linkers between the bead and streptavidin. The binding of human monoclonal anti-P1 antibodies as well as plasma antibodies from healthy donors to modified beads was assayed by SGA. The results (Fig. 5A) showed that binding of monoclonal anti-P1 antibodies and plasma antibodies to all three types of beads, i.e. regular P1-beads and P1-beads modified with both heterobifunctional PEG, was comparable, indicating that neither the bead modification with heterobifunctional PEGs in general nor the PEG length affected antibody binding to P1. This is in contrast to the PEGylated (different PEG chain lengths) glycopolymers (Fig.