Avidin is a 65 kDa protein found in egg whites. It consists of 4 identical subunits, each with a high-affinity binding site for biotin (Vitamin H). The strength of the binding (Ka = 1015) results in an essentially irreversible interaction. This interaction has been exploited for immunolabeling of antigens in histochemical, blotting, and multiwell assays. Biotinylated probes, which may be secondary antibodies, lectins, or other bioactive compounds, bind to targets on tissue samples, microtiter plates, or membranes. Avidin conjugated to enzymes, fluorochromes, or other labels binds to the biotinylated probes for visualization, either by detection of fluorescence or enzymatic conversion of substrate to produce a visible end product. In this system avidin serves as a secondary probe, attaching to the primary biotinylated probe at several sites and amplifying the signal. Use of an avidin-enzyme conjugate provides further amplification by conversion of substrate by the enzyme, which will continue to produce a visible product until the substrate is exhausted or the reaction is stopped.
Avidin has been reported to exhibit non-specific binding to membranes and tissues. For applications where nonspecific binding of avidin is a problem, Sigma offers Streptavidin and ExtrAvidin®.
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