Streptavidin, a bacterial protein similar to egg-white avidin in its ability to bind biotin, has been used as a replacement for egg-white avidin because of its more favorable chemical properties. It has almost no net charge at neutral pH, does not contain carbohydrate, and exhibits lower non-specific binding. Egg-white avidin has a net positive charge at neutral pH and contains about 7% carbohydrate. Therefore, it may give higher background due to non-specific binding to some cells or other surfaces.

All conjugates of streptavidin are recommended for use with Biotin-SP (long spacer)-conjugated, affinity-purified, secondary antibodies and Biotin-SP-conjugated ChromPure proteins. Our research indicates that this system of covalently conjugated reagents is more stable, gives less background, and is more sensitive than the avidin-biotin-enzyme complex (ABC) method. This has been independently verified by Shi et al. (J. Histochem. Cytochem. 1988. 36, 317) and Milde et al. (J. Histochem. Cytochem. 1989. 37, 1609). The increased sensitivity may be due to enhanced tissue penetration and less steric hindrance since nominal molecular weights for all components of the conjugated streptavidin system are less than 200,000, which are considerably lower than those of ABC.

We offer a comprehensive list of fluorophores and enzymes conjugated to streptavidin for use in enzyme immunoassays, immunohistochemistry, flow cytometry, in situ hybridization, and immunoblotting procedures. Most streptavidin products are freeze dried in buffer containing stabilizers and preservatives.