Biotin (also known as vitamin H) occurs in all living cells. Although the presence of biotin in biological samples is irrelevant
in many assay systems, it can cause high background when making use of tetrameric biotin-binding proteins (avidin,
streptavidin or Thermo Scientific NeutrAvidin™ Protein) to detect or purify biotin-labeled probes. The following protocol
may be used in immunohistochemical staining, Western blotting or ELISA methods to eliminate background caused by
endogenous biotin by blocking it before adding the biotin-labeled probe.
Two basic steps are involved in this blocking procedure:
1. Bind all endogenous biotin moieties with excess streptavidin (or equivalent biotin-binding protein); wash thoroughly.
2. Block remaining streptavidin biotin-binding sites with free biotin; wash thoroughly.
The second step is necessary because streptavidin, avidin and NeutrAvidin Proteins are tetrameric proteins, having four
biotin-binding sites per molecule. Therefore, the first step in which endogenous biotin is blocked by excess streptavidin must
be followed by a second step in which excess biotin-binding sites on the bound streptavidin are blocked with free biotin. If
this second step were not performed, the blocking streptavidin would undesirably bind the biotin-labeled probe used in the
assay. After the final wash, the result is a sample in which all biotin molecules are bound by streptavidin and all biotinbinding sites on streptavidin are bound by biotin.