Immunoassay avidin-biotin interaction

Liposome conjugates may be used in various immunoassay procedures. The lipid vesicle can provide a multivalent surface to accommodate numerous antigen-antibody interactions and thus increase the sensitivity of an assay. At the same time, it can function as a vessel to carry encapsulated detection components needed for the assay system. This type of enzyme-linked immunosorbent assay (ELISA) is called a liposome immunosorbent assay or LISA. One method of using liposomes in an immunoassay is to modify the surface so that it can interact to form biotin-avidin or biotin-streptavidin complexes. The avidin-biotin interaction can be used to increase detectability or sensitivity in immunoassay tests (Chapter 23) (Savage et al., 1992). 

One method of using liposomes in an immunoassay is to modify the surface so that it can interact to form biotin—avidin or biotin—streptavidin complexes. The avidin— biotin interaction can be used to increase detectability or sensitivity in immunoassay tests (Chapter 13) (Savage et al., 1992). 

Avidin (an egg yolk protein) exhibits an extremely strong afEnity (ca. lO L/mol) for biotin (vitamin H). This irreversible interaction can be utilized in the design of heterogeneous immunoassays with biotinylated. antigen-specific antibodies [68], [69]. Biotinylation can be performed without interfering with antibody function. The antibodies are applied in the usual manner in a competitive immunoassay (Fig. 8). In the last step, enzyme-labeled avidin is added in excess. Since there are four possibilities for the binding of avidin... 

Antibodies form complexes with their respective antigens (here analytes). These complexes can show a particular strength, which can be quantified by the affinity constant (or equilibrium constant). This affinity constant is about 10 -10 L mol for most analytically useful antibodies. The higher this number, the more stable is the complex. The highest known affinity in the biochemical field is the interaction between avidin (an egg protein) and biotin (vitamin H), for which a value of around 10 L mol has been determined. The affinity constant plays an important role in immunoassays and other immunological techniques. The development of new methods is greatly facilitated if this constant is known. 

---