Immunoassay liposome conjugates

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). 

Flow injection liposome immunoanalysis (FILIA). Flow injection liposome immunoanalysis is an approach to bioanalytical analysis that is a combination of FIA and immunoassay in conjugation of the utilization of liposome biolabels. 

Homogeneous liposome immunoassays using lytic agents other than complement and mellitin have also been used. Phospholipase C catalyzes the dephosphorylation of phospholipids, which in turn destabilizes the liposome. The assay is based on the inhibition of the lytic activity by an antibody binding to an antigen conjugated to phospholipase Gentamicin is analyzed by this method. 

As an alternative to the catalytic activity of a conjugated enzyme, the complement-mediated immunolysis of liposomes can be used to amplify the measuring signals of immunoassays. The liberation of liposome-entrapped markers depends on the amount of antibody adhering to the lipid membrane and serves to measure the concentration of antigen. Such liposomes may be filled with quarternary ammonium ions detectable by ion sensitive electrodes (Umezawa et al., 1983), with enzyme substrates, e.g. glucose, which can be measured with enzyme electrodes... 

Fig. 3. Competitive Immunoassay (A) In the absence of analyte, analyte-conjugated liposomes are captured by an analyte-specific antibody at the competition zone. (B) In the presence of analyte, analyte-tagged liposomes compete with analyte present in the sample for the antibody immobilized at the competition zone yielding a signal that is inversely proportional to the analyte concentration.

Other homogeneous immunoassays Several further homogenous assay formats have been developed that capitalize on the ability of antigen-antibody complexes to form detectable clusters or networks. Examples include the latex particle agglutination assay and the latex particle agglutination inhibition assay, in which the analyte or the antibody is conjugated to latex beads, and the analyte is quantified by virtue of its ability to promote or disrupt agglutination. Another example is the liposome immunoassay, in which analyte molecules are coupled to lipids,... 

Liposomes are vesicles produced from phospholipids, cholesterol, fatty acids, etc. In typical liposome immunoassays, a fluorophore, mostly carboxy-fluorescein, is encapsulated in the liposomes. The fluorophore can be released by addition of a lysing component such as serum complement or a surfactant. In several liposome based immunoassay formats the hapten is linked to a cytolysin such as mellitin. After the immunoreaction the only free portion of the conjugate is capable of disrupting the liposomes releasing the fluorophore. 

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