Liposome, immunoassay

Immunoassays have been based on the potentiometric measurement of marker ions such tetrapentylammonium ion (TPA ) that are loaded in phospholipid liposomes Complement mediated immunolysis of these loaded vesicles is caused... 

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). 

Figure 22.18 Biotinylated liposomes may be used in immunoassay systems to enhance the signal for detection or measurement of specific analytes. The liposome components may be constructed to include fluorescent molecules to facilitate detection of antigens within tissue sections.

Plant, A.L., Brizgys, M.V., Lacasio-Brown, L., and Durst, R.A. (1989) Generic liposome reagent for immunoassays. Anal. Biochem. 176, 42CM-26. 

Rongen HAH, Bult A, van Bennekom WP (1997) Liposomes and immunoassays. J Immunol Methods 204 105-133... 

J. P. O Connell, R. L. Campbell, B. M. Fleming, T. J. Mercolino, M. D. Johnson, and D. A. McLaurin, A highly sensitive immunoassay system involving antibody-coated tubes and liposome-entrapped dye, clin. Chem. 31, 1424-1426 (1985). 

M. A. Gerber, M. F. Randolph, and K. K. DeMeo, Liposome immunoassay for rapid identification of group A streptococci directly from throat swabs, J. clin, Microbiol. 28, 1463-1464 (1990). 

L. Locascio-Brown, A. L. Plant, V. Horvath, and R. A. Durst, Liposome flow injection immunoassay Implications for sensitivity, dynamic range, and antibody regeneration. Anal. Chem. 62, 2587-2593... 

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). 

Since none of the liposomal immunoassay approaches described in the scientific literature thus far took advantage of surface immobilization techniques, one could envision a double-amplification biosensor in which surface modification plays an important role [35]. For example, consider a dehydrogenase enzyme marker system which requires an electroactive cofactor such as NAD+. In the enzymatic reaction scheme ... 

This method was developed by Collaborative Research Inc. (F3). The technique, based on the use of immunoreactive liposomes, may be classified as another type of homogeneous immunoassay. The liposomes are microscopic vesicles (200-1000 nm in diameter) consisting of a relatively impermeable lipid bilayer that delineates and separates an internal aqueous compartment from the external aqueous medium. The principle is as follows (Fig. 3 and Table 6). An enzyme, alkaline phosphatase, is encapsulated in the liposomes [E] and sequestered from the substrate, p-nitrophenyl phos-... 

Haga et al. developed another type of immunosensor by combining an enzyme membrane immunoassay and an enzyme sensor using oxygen electrodes (HI). In this assay antigen molecules (theophylline) are attached on the surface of the liposomes and an enzyme (horseradish peroxidase) is encapsulated in the sensitized liposome. When antibody (antitheophylline antibody) and complement are added, the enzyme is released by the liposome lysis. The enzyme activity with the NADH-NAD reaction can be determined by the oxygen electrode. When antigen is added, it competitively binds to antibodies, then liposome lysis and enzyme activity are decreased. The sensitivity of this method for theophylline determination was reported as 0.7 ng/ml. 

F2. Freytag, J. W., and Litchfield, W. J., Liposome-mediated immunoassays for small haptens (digoxin) independent of complement. J. Immunol. Methods 70, 133-140 (1984). 

Interest in analytical uses of liposomes is growing steadily as shown by the publication of large number of articles on this topio [71,81]. Applioations have used various techniques including liquid chromatography, oapillary eleotrophoresis, immunoassay and sensors. The results warrant some interesting oonclusions from the bioanalytical, medical and pharmaceutical points of view. 

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