Cofactor-labeled antigens

Homogeneous competitive enzyme immunoassays using cofactor-labeled antigen When an indispensable cofactor is conju-... 

Fig. 2.4. Competitive, homogeneous EIA. The symbol E represents enzyme, A antigen or hapten, S and CO substrate and enzyme cofactor, and Av and B avidin and biotin. Enzymes indicated with have modulated (increased or decreased) activities. In (a) hapten is conjugated to the enzyme and reaction with the antibody (shaded structure) modulates the enzyme activity. This interaction is, however, prevented if antigen is present in the sample. Similar principles apply if substrate (b) or cofactor (c) are conjugated with the antigen. In (d) antibody is directly linked to the enzyme, and the reaction of this antibody, particularly with high-molecular weight antigens, will block the active site of the enzyme. In (e) antibody, directed to haptens labeled to avidin, prevents avidin from inhibiting the biotin-containing enzyme, unless antibody is neutralized by the same hapten present in the sample.

Enzymes and antibodies are proteins, with the ability to bind appropriate ligands very strongly and specifically. Enzymes frequently need cofactors or metal ions for (full) activity. Antibodies are multifunctional molecules, whose ability to bind to cell surfaces and to other proteins, at sites distinct from the antigen/hapten-binding sites, extends the range of labeling and detection methods available in immunoassays. 

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