Enzyme-Linked Competitive Binding Assays for Digoxin

Enzymes have emerged as the most powerful alternatives to radioisotopes in the development of binding assay meth-ods for the selective detection of various physiological, bio-logical and environmental substances at trace levels.1-6 Such methods may be classified as either heterogeneous (solid-phase) or homogeneous (separation-free). The heterogeneous arrangements such as the enzyme-linked immunosorbent assay (ELISA) are much slower, but prevalent. The homogeneous types such as the enzyme-multiplied immunoassay technique (EMIT) are much faster because there is no need for separa-tion of free and bound enzyme labels.5,6 In this method, analytical signals result typically from inhibition of enzyme-conjugate catalytic activity in solution by antibody (or bind-ing protein) interactions with the conjugate. We have previously devised a generic type of the homo