Enzyme‐Linked Immunoassay

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Abstract
Rabbit immunoglobulin G (IgG) was reduced by incubating with 10 mM 2‐mercaptoethylamine and then treated with excess amounts of N,N′‐O‐phenylenedimaleimide. As a result, maleimide residues were introduced into rabbit IgG molecules. Rabbit IgG containing maleimide residues could be coupled to β‐d‐galactosidase from Escherichia coli which has sulfhydryl groups in the molecule. The resulting rabbit IgG (antibody)–enzyme complex may be useful for immunoassay of antigens. As an example, human IgG was assayed by the sandwich method using the rabbit anti‐(human IgG) IgG‐β‐d‐galactosidase complex and amounts of human IgG as small as 3 fmoles were measurable.