title: A colorimetric immunoassay based on an enzyme inhibitor method.
abstract: A competitive binding immunoassay was developed using an enzyme inhibitor for labeling the analyte. Thyroxine was labeled by covalent coupling of the alpha-amino group to the gamma-carboxyl of the glutamyl residue of methotrexate. This thyroxine-methotrexate conjugate was a potent inhibitor of dihydrofolate reductase. When antibody was bound to the thyroxine moiety, the inhibitor was inactivated. Thus, a competitive binding immunoassay for thyroxine was demonstrated based on colorimetric measurement of dihydrofolate reductase activity.
mesh_terms: D000818:Animals; D000906:Antibodies / Q000032:analysis; D001666:Binding Sites, Antibody; D001667:Binding, Competitive; D002417:Cattle; D003124:Colorimetry; D005493:Folic Acid Antagonists; D007124:Immunoenzyme Techniques; D008727:Methotrexate / Q000494:pharmacology*; D013974:Thyroxine / Q000276:immunology / Q000494:pharmacology*
chemical_list: D000906:Antibodies; D005493:Folic Acid Antagonists; D013974:Thyroxine; D008727:Methotrexate