Double-antibody enzyme immunoassay for nortriptyline.

beta-D-Galactosidase (EC 3.2.1.23) from Escherichia coli was conjugated to desmethylnortriptyline by means of a bifunctional cross-linking reagent, dimethyl adipimidate, and used in a double-antibody immunoassay for nortriptyline. Eighty percent of the enzyme activity was retained after conjugation; 75% of the enzyme was conjugated to desmethylnortriptyline. In the final immunoassay the enzyme activity of the bound fraction was determined with o-nitrophenyl-beta-D-galactopyranoside as substrate. The sensitivity, precision, and simplicity of the enzyme immunoassay compared favorable with that of a published radioimmunoassay method. Results for nortriptyline in plasma samples correlated well with those determined by either radioimmunoassay or gas-chromatography.