HOMOGENEOUS ENZYME-IMMUNOASSAY FOR LIPOIC ACID BASED ON THE PYRUVATE-DEHYDROGENASE COMPLEX - A MODEL FOR AN ASSAY USING A CONJUGATE WITH ONE LIGAND PER SUBUNIT

A homogeneous enzyme immunoassay for lipoic acid was developed by using an enzyme-ligand conjugate containing only one ligand per enzyme subunit. Theoretical studies have shown that the traditional use of multisubstituted enzyme-ligand conjugates has limited the detection limits and sensitivity obtainable with these assays. The use of conjugates with a smaller number of ligands should allow for improved assays. The pyruvate dehydrogenase complex was chosen for this study because each polypeptide chain of dihydrolipoyl transacetylase contains one lipoic acid as a covalently attached prosthetic group. Thus, the naturally occurring enzyme can be considered as an enzyme-lipoic acid conjugate. Anti-lipoic acid antibodies were developed in New Zealand White rabbits to be used as the analyte-specific binders. Association and binder dilution curves were prepared in order to optimize the reagent concentrations and the analytical conditions. Unexpected inhibition by free lipoic acid resulted in a biphasic dose-response curve with a detection limit of 5 × 10-6 m lipoic acid. This technique has several advantages over previous electrochemical and chromatographic techniques for lipoic acid determination. © 1991.