High-performance liquid chromatography methods for determination of lipoic and dihydrolipoic acid in human plasma

This assay method was applied to determine plasma levels of lipoic acid in humans. The method consists of enzymatic hydrolysis to release the protein-bound lipoic acid, solid-phase extraction, and electrochemical detection at a potential of +1.1 V. Previous methods did not provide adequate sensitivity for these studies or required procedural modifications for detection of low levels of plasma lipoic acid. The chromatographic system is capable of separating lipoic acid from dihydrolipoic acid. Both reduced and oxidized lipoic acid can be detected. Therefore, oxidation of dihydrolipoic acid must be prevented. In the described procedure, we do not prevent oxidation and the whole content is measured as lipoic acid. The method does not detect lipoic acid covalently bound to lysine. The detection limit for this method is 1 ng of lipoic acid per milliliter of plasma.