Determination of N-acetylcysteine, intact and oxidized, in plasma by column liquid chromatography and post-column derivatization.

N-Acetylcysteine in plasma may exist as intact N-acetylcysteine (NAC) or be oxidized to disulphides, either as a dimer or mixed with other thiol-containing compounds. To prevent oxidation of NAC, whole blood was immediately centrifuged after collection and the plasma proteins were precipitated with perchloric acid. NAC was measured by direct injection of the supernatant into the chromatographic system and the oxidized forms, coupled to small sulphides (ONACS), were determined after reductive cleavage of all NAC disulphides in the supernatant with dithiothreitol before injection. The total plasma concentration of the compound, i.e., including the fraction coupled to proteins (ONACP), was assayed after an initial reduction of the disulphide linkages in plasma. After subsequent precipitation of proteins, the supernatant was directly injected. The chromatographic system was a reversed-phase column (C18) with an acidic mobile phase. After a fast (less than 6 s) post-column reaction with pyrenemaleimide, NAC was detected by fluorimetry. The contribution to the band broadening by the reactor was about 10%. The limit of quantification of NAC in plasma was 240 nM, with an intra-assay precision of 14%.