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First studies on the occurrence of nitrated fatty acids in plasma of healthy subjects revealed basal concentrations of 600 nM for free/nonesterified nitro-oleic acid (NO(2)-OA) as measured by liquid chromatography tandem mass spectrometry (LC-MS/MS). We recently showed by a gas chromatography tandem mass spectrometry (GC-MS/MS) method the physiological occurrence of two isomers, i.e., 9-NO(2)-OA and 10-NO(2)-OA, at mean basal plasma concentrations of 880 and 940 pM, respectively. In consideration of this large discrepancy we modified our originally reported method by replacing solid-phase extraction (SPE) by solvent extraction with ethyl acetate and by omitting the high-performance liquid chromatography (HPLC) step for a more direct detection and with the potential for lipidomics studies. Intra-assay imprecision and accuracy of the modified method in human plasma were 1-34% and 91-221%, respectively, for added NO(2)-OA concentrations in the range 0-3,000 pM. This method provided basal plasma concentrations of 306 +/- 44 pM for 9-NO(2)-OA and 316 +/- 33 pM for 10-NO(2)-OA in 15 healthy subjects. Nitro-arachidonic acid and nitro-linolenic acid were not detectable in the plasma samples. In summary, our studies show 9-NO(2)-OA and 10-NO(2)-OA as endogenous nitrated fatty acids in human plasma in the pM range; HPLC is recommendable as a sample clean-up step for reliable quantification of nitro-oleic acids by GC-MS/MS.

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