Characterization of Linoleic Acid Nitration in Human Blood Plasma by Mass Spectrometry

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Abstract
Nitric oxide (•NO) is a pervasive free radical species that concentrates in lipophilic compartments to serve as a potent inhibitor of lipid and low-density lipoprotein oxidation processes. In this study, we synthesized, characterized, and detected nitrated derivatives of linoleic acid (18:2) in human blood plasma using high-pressure liquid chromatography coupled with electrospray ionization tandem mass spectrometry. While the reaction of nitronium tetrafluoroborate with 18:2 presented ions with a mass/charge (m/z) ratio of 324 in the negative ion mode, characteristic of nitrolinoleate (LNO2), the reaction of nitrite (NO2-) with linoleic acid hydroperoxide yielded nitrohydroxylinoleate (LNO2OH, m/z 340). Further analysis by MS/MS gave a major fragment at m/z 46, characteristic of a nitro group (−NO2) present in the parent ion. This was confirmed by using [15N]O2, which gave products of m/z 325 and 341, that after fragmentation yielded a daughter ion at m/z 47. Moreover, a C−NO2 structure was also demonstrated in LNO2OH by nuclear magnetic resonance spectroscopy (15N NMR, δ 375.9), as well as by infrared analysis in both LNO2OH (νmax = 3427, 1553, and 1374 cm-1) and LNO2 (νmax = 1552 and 1373 cm-1). Stable products with m/z of 324 and 340, which possessed the same chromatographic characteristics and fragmentation pattern as synthesized standards, were found in human plasma of normolipidemic and hyperlipidemic donors. The presence of these novel nitrogen-containing oxidized lipid adducts in human plasma could represent “footprints” of the antioxidant action of •NO on lipid oxidation and/or a pro-oxidant and nitrating action of •NO-derived species.

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