Nitrogen dioxide induces cis-trans-isomerization of arachidonic acid within cellular phospholipids - Detection of trans-arachidonic acids in vivo

Oxygen free radicals oxidize arachidonic acid to a complex mixture of metab olites termed isoeicosanoids that share structural similarity to enzymatica lly derived eicosanoids. However, little is known about oxidations of arach idonic acid mediated by reactive radical nitrogen oxides. We have studied t he reaction of arachidonic acid with NO2, a free radical generated by nitri c oxide and nitrite oxidations, A major group of products appeared to be a mixture of arachidonic acid isomers having one trans-bond and three cis-dou ble bonds, We have termed these new products trans-arachidonic acids, These isomers were chromatographically distinct from arachidonic acid and produc ed mass spectra that were nearly identical with mass spectra of arachidonic acid. The lack of ultraviolet absorbance above 205 nm and the similarity o f mass spectra of dimethyloxazoline derivatives suggested that the trans-bo nd was not conjugated with any of the cis-bonds, and the C=C bonds were loc ated at carbons 5, 8, 11, and 14, Further identification was based on compa rison of chromatographic properties with synthetic standards and revealed t hat NO2 generated 14-trans-eicosatetraenoic acid and a mixture containing 1 1-trans-, 8-trans, and 5-trans-eieosatetraenoic acids. Exposure of human pl atelets to submicromolar levels of NO2 resulted in a dose-dependent formati on of 14-trans-eicosatetraenoic acid and other isomers within platelet glyc erophospholipids. Using a sensitive isotopic dilution assay we detected tra ns-arachidonic acids in human plasma (50.3 +/- 10 ng/ml) and urine (122 +/- 50 pg/ml), We proposed a mechanism of arachidonic acid isomerization that involves a reversible attachment of NO2 to a double bond with formation of a nitroarachidonyl radical. Thus, free radical processes mediated by NO2 le ad to generation of trans-arachidonic acid isomers, including biologically active 14-trans-eicosatetraenoic acid, within membrane phospholipids from w hich they can be released and excreted into urine.