Formation of novel D-ring and E-ring isoprostane-like compounds (D-4/E-4-neuroprostanes) in vivo from docosahexaenoic acid

Free radical-mediated oxidant injury and lipid peroxidation have been impli cated in a number of neural disorders. We have reported that bioactive pros taglandin D-2/E-2-like compounds, termed D-2/E-2-isoprostanes, are produced in vivo by the free radical-catalyzed peroxidation of arachidonic acid. Do cosahexaenoic acid, in contrast to arachidonic acid, is the most abundant u nsaturated fatty acid in brain. We therefore questioned whether D/E-isopros tane-like compounds (D-4/E-4-neuroprostanes) are formed from the oxidation of docosahexaenoic acid. Levels of putative D-4/E-4-neuroprostanes increase d 380-fold after oxidation of docosahexaenoic acid in vitro from 15.2 +/- 6 .3 to 5773 +/- 1024 ng/mg of docosahexaenoic acid. Subsequently, chemical a pproaches and liquid chromatography electrospray ionization tandem mass spe ctrometry definitively identified these compounds as D-4/E-4-neuroprostanes . We then explored the formation of D-4/E-4-neuroprostanes from a biologica l source, rat brain synaptosomes. Basal levels of D-4/E-4-neuroprostanes we re 3.8 +/- 0.6 ng/mg of protein and increased 54-fold after oxidation (n = 4). We also detected these compounds in fresh brain tissue from rats at lev els of 12.1 +/- 2.4 ng/g of brain tissue (n = 3) and in human brain tissue at levels of 9.2 +/- 4.1 ng/g of brain tissue (n = 4). Thus, these studies have identified novel D/E-ring isoprostane-like compounds that are derived from docosahexaenoic acid and that are formed in brain in vivo. The fact th at they are readily detectable suggests that ongoing oxidative stress is pr esent in the central nervous system of humans and animals. Further, identif ication of these compounds provides a rationale for examining their role in neurological disorders associated with oxidant stress.