Quantification of F-ring isoprostane-like compounds (F₄- neuroprostanes) derived from docosahexaenoic acid in vivo in humans by a stable isotope dilution mass spectrometric assay

Lipid peroxidation has been implicated in the pathophysiological sequelae of human neurodegenerative disorders. It is recognized that quantification of lipid peroxidation is best assessed in vivo by measuring a series of prostaglandin (PG) F₂-like compounds termed F₂- isoprostanes (IsoPs) in tissues in which arachidonic acid is abundant. Unlike other organs, the major polyunsaturated fatty acid (PUFA) in the brain is docosahexaenoic acid (DHA, C22:6 ω-6), and this fatty acid is particularly enriched in neurons. We have previously reported that DHA undergoes oxidation in vitro and in vivo resulting in the formation of a series of F₂-IsoP-like compounds termed F₄-neuroprostanes (F ₄-NPs). We recently chemically synthesized one F₄-NP, 17-F_4c-NP, converted it to an ¹⁸O-labeled derivative, and utilized it as an internal standard to develop an assay to quantify endogenous production of F₄-NPs by gas chromatography (GC)/negative ion chemical ionization (NICI) mass spectrometry (MS). The assay is highly precise and accurate. The lower limit of sensitivity is approximately 10pg. Levels of F₄-NPs in brain tissue from rodents were 8.7±2.0ng/g wet weight (mean±S.D.). Levels of the F₄-NPs in brains from normal humans were found to be 4.9±0.6ng/g (mean±S.D.) and were 2.1-fold higher in affected regions of brains from humans with Alzheimer's disease (P=0.02). Thus, this assay provides a sensitive and accurate method to assess oxidation of DHA in animal and human tissues and will allow for the further elucidation of the role of oxidative injury to the central nervous system in association with human neurodegenerative disorders.