Isoprostane phospholipids

Docosahexaenoic-acid-containing phospholipids are targets for lipid peroxidation. As a result of free-radical-catalyzed peroxidation, F4-isoprostanes are formed [75, 76]. F2-isoprostanes are also derived from free-radical-catalyzed peroxidation, although from AA instead [77]. 

F4-isoprostanes are found esterified in phospholipids and it has been reported that their content is increased in the brains of patients with Alzheimer s disease [78],... 

Morrow JD, Awad JA, Boss HJ, Blair IA, Roberts LJ II. Non-cyclooxygenase-derived prostanoids (F2-isoprostanes) are formed in situ on phospholipids. Proc Nat Acad Sci USA 1992 89(22) 10721-10725. 

Figure 12.1. Overview of eicosanoid metabolism, a Stmetures of eicosanoid precursor fatty acids, and their occurrence in membrane phospholipids (PC is shown as an example). Araehidonie acid is the prototypic precursor eicosatrienoic and eicosapen-tanoic acid differ from it by the a lackingor an additional double bond, respectively, b Conversion of preemsor fatty acids occurs by various enzymes, notably cyclooxygenases and lipoxygenases. Isoprostanes are non-enzymahe derivatives that may form in vivo at appreciable rates one characterishc featme is that they occur as racemic mixtures. Their physiological significance is not entirely clear.

Stafforini DM, Sheller JR, Blackwell TS, Sapirstein A, Yuli FE, McIntyre TM, Bonventre JV, Prescott SM, Roberts LJ 2nd. Release of free F2-isoprostanes from esterified phospholipids is catalyzed by intracellular and plasma platelet-activating factor acetylhydrolases. J. Biol. Chem. 2006 281 4616. 

Fig. 13.1 Chemical structures of oxidized phospholipids. (A) Free radical-induced oxidation of l-palmitoyl-2-arachidonoyl-jn-glycero-3-phosphocholine (PA-PC) leads to a plethora of different oxidation products such as peroxidized phospholipids (not shown), truncated phospholipids, isoprostanes, isolevuglandins, and isothromboxanes.

A relatively new class of oxidized arachidonic acid derivatives with potential relevance to atherosclerosis are F2 isoprostanes [24] (Fig. 6) (Chapter 12). These compounds form as a result of non-enzymatic, free-radical attack of the fatty acid moieties of cellular or lipoprotein phospholipids, followed by release of the isoprostanes from the phospholipids by a phospholipase. 8-wo-prostaglandin-F2 may also be formed by the action of COX-1 or -2 in platelets or monocytes, respectively, but the significance of COX-dependent 8-W0-PGF2 formation in vivo is unproven. Fj isoprostanes circulate in the plasma and appear in the urine as free compounds or esterified to phospholipids, and... 

F. 35.14. Generation of an isoprostane. Radical damage to a phospholipid on the arachidonic acid residue at position 2 generates an isoprostane, which is then removed from the damaged phospholipid by phospholipase A2. The example of an isoprostane shown in this figure is just one of many that can be produced. 

It should not be assumed that hydroxy fatty acids are biologically inactive. Hydroxy fatty acids are chemotactic and vasoactive. Such fatty acids could perturb phospholipids in membranes. For instance, cardiolipin containing hydroxy-linoleic acid does not support the electron transport coupled to ATP production of the mitochondrion. 5-Hydroxy de-canoic acid is a well-known inhibitor of the K -ATP channel. Isoprostanes, trihydroxy oxidation products of arachi-donic acid, are vasoconstrictors (76). 13-Hydroxy linoleic acid (13-HODE) is a lipoxygenase-derived metabolite that influences the thromboresistant properties of endothelial cells in culture (77). However, there is some doubt about the tme nature of these hydroxy-fatty acids generated by the cells, as there are several GSH- and NADPH-dependent pathways that can immediately reduce hydroperoxy- to hydroxy-fatty acids. Furthermore, the reduction step of the analytical method would have converted the hydroperoxy- to a hydroxy-group. Nevertheless, much work remains to be done to determine the relative contribution of hydroperoxy- and hydroxy- to the biological effects of fried fat, and in particular their role in endothelial dysfunction and activation of factor VII. There have been earlier suggestions that a diet rich in lipid peroxidation products may lead to atherosclerosis and CHD (34,78). 

In many instances, oxidized products are considerably more stable when formed in complex lipids, like phospholipids or cholesteryl esters, and alkaline hydrolysis is required to release the oxidized lipid for analysis. Separation and quantitation of mono-hydroxyicosanoids by HPLC has been regularly used for the last 30 years. The procedures have become routine and there are now published procedures for automating the analysis [60]. A recent review by Yin et al. [36] describes in detail how to quantify both mono-hydroxyeicosatetraenoates and F2-isoprostanes. Similar procedures can be used to separate and identify products derived from (5Z,8Z,llZ,14Z,17Z)-eicosa-5,8,11,14,17-pentenoic acid, (7Z,10Z,13Z,16Z,19Z)-docosa-7,10,13,16,19-pentaenoic acid, and (4Z,7Z,10Z,13Z,16Z,19Z)-docosa-4,7,10,13,16,19-hexaenoic acid. 

Koch et al. determined total cholesterol, phospholipids, and fatty acids in CSF samples from 216 individuals in order to establish the lipid and apo-lipoprotein levels in Cerebrospinal Fluid (CSF) in a large group of individuals, on the basis of which a classification of CSF lipoproteins was made. The cholesterol and phospholipids are measured enzymatically by fluorometric detection of the reaction products. Earlier work had shown reduced levels of cholesterol, phospholipids, and free fatty acids in Cerebrospinal Fluid (CSF) of Alzheimer disease patients. Urine levels of F2-isoprostanes or their major metabolite were not significantly different between Alzheimer s disease patients and controls. In addition, urine and CSF F2-isoprostane levels in Alzheimer s disease patients did not correlate. These results indicate that plasma and urine F2-isoprostanes and F2-neuroprostanes do not accurately reflect central nervous system levels of these biomarkers and are not reproducibly elevated in body fluids outside of the central nervous system in Alzheimer s disease patients. 

The complex mixture of regioisomers and stereoisomers structurally related to the endoperoxides and thromboxanes (prostaglandin-like compounds) produced from arachidonic acid and esterified to phospholipids and cholesterol are classified as isoprostanes. These compounds are used as markers of in vivo oxidative injury in biological systems and considered reliable because they are specific, stable and not affected by the lipid content of the diet (Chapter 13. E.4). However, these secondary oxidation products are produced in relatively small concentrations because they are derived from arachidonic acid, which is a minor polyunsaturated lipid compared to linoleic acid in biological samples. Very sensitive gas chromatography-mass spectrometric (GC-MS) and liquid chromatography-MS techniques are required to analyse isoprostanes in biological samples. 

Witztum, J.L. and Berliner, J.A. 1998. Oxidized phospholipids and isoprostanes in aterosclerosis. Curr OpinLipidol 9 441 48. 

The isoprostanes are a family of prostaglandin analogs formed in vivo through the non-enzymatic peroxidation of arachidonate, catalyzed by free-radicals. This reaction may occur primarily in phospholipids, which are then cleaved by phospholipase activities, releasing the free isoprostanes. The physiological function, if any, of these compounds is not known, but their production in vivo and the biological actions associated with at least one of them, 8-epi-PGF2a (a vasoconstrictor in kidney. Fig. 4.14),... 

Kayganich-Harrison KA, Rose DM, Murphy RC, Morrow JD, Roberts LJ. Collision-induced dissociation of F2 -isoprostanes-con-taining phospholipids. J Lipid Res 1993 34 1229-1235. 

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