Metabolism of hydroperoxy-phospholipids in human hepatoma HepG2 cells

Hydroperoxide-Reducing Enzymes in the Regulation of Free-Radical Processes

The review presents current concepts of the molecular mechanisms of oxidative stress development and describes main stages of the free-radical reactions in oxidative stress. Endogenous and exogenous factors of the oxidative stress development, including dysfunction of cell oxidoreductase systems, as well as the effects of various external physicochemical factors, are discussed. The review also describes the main components of the antioxidant defense system and stages of its evolution, with a special focus on peroxiredoxins, glutathione peroxidases, and glutathione S-transferases, which share some phylogenetic, structural, and catalytic properties. The substrate specificity, as well as the similarities and differences in the catalytic mechanisms of these enzymes, are discussed in detail. The role of peroxiredoxins, glutathione peroxidases, and glutathione S-transferases in the regulation of hydroperoxide-mediated intracellular and intercellular signaling and interactions of these enzymes with receptors and non-receptor proteins are described. An important contribution of hydroperoxide-reducing enzymes to the antioxidant protection and regulation of such cell processes as growth, differentiation, and apoptosis is demonstrated.

Metabolism and cytotoxic effects of phosphatidylcholine hydroperoxide in human hepatoma HepG2 cells

In this study, we investigated cellular uptake and metabolism of phosphatidylcholine hydroperoxide (PCOOH) in human hepatoma HepG2 cells by high performance liquid chromatography-tandem mass spectrometry, and then evaluated whether PCOOH or its metabolites cause pathophysiological effects such as cytotoxicity and apoptosis. Although we found that most PCOOH was reduced to PC hydroxide in HepG2 cells, the remaining PCOOH caused cytotoxic effects that may be mediated through an unusual apoptosis pathway. These results will enhance our fundamental understanding of how PCOOH, which is present in oxidized low density lipoproteins, is involved in the development of atherosclerosis.

Hepatoprotective activity of quercetin against acrylonitrile-induced hepatotoxicity in rats

Acrylonitrile is a potent hepatotoxic, mutagen, and carcinogen. A role for free radical-mediated lipid peroxidation in the toxicity of acrylonitrile has been suggested. The present study was designed to assess the hepatoprotective effect of quercetin against acrylonitrile-induced hepatotoxicity in rats. Liver damage was induced by oral administration of acrylonitrile (50 mg/kg/day/5 weeks). Acrylonitrile produced a significant elevation of malondialdehyde (138.9%) with a marked decrease in reduced glutathione (72.4%), and enzymatic antioxidants; superoxide dismutase (81%), and glutathione peroxidase (53.2%) in the liver. Serum aspartate aminotransferase, alanine aminotransferases, direct bilirubin, and total bilirubin showed a significant increase in acrylonitrile alone treated rats (115.5%, 110.8%, 1006.8%, and 1000.8%, respectively). Pretreatment with quercetin (70 mg/kg/day/6 weeks) and its coadministration with acrylonitrile prevented acrylonitrile-induced alterations in hepatic lipid peroxides and enzymatic antioxidants as well as serum aminotransferases and bilirubin. Histopathological findings supported the biochemical results. We suggest that querectin possess hepatoprotective effect against acrylonitrile-induced hepatotoxicity through its antioxidant activity.

Phospholipid hydroperoxides are detoxified by phospholipase A2 and GSH peroxidase in rat gastric mucosa

The aim of this study was to determine the metabolic fate of phospholipid hydroperoxides (PLOOH) in rat gastric mucosa. Here we report evidence concerning the mechanism for PLOOH detoxification in gastric mucosa homogenate. Analysis by the TLC blot technique showed that the gastric mucosa has the highest potential to eliminate 1-palmitoyl-2-linoleoyl-phosphatidylcholine hydroperoxides (PL-PtdChoOOH) compared with the intestinal mucosa and liver. Major products detected after incubation with gastric mucosa were the partially reduced linoleic acid hydroperoxides (LAOOH) and lysophosphatidylcholine, indicating the involvement of phospholipase A2 (PLA2) in the elimination pathway. Using unilamellar vesicles, we demonstrated that gastric mucosal PLA2 does not distinguish between PLOOH and intact phospholipids. Although gastric mucosal PLA2 activity efficiently eliminated excess amounts of PLOOH, the complete reduction of LAOOH was dependent on the supply of exogenous GSH. In a separate experiment, administration of egg yolk PtdChoOOH to rats for 6 d significantly elevated GSH peroxidase (GPx) activity in the gastric mucosa. We concluded that excess amounts of PLOOH are efficiently eliminated through the hydrolysis by PLA2, and the subsequent reduction of FA hydroperoxide by GPx is the critical step for complete detoxification of oxidized phospholipids in the stomach.

Photosensitized oxidation of membrane lipids: reaction pathways, cytotoxic effects, and cytoprotective mechanisms

Unsaturated lipids in cell membranes, including phospholipids and cholesterol, are well-known targets of oxidative modification, which can be induced by a variety of stresses, including ultraviolet A (UVA)- and visible light-induced photodynamic stress. Photodynamic lipid peroxidation has been associated with pathological conditions such as skin phototoxicity and carcinogenesis, as well as therapeutic treatments such as antitumor photodynamic therapy (PDT). Lipid hydroperoxides (LOOHs), including cholesterol hydroperoxides (ChOOHs), are important non-radical intermediates of the peroxidative process which can (i) serve as in situ reporters of type I vs. type II chemistry; (ii) undergo one-electron or two-electron reductive turnover which determines whether peroxidative injury is respectively intensified or suppressed; and (iii) mediate signaling cascades which either fortify antioxidant defenses of cells or evoke apoptotic death if oxidative pressure is too great. The purpose of this article is to review current understanding of photodynamic (UVA- or visible light-induced) lipid peroxidation with a special focus on LOOH generation and reactivity. Future goals in this area, many of which depend on continued development of state-of-the-art analytical techniques, will also be discussed.

alpha-Tocopherol enhances the peroxidase activity of hemoglobin on phospholipid hydroperoxide

We have used direct separation of phospholipid hydroperoxide and phospholipid hydroxide by high performance liquid chromatography to examine the phospholipid hydroperoxide peroxidase activity of hemoglobin (Hb) in the presence of hydrogen donors. Hb exhibits phospholipid hydroperoxide peroxidase activity and rapidly breaks down phospholipid hydroperoxide to thiobarbituric acid-reactive substances. However, in the presence of alpha-tocopherol, some phospholipid hydroperoxide is converted to phospholipid hydroxide, which is more stable than the hydroperoxide and is much less reactive with thiobarbituric acid. Other electron donors such as glutathione and ascorbate are less effective than alpha-tocopherol. Free cysteine also shows some ability to reduce phospholipid hydroperoxides to corresponding hydroxides, but cys-93 beta of Hb did not participate in the reaction, as shown by N-ethylmaleimide modification. Hemin alone catalysed the reaction, in the absence of protein. The results therefore show that Hb catalyses an apparent phospholipid hydroperoxide alpha-tocopherol peroxidase reaction due to bound hemin, and that the reduction depends on the ability of hydrogen donors to react with the intermediate phospholipid alkoxyl radical and does not involve reduction by deprotonated sulfhydryl groups.

Reduction of thymine hydroperoxide by phospholipid hydroperoxide glutathione peroxidase and glutathione transferases

Thymine hydroperoxide (5-hydroperoxymethyluracil), a model compound representing products of oxidative damage to DNA, is a substrate for glutathione peroxidase and some isoforms of glutathione transferase. In this paper, we show that selenium-dependent human phospholipid hydroperoxide glutathione peroxidase (Se-PHGPx) exhibits about four orders of magnitude higher activity on thymine hydroperoxide than that of other human enzymes such as selenium-dependent glutathione peroxidase and various representatives of glutathione transferases. The results indicate that Se-PHGPx may be an important enzyme in repairing oxidatively damaged DNA.