Enzymes Of Arachidonic Acid Cascade Research Articles

Structure-based screening for the discovery of 1,2,4-oxadiazoles as promising hits for the development of new anti-inflammatory agents interfering with eicosanoid biosynthesis pathways.

The multiple inhibition of biological targets involved in pro-inflammatory eicosanoid biosynthesis represents an innovative strategy for treating inflammatory disorders in light of higher efficacy and safety. Herein, following a multidisciplinary protocol involving virtual combinatorial screening, chemical synthesis, and invitro and invivo validation of the biological activities, we report the identification of 1,2,4-oxadiazole-based eicosanoid biosynthesis multi-target inhibitors. The multidisciplinary scientific approach led to the identification of three 1,2,4-oxadiazole hits (compounds 1, 2 and 5), all endowed with IC50 values in the low micromolar range, acting as 5-lipoxygenase-activating protein (FLAP) antagonists (compounds 1 and 2), and as a multi-target inhibitor (compound 5) of arachidonic acid cascade enzymes, namely cyclooxygenase-1 (COX-1), 5-lipoxygenase (5-LO) and microsomal prostaglandin E2 synthase-1 (mPGES-1). Moreover, our invivo results demonstrate that compound 5 is able to attenuate leukocyte migration in a model of zymosan-induced peritonitis and to modulate the production of IL-1β and TNF-α. These results are of interest for further expanding the chemical diversity around the 1,2,4-oxadiazole central core, enabling the identification of novel anti-inflammatory agents characterized by a favorable pharmacological profile and considering that moderate interference with multiple targets might have advantages in re-adjusting homeostasis.

Evaluation of inhibitors of the arachidonic acid cascade with intact platelets using an on-line dilution and on-line solid phase extraction HPLC–MS method

An automatic on-line dilution/on-line solid phase extraction (SPE) system has been developed for the detection of metabolites of the arachidonic acid cascade in platelets. The method allows the direct injection of larger quantities of centrifugates from cell suspensions previously treated with an equal volume of an acetonitrile/methanol mixture for protein precipitation. The method was used to study the effect of inhibitors of platelet arachidonic acid cascade enzymes (cytosolic phospholipase A2α, cyclooxygenase-1, thromboxane synthase, 12-lipoxygenase) and related targets (cyclooxygenase-2, microsomal prostaglandin E synthase-1, 5-lipoxygenase) in intact platelets after stimulation with calcium ionophore A23187. In addition to enzyme inhibition, the cell-damaging properties of the test compounds was determined by measuring the release of serotonin from the platelets into the incubation buffer.

Impact of food polyphenols on oxylipin biosynthesis in human neutrophils

The intake of food polyphenols is associated with beneficial impacts on health. Besides anti-oxidative effects, anti-inflammatory properties have been suggested as molecular modes of action, which may result from modulations of the arachidonic acid (AA) cascade. Here, we investigated the effects of a library of food polyphenols on 5-lipoxygenase (5-LOX) activity in a cell-free assay, and in human neutrophils. Resveratrol, its dimer (ε-viniferin), and its imine analogue (IRA) potently blocked the 5-LOX-mediated LT formation in neutrophils with IC50 values in low μM-range. Among the tested flavonoids only the isoflavone genistein showed potent 5-LOX inhibition in neutrophils (IC50 = 0.4 ± 0.1 μM), however was ineffective on isolated 5-LOX. We exclude an interference with the 5-LOX-activating protein (FLAP) in HEK_5-LOX/±FLAP cells and suggest global effects on intact immune cells. Using LC-MS based targeted oxylipin metabolomics, we analyzed the effects of 5-LOX-inhibiting polyphenols on all branches of the AA cascade in Ca2+-ionophore-challenged neutrophils. While ε-viniferin causes a clear substrate shunt towards the remaining AA cascade enzymes (15-LOX, cyclooxygenase – COX-1/2, cytochrome P450), resveratrol inhibited the COX-1/2 pathway and showed a weak attenuation of 12/15-LOX activity. IRA had no impact on 15-LOX activity, but elevated the formation of COX-derived prostaglandins, having no inhibitory effects on COX-1/2. Overall, we show that food polyphenols have the ability to block 5-LOX activity and the oxylipin pattern is modulated with a remarkable compound/structural specificity. Taken the importance of polyphenols for a healthy diet and their concentration in food supplements into account, this finding justifies further investigation.

Dimethyl ester of bilirubin exhibits anti-inflammatory activity through inhibition of secretory phospholipase A2, lipoxygenase and cyclooxygenase

Overproduction of arachidonic acid (AA) mediated by secretory phospholipase A2 group IIA (sPLA2IIA) is a hallmark of many inflammatory disorders. AA is subsequently converted into pro-inflammatory eicosanoids through 5-lipoxygenase (5-LOX) and cyclooxygenase-1/2 (COX-1/2) activities. Hence, inhibition of sPLA2IIA, 5-LOX and COX-1/2 activities is critical in regulating inflammation. We have previously reported unconjugated bilirubin (UCB), an endogenous antioxidant, as sPLA2IIA inhibitor. However, lipophilic UCB gets conjugated in liver with glucuronic acid into hydrophilic conjugated bilirubin (CB). Since hydrophobicity is pre-requisite for sPLA2IIA inhibition, conjugation reduces the efficacy of UCB. In this regard, UCB was chemically modified and derivatives were evaluated for sPLA2IIA, 5-LOX and COX-1/2 inhibition. Among the derivatives, BD1 (dimethyl ester of bilirubin) exhibited ∼ 3 fold greater inhibitory potency towards sPLA2IIA compared to UCB. Both UCB and BD1 inhibited human 5-LOX and COX-2 activities; however only BD1 inhibited AA induced platelet aggregation. Molecular docking studies demonstrated BD1 as better inhibitor of aforesaid enzymes than UCB and other endogenous antioxidants. These data suggest that BD1 exhibits strong anti-inflammatory activity through inhibition of AA cascade enzymes which is of great therapeutic importance.

Polypharmacology of Small-Molecule Modulators of the 5-Lipoxygenase Activating Protein (FLAP) Observed via a High-throughput Lipidomics Platform

Leukotrienes (LTs) and related species are proinflammatory lipid mediators derived from arachidonic acid (AA) that have pathological roles in autoimmune and inflammatory conditions, cardiovascular diseases, and cancer. 5-Lipoxygenase activating protein (FLAP) plays a critical accessory role in the conversion of AA to LTA4, and its subsequent conversion to LTC4 by LTC4 synthase. Pharmacological inhibition of FLAP results in a loss of LT production by preventing the biosynthesis of both LTB4 and LTC4, making it an attractive target for the treatment of inflammatory diseases in which LTs likely play a role. Small-molecule (SM) drugs often exhibit polypharmacology through various pathways, which may explain the differential therapeutic efficacies of compounds sharing structural similarity. We have profiled a series of SM FLAP modulators for their selectivity across enzymes of AA cascade in human whole blood (HWB), using a recently developed LC/MS (liquid chromatography–mass spectrometry)-based high-throughput lipidomics platform that monitors 122 eicosanoids in multiplex. Highly efficient data acquisition coupled with fast and accurate data analysis allowed facile compound profiling from ex vivo study samples. This platform allowed us to quantitatively map the effects of those SMs on the entire AA cascade, demonstrating its potential to discriminate structurally related compounds.

Phospholipase A2: A Potential Therapeutic Target in Inflammation and Cancer (In silico, In vitro, In vivo and Clinical Approach)

Phospholipase A2 (PLA2) (EC 3.1.1.4) is the initial enzyme of arachidonic acid cascade, has key role in inflammation and cancer. Hence, PLA2 inhibitors have wide medicinal importance. This short review focused on PLA2 structure, function and role in inflammation and cancer. Further we tried to collect PLA2 inhibitors from the previous literature and explained the possibility of their utility as anti-inflammatory and anticancer agents.

Covering the Cover

Covering the Cover

Abstract 5174: Involvement of COX-2 in p53 induction and Jnk activation following DNA damage.

Abstract Cyclooxygenase (COX) is a rate-limiting enzyme in arachidonic acid cascade and exists as two related but unique isoforms, COX-1 and COX-2. While COX-1 is constitutively expressed in many tissues and is believed to play a role in tissue homeostasis, COX-2 is highly inducible by various stimuli including inflammatory cytokines, growth factors, and DNA damage. In addition, aberrant COX-2 expression is frequently observed in many types of cancer, suggesting the role of COX-2 in tumor development and progression. p53 is a well-known tumor suppressor and plays a key role in the control of normal cell's response to environmental or cellular stresses. Although p53 has been shown to regulate COX-2 expression, the modulation of p53 levels by COX-2 has not been determined. In the present study, we have examined the role of COX-2 as an upstream regulator of p53 levels using COX-2 knockout (KO) mouse embryonic fibroblasts (MEFs). DNA damage-induced increase in p53 protein levels were greatly reduced in COX-2 KO MEFs compared to wild type (WT) or COX-1 KO MEFs. Furthermore, a COX-2 specific inhibitor suppressed doxorubicin-induced p53 accumulation and cell death. However, initial induction of p53 accumulation in response to doxorubicin was not altered by deficiency or inhibition of COX-2, suggesting that COX-2 may be involved in maintenance of p53 stability. Doxorubicin-induced accumulation of p53 appeared to be mediated through the activation of Jun-N-terminal Kinase (Jnk). Activation of Jnk by doxorubicin was decreased in COX-2 KO MEFs or by COX-2 inhibition, suggesting that DNA damage induces activation of Jnk by a mechanism that involves COX-2. Our results suggest the existence of novel cross-talk between COX-2 and p53 and identify COX-2 as a potential target for reducing the side effects from chemotherapy. Citation Format: Joohwee Kim, Hyunju Lee, Kawngil Park, Thomas E. Eling, Minsub Shim. Involvement of COX-2 in p53 induction and Jnk activation following DNA damage. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5174. doi:10.1158/1538-7445.AM2013-5174

The effects of polyphenols on oxidative stress and the arachidonic acid cascade. Implications for the prevention/treatment of high prevalence diseases

Redox state unbalance and the activation of the arachidonic acid (AA) cascade, contribute to the pathogenesis of cardiovascular disease (CVD) and cancer. Inflammatory cells that infiltrate the atheroma plaque or tumor are a major source of reactive oxygen species and eicosanoids. The human antioxidant defense network is complex and interlocking and there is controversy surrounding the beneficial effects of diet-derived antioxidants in vivo. However, epidemiological studies indicate that populations that consume high levels of plant-derived foods containing phenolic compounds have low rates of CVD and cancer. The molecular mechanisms for these effects are multi-faceted. They include the regulation of transcription factors and consequently the modulation of genes (cytokines, growth factors and adhesion molecules), and growth factor-receptor interactions and cell signaling cascades, which determine the expression of genes involved in cell cycle, cell survival and apoptosis, as well as adhesiveness/invasiveness and angiogenesis. The present paper also focuses on the effects of phenolic compounds on AA cascade enzymes (cyclooxygenases and lipoxygenases) and the subsequent synthesis of eicosanoids, which are involved in CVD and cancer. A better understanding of these processes could explain the beneficial effects of polyphenols on the most prevalent diseases in Western societies. This commentary shows that antioxidants under evaluation include structural modifications of low-molecular-mass polyphenols, which could lead to a valuable strategy for modulating the generation of inflammatory mediators involved in these chronic diseases.

Altered neuroinflammatory, arachidonic acid cascade and synaptic markers in postmortem Alzheimer's disease brain.

Alzheimer's disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A recent positron emission tomography imaging study demonstrated upregulated brain arachidonic acid (AA) metabolism in AD patients. Further, a mouse model of AD shows an increase in AA-releasing cytosolic phospholipase A2 (cPLA2) in brain, and a reduction in cPLA2 activity ameliorated cognitive deficits. These observations led us to hypothesize that there is an upregulation of AA cascade and neuroinflammatory markers in the brain of AD patients. To test this hypothesis, we measured protein and mRNA levels of AA cascade, neuroinflammatory and synaptic markers in postmortem frontal cortex from 10 AD patients and 10 age-matched controls. Consistent with our hypothesis, AD frontal cortex showed significant increases in protein and mRNA levels of cPLA2-IVA, secretory sPLA2-IIA, cyclooxygenase-1 and -2, membrane prostaglandin (PG) synthase-1 and lipoxygenase-12 and -15. Calcium-independent iPLA2-VIA and cytosolic PGE2 synthase were decreased. In addition, interleukin-1β, tumor necrosis factor-α, glial fibrillary acidic protein and CD11b were increased. AD postmortem brain also showed signs of cellular injury, including decreased synaptophysin and drebrin, pre- and postsynaptic markers. These results indicate that increased AA cascade and inflammatory markers could contribute to AD pathology. Altered brain AA cascade enzymes could be considered therapeutic targets for future drug development.

Altered arachidonic acid cascade enzymes in postmortem brain from bipolar disorder patients

Mood stabilizers that are approved for treating bipolar disorder (BD), when given chronically to rats, decrease expression of markers of the brain arachidonic metabolic cascade, and reduce excitotoxicity and neuroinflammation-induced upregulation of these markers. These observations, plus evidence for neuroinflammation and excitotoxicity in BD, suggest that arachidonic acid (AA) cascade markers are upregulated in the BD brain. To test this hypothesis, these markers were measured in postmortem frontal cortex from 10 BD patients and 10 age-matched controls. Mean protein and mRNA levels of AA-selective cytosolic phospholipase A(2) (cPLA(2)) IVA, secretory sPLA(2) IIA, cyclooxygenase (COX)-2 and membrane prostaglandin E synthase (mPGES) were significantly elevated in the BD cortex. Levels of COX-1 and cytosolic PGES (cPGES) were significantly reduced relative to controls, whereas Ca(2+)-independent iPLA(2)VIA, 5-, 12-, and 15-lipoxygenase, thromboxane synthase and cytochrome p450 epoxygenase protein and mRNA levels were not significantly different. These results confirm that the brain AA cascade is disturbed in BD, and that certain enzymes associated with AA release from membrane phospholipid and with its downstream metabolism are upregulated. As mood stabilizers downregulate many of these brain enzymes in animal models, their clinical efficacy may depend on suppressing a pathologically upregulated cascade in BD. An upregulated cascade should be considered as a target for drug development and for neuroimaging in BD.

Brain Arachidonic Acid Cascade Enzymes are Upregulated in a Rat Model of Unilateral Parkinson Disease

Arachidonic acid (AA) signaling is upregulated in the caudate-putamen and frontal cortex of unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, a model for asymmetrical Parkinson disease. AA signaling can be coupled to D(2)-like receptor initiated AA hydrolysis from phospholipids by cytosolic phospholipase A(2) (cPLA(2)) and subsequent metabolism by cyclooxygenase (COX)-2. In unilaterally 6-OHDA- and sham-lesioned rats, we measured brain expression of cPLA(2), other PLA(2) enzymes, and COX-2. Activity and protein levels of cPLA(2) were significantly higher as was COX-2-protein in caudate-putamen, frontal cortex and remaining brain on the lesioned compared to intact side of the 6-OHDA lesioned rats, and compared to sham brain. Secretory sPLA(2) and Ca(2+)-independent iPLA(2) expression did not differ between sides or groups. Thus, the tonically increased ipsilateral AA signal in the lesioned rat corresponds to upregulated cPLA(2) and COX-2 expression within the AA metabolic cascade, which may contribute to symptoms and pathology in Parkinson disease.

Mood-Stabilizers Target the Brain Arachidonic Acid Cascade

Bipolar disorder (BD) is a severe psychiatric illness characterized by recurrent manic and depressive episodes, without a characteristic neuropathology or clear etiology. Drugs effective in BD target many key signaling pathways in animal and cell studies. However, their mode of action in the BD brain remains elusive. In the rat brain, some of the mood stabilizers effective in treating mania (lithium, carbamazepine, valproate) or depression (lamotrigine) in BD are reported to decrease transcription of cytosolic phospholipase A(2) and cyclooxygenase-2 and to reduce levels of AP-2 and NF-kappaB, transcription factors of the two enzymes. The anti-manic drugs also decrease arachidonic acid (AA) turnover in brain phospholipids when given chronically to rats. Thus, drugs effective in BD commonly target AA cascade kinetics as well as AA cascade enzymes and their transcription factors in the rat brain. These studies suggest that of BD is associated with increased AA signaling in the brain. Developing therapeutic agents that suppress brain AA signaling could lead to additional treatments for BD. In this review, we discuss the mechanisms of action of mood stabilizers and the effects of docosahexaenoic acid on AA cascade enzymes in relation to BD.

Prostaglandin E2 is an enhancer for IL-1β-induced expression of membrane-associated prostaglandin E synthase in rheumatoid synovial fibroblasts

Membrane-associated prostaglandin E synthase (mPGES) is a recently identified terminal enzyme of arachidonic acid cascade that catalyzes prostaglandin (PG)H2 to PGE2. We recently reported that expression of mPGES in rheumatoid synovial fibroblasts (RSF) was upregulated by IL-1β, and that the time-course of its expression was delayed and prolonged when compared with that of cyclooxygenase-2 (COX-2).

Altered eicosanoid biosynthesis in selenium-deficient endothelial cells

Selenium (Se) is an integral part of the Se-dependent glutathione peroxidase (Se-GSH-Px) catalytic domain. By modulating the cellular levels of fatty acid hydroperoxides, Se-GSH-Px can influence key enzymes of arachidonic acid cascade, in this case cyclooxygenase (COX) and lipoxygenase (LOX). To investigate this phenomenon, the effects of cellular Se status on the enzymatic oxidation of arachidonic acid were investigated in bovine mammary endothelial cells (BMEC), which were cultured in either Se-deficient (−Se) or Se-adequate (+Se) media. When stimulated with calcium ionophore A23187, BMEC produced eicosanoids of both COX and LOX pathways. Compared with the Se-adequate cells, the production of prostaglandin I 2 (PGI 2), prostaglandin F 2 (PGF 2α), and prostaglandin E 2 (PGE 2) was significantly decreased in Se-deficient cells, whereas the production of thromboxane A 2 (TXA 2) was markedly increased in the −Se BMEC cultures. Although the enzymatic oxidation of arachidonic acid by the LOX pathway was found to be relatively less than by the COX pathway, the BMEC cultured in −Se media produced significantly more 15-hydroperoxyeicosatetraenoic acid (15-HPETE) than the +Se cells produced. Based on these results, we postulate that cellular Se status plays an important regulatory role in the enzymatic oxidation of arachidonic acid by the COX and LOX pathways. The altered eicosanoid biosynthesis, especially the overproduction of 15-HPETE, in −Se BMEC may be one of the underlying biochemical phenomena responsible for vascular dysfunction during Se deficiency.

Mechanisms of platelet function alterations induced by low-intensity laser radiation

Effects of laser radiation (wavelength 630 nm, power 1 mW) on platelet aggregation in human bloodin vitro andin vivo and on platelet sensitivity to thromboxane A as well as on cyclooxygenase and thromboxane synthetase activities were studied. Changes in platelet functional activity were shown to be related to platelet transformation into inactive discoid form, suppression of their sensitivity to endogenous activators, and decreased activity of the arachidonic acid cascade enzymes.

Peritoneal lavage fluid alters patterns of eicosanoid production in murine bone marrow-derived and peritoneal macrophages: dependency on inflammatory state of the peritoneum.

Murine resident macrophages produce an abundance of eicosanoids, whereas elicited macrophages produce lesser quantities of eicosanoids in general, and leukotriene C4 (LTC4) and prostacyclin (PGI2) in particular. Macrophage precursors derived from bone marrow cells produce even smaller amounts. We postulated that these differences may be regulated by substances found in the microenvironment of the cell, which may alter arachidonate release from phospholipid and its subsequent metabolism to eicosanoids. To examine if inherent differences in phospholipid availability contributed to the observed differences in eicosanoid synthesis among these three groups of macrophages, we monitored uptake and release of arachidonic acid (AA) in resident and elicited peritoneal macrophages and in bone marrow-derived macrophages (BMDM). Although differences existed in the extent of arachidonate release (37% vs. 22% vs. 27% release), the differences were not enough to explain the much larger differences in eicosanoid production. We therefore determined whether the AA cascade enzymes, including phospholipase A2 (PLA2) were intact by adding exogenous AA to the three cell types. PGI2 synthesis was not significantly increased in either elicited or BMDM. However, the enzymes necessary for LTC4 production appeared intact in elicited cells but not in BMDM. To further characterize the differences in eicosanoid synthesis between resident and elicited peritoneal macrophages and BMDM, we determined if a variety of exogenous substances [growth factors, cytokines, and noninflammatory and inflammatory peritoneal lavage fluid (NPLF and IPLF)] could enhance the production of LTC4 and PGI2 in those macrophage groups. The addition of granulocyte-macrophage colony stimulating factor (GM-CSF) slightly increased LTC4 production by BMDM and elicited macrophages. In contrast, NPLF increased the production of both LTC4 and PGI2 from BMDM, while IPLF had no effect. A similar effect of NPLF was seen on LTC4 (but not PGI2) production from elicited peritoneal cells, while IPLF decreased both LTC4 and PGI2 production from resident peritoneal macrophages. These studies indicate that substances found in the peritoneum of mice can enhance or diminish the production of LTC4 and PGI2 from the macrophage. This regulation appears to depend on the inflammatory state of the peritoneum.

Effects of copper, iron and zinc on oedema formation induced by phospholipase A 2

1. 1. Intradermal or subplantar injection of soluble snake venom phospholipase A 2(PLA 2) evoked a brisk inflammatory response, with cutaneous vascular permeability increase and paw oedema. 2. 2. These inflammatory processes are mainly the result of arachidonic acid cascade activation and mast cell degranulation. 3. 3. Copper, iron and zinc have an inhibitory effect on vascular permeability increase and paw oedema induced by PLA 2. 4. 4. Copper and iron could have not only a direct effect on PLA 2 but on enzymes of arachidonic acid cascade. 5. 5. However, zinc have a moderate antiinflammatory activity. This effect could be the result to inhibit PLA 2 induced mast cell degranulation.

Effects of inhibition of thromboxane A 2 synthesis in aspirin-induced asthma

Since inhibition of cyclooxygenase precipitates asthmatic attacks in patients with aspirin idiosyncrasy, we have evaluated the effects of pharmacologic inhibition of thromboxane A2 (TXA2) synthetase, next to cyclooxygenase enzyme in arachidonic acid cascade. Sixteen patients with aspirin-induced asthma received increasing doses on 3 days (25 to 400 mg) of an imidazole derivative, OKY-046, which specifically blocks TXA2 synthetase. Twenty-three healthy control subjects received a single dose of 400 mg of OKY-046. In both patients and control subjects, the inhibitor at a dose of 400 mg produced (1) a pronounced fall in thromboxane B2 serum levels, (2) a rise in serum 6-keto-prostaglandin F1 alpha, and (3) a depression in platelet aggregability to arachidonic acid and adenosine diphosphate. The drug, however, neither precipitated attacks of asthma nor impaired pulmonary function tests throughout a 24-hour observation period. Five patients, but none of the control subjects, developed transient nasal congestion about 1 hour after taking the drug. Thus, inhibition of TXA2 synthetase, contrary to inhibition of cyclooxygenase, does not affect bronchopulmonary function in patients with asthma and aspirin intolerance.

The role of selenium peroxidases in the protection against oxidative damage of membranes

The present review deals with the chemical properties of selenium in relation to its antioxidant properties and its reactivity in biological systems. The interaction of selenite with thiols and glutathione and the reactivity of selenocompounds with hydroperoxides are described. After a short survey on distribution, metabolism and organification of selenium, the role of this element as a component of the two seleno-dependent glutathione peroxidases is described. The main features of glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase are also reviewed. Both enzymes reduce different hydroperoxides to the corresponding alcohols and the major difference is the reduction of lipid hydroperoxides in membrane matrix catalyzed only by the phospholipid hydroperoxide glutathione peroxidase. However, in spite of the different specificity for the peroxidic substrates, the kinetic mechanism of both glutathione peroxidase and phospholipid hydroperoxide glutathione peroxidase seems identical and proceeds through a tert-uni ping pong mechanism. In the reaction cycle, indeed, as supported by the kinetic data, the oxidation of the ionized selenol by the hydroperoxide yields as selenenic acid that in turn is reduced back by two reactions with reduced glutathione. Special emphasis has been given to the role of selenium-dependent glutathione peroxidases in the prevention of membrane lipid peroxidation. While glutathione peroxidase is able to reduce hydrogen peroxide and other hydroperoxides possibly present in the soluble compartmet of the cell, this enzyme fails to inhibit microsomal lipid peroxidation induced by NADPH or ascorbate and iron complexes. On the other hand, phospholipid hydroperoxide glutathione peroxidase, by reducing the phospholipid hydroperoxides in the membranes, actively prevents lipid peroxidation, provided a normal content of vitamin E is present in the membranes. In fact, by preventing the free radical generation from lipid hydroperoxides, phospholipid hydroperoxide glutathione peroxidase decreases the vitamin E requirement necessary to inhibit lipid peroxidation. Finally, the possible regulatory role of the selenoperoxidases on the arachidonic acid cascade enzymes (cyclooxygenase and lipoxygenase) is discussed.