Zur photosensibilisierten Autoxydation der Steroide. Darstellung von Steroid-Hydroperoxyden Mittels Phototoxischer Photosensibilisatoren

Electrophilic oxysterols: generation, measurement and protein modification

Cholesterol is an essential component of mammalian plasma membranes. Alterations in sterol metabolism or oxidation have been linked to various pathological conditions, including cardiovascular diseases, cancer, and neurodegenerative disorders. Unsaturated sterols are vulnerable to oxidation induced by singlet oxygen and other reactive oxygen species. This process yields reactive sterol oxidation products, including hydroperoxides, epoxides as well as aldehydes. These oxysterols, in particular those with high electrophilicity, can modify nucleophilic sites in biomolecules and affect many cellular functions. Here, we review the generation and measurement of reactive sterol oxidation products with emphasis on cholesterol hydroperoxides and aldehyde derivatives (electrophilic oxysterols) and their effects on protein modifications.

Association Between Oxidative Stress and Altered Cholesterol Metabolism in Alzheimer's Disease Patients

BACKGROUND

Oxidative stress is the main feature of several diseases including Alzheimer's disease (AD). The involvement of oxysterols derivates has been recently reported.

OBJECTIVE

The aim of this study was to evaluate the implication of oxidative stress in cholesterol impairment in AD patients.

METHODS

A case-control study was conducted on 56 AD patients and 97 controls. Levels of oxidative biomarkers, including lipid peroxidation products and antioxidant enzyme activities were measured with spectrophotometric methods on red blood cells (RBCs) and plasma. Cholesterol precursors and oxysterols (7-Ketocholeterol (7KC), 7α-hydroxycholesterol (7α-OHC), 7β-hydroxycholesterol (7β-OHC), 24Shydroxycholesterol (24S-OH), 25-hyroxycholesterol (25-OHC), and 27-hydroxycholesterol (27-OHC), in plasma were quantified by gas chromatography coupled with mass spectrometry.

RESULTS

In RBCs and plasma of AD patients, a significant decrease of glutathione peroxidase (GPx) activity was detected associated with raised levels of malondialdehyde (MDA). A decreased level of lanosterol and an accumulation of 7β-OHC, 24S-OHC, 27-OHC, and 25-OHC that were higher in plasma of AD patients, compared to controls, were also observed in AD patients. Mini-Mental State Examination (MMSE) score was correlated with MDA and conjugated dienes (CD) levels in plasma. Besides, the MDA level in RBCs was correlated with 7β-OHC. Binary logistic regression revealed an association between GPx activity and AD (OR=0.895, 95%CI: 0.848-0.945. P<0.001).

CONCLUSION

Our data consolidate the relationship between the rupture of redox homeostasis and lipid and cholesterol oxidation in AD.

Cholesterol Peroxidation as a Special Type of Lipid Oxidation in Photodynamic Systems

Like other unsaturated lipids in cell membranes and lipoproteins, cholesterol (Ch) is susceptible to oxidative modification, including photodynamic oxidation. There is a sustained interest in the pathogenic properties of Ch oxides such as those generated by photooxidation. Singlet oxygen (¹ O₂ )-mediated Ch photooxidation (Type II mechanism) gives rise to three hydroperoxide (ChOOH) isomers: 5α-OOH, 6α-OOH and 6β-OOH, the 5α-OOH yield far exceeding that of the others. 5α-OOH detection is relatively straightforward and serves as a definitive indicator of ¹ O₂ involvement in a reaction, photochemical or otherwise. Like all lipid hydroperoxides (LOOHs), ChOOHs can disrupt membrane or lipoprotein structure/function on their own, but subsequent light-independent reactions may either intensify or attenuate such effects. Such reactions include (1) one-electron reduction to redox-active free radical intermediates, (2) two-electron reduction to redox-silent alcohols and (3) translocation to other lipid compartments, where (1) or (2) may take place. In addition to these effects, ChOOHs may act as signaling molecules in reactions that affect cell fates. Although processes a-c have been well studied for ChOOHs, signaling activity is still poorly understood compared with that of hydrogen peroxide. This review focuses on these various aspects Ch photoperoxidation and its biological consequences.

Involvement of oxysterols in age-related diseases and ageing processes

Ageing is accompanied by increasing vulnerability to major pathologies (atherosclerosis, Alzheimer's disease, age-related macular degeneration, cataract, and osteoporosis) which can have similar underlying pathoetiologies. All of these diseases involve oxidative stress, inflammation and/or cell death processes, which are triggered by cholesterol oxide derivatives, also named oxysterols. These oxidized lipids result either from spontaneous and/or enzymatic oxidation of cholesterol on the steroid nucleus or on the side chain. The ability of oxysterols to induce severe dysfunctions in organelles (especially mitochondria) plays key roles in RedOx homeostasis, inflammatory status, lipid metabolism, and in the control of cell death induction, which may at least in part contribute to explain the potential participation of these molecules in ageing processes and in age related diseases. As no efficient treatments are currently available for most of these diseases, which are predicted to become more prevalent due to the increasing life expectancy and average age, a better knowledge of the biological activities of the different oxysterols is of interest, and constitutes an important step toward identification of pharmacological targets for the development of new therapeutic strategies.

The Involvement of Phycocyanin Pigment in the Photodecomposition of the Cyanobacterial Toxin, Microcystin-LR

While investigating the destruction of the cyanobacterial hepatotoxin microcystin-LR in the presence of phycocyanin pigment via semiconductor photocatalysis, it became apparent that the pigment was catalysing the toxin decomposition. The mechanism of this process in terms of phycocyanin acting as a photo-oxygenation sensitizer via singlet oxygen and superoxide attack is explored. The absorption and fluorescence spectra of phycocyanin have been obtained and data on the properties of the excited state calculated. The established photo-oxygenation sensitizer rose bengal was also used as a catalyst for the photolytic decomposition of microcystin-LR to help elucidate the decomposition mechanism.

Pathways of cholesterol oxidation via non-enzymatic mechanisms

Cholesterol has many functions, including those that affect biophysical properties of membranes, and is a precursor to hormone synthesis. These actions are governed by enzymatic pathways that modify the sterol nucleus or the isooctyl tail. The addition of oxygen to the cholesterol backbone produces its derivatives known as oxysterols. In addition to having an enzymatic origin, oxysterols can be formed in the absence of enzymatic catalysis in a pathway usually termed "autoxidation," which has been known for almost a century and observed under various experimental conditions. Autoxidation of cholesterol can occur through reactions initiated by free radical species, such as those arising from the superoxide/hydrogen peroxide/hydroxyl radical system and by non-radical highly reactive oxygen species such as singlet oxygen, HOCl, and ozone. The susceptibility of cholesterol to non-enzymatic oxidation has raised considerable interest in the function of oxysterols as biological effectors and potential biomarkers for the non-invasive study of oxidative stress in vivo.

Surprising inability of singlet oxygen-generated 6-hydroperoxycholesterol to induce damaging free radical lipid peroxidation in cell membranes

Singlet oxygen attack on cholesterol (Ch), a prominent monounsaturated lipid of mammalian cell plasma membranes, gives rise to three hydroperoxide (ChOOH) isomers, 5alpha-OOH, 6alpha-OOH and 6beta-OOH, the latter two in lower yield than 5alpha-OOH, and 6alpha-OOH in lowest yield. A third possible positional isomer, 7alpha-OOH and 7beta-OOH, is produced by free radical attack. In the presence of iron and ascorbate (Fe/AH), 5alpha-OOH or 6beta-OOH in phosphatidylcholine/Ch/ChOOH (20:15:1 by mol) liposomes was reduced to its corresponding alcohol, the rate constant being approximately the same for both ChOOHs. Using [(14)C]Ch as an in situ probe, we found that liposomal 5alpha-OOH readily set off free radical-mediated (chain) peroxidation reactions when exposed to Fe/AH, whereas 6beta-OOH under the same conditions did not. Moreover, liposomal 5alpha-OOH triggered robust chain peroxidation in [(14)C]Ch-labeled L1210 cells, leading to cell death, whereas 6beta-OOH was essentially inert in this regard. Thus, 5alpha-OOH and 6beta-OOH undergo iron-catalyzed reductive turnover, but only the former can provoke toxic membrane damage. These novel findings have important implications for UVA-induced photodamage in Ch-rich tissues like skin and eye, where (1)O(2) often plays a major role.

Translocation as a means of disseminating lipid hydroperoxide-induced oxidative damage and effector action

Lipid hydroperoxides (LOOHs) generated in cells and lipoproteins under oxidative pressure may induce waves of damaging chain lipid peroxidation near their sites of origin if O2 is readily available and antioxidant capacity is overwhelmed. However, recent studies have demonstrated that chain induction is not necessarily limited to a nascent LOOH's immediate surroundings but can extend to other cell membranes or lipoproteins by means of LOOH translocation through the aqueous phase. Mobilization and translocation can also extend the range of LOOHs as redox signaling molecules and in this sense they could act like the small, readily diffusible inorganic analogue H2O2, which has been studied much more extensively in this regard. In this article, basic mechanisms of free-radical- and singlet-oxygen-mediated LOOH formation and one-electron and two-electron LOOH reduction pathways and their biological consequences are reviewed. The first studies to document spontaneous and protein-assisted LOOH transfer in model systems and cells are described. Finally, LOOH translocation is discussed in the context of cytotoxicity vs detoxification and expanded effector action, i.e., redox signaling activity.

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.

Nitric oxide inhibition of free radical-mediated cholesterol peroxidation in liposomal membranes

The ability of nitric oxide ((*)NO) to inhibit propagative lipid peroxidation was investigated using unilamellar liposomes (LUVs) constituted with egg phosphatidylcholine (PC) or 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), [(14)C]cholesterol (Ch), and a nonregenerable singlet oxygen-derived primer, 5alpha-hydroperoxycholesterol (5alpha-OOH). Exposing LUVs to ascorbate and a lipophilic iron chelate at 37 degrees C resulted in an exponential decay of 5alpha-OOH and accumulation of free radical-derived 7alpha- and 7beta-hydroperoxycholesterol (7alphabeta-OOH), as detected by high-performance liquid chromatography with electrochemical detection. Thiobarbituric acid-reactive species (TBARS) were generated concurrently in egg PC-containing LUVs. Including the (*)NO donor spermine NONOate (SPNO, 5-50 microM) or S-nitroso-N-acetyl-D,L-penicillamine (SNAP, 50-100 microM) in the reaction mixture had no effect on 5alpha-OOH decay (suggesting that iron was not redox-inhibited) but slowed TBARS and 7alphabeta-OOH accumulation in a strongly dose-dependent fashion. Decomposed SPNO or SNAP had no such effects, implying that (*)NO was the responsible agent. Accumulation of several [(14)C]Ch oxidation products, detected by high-performance thin-layer chromatography with phosphorimaging, was similarly diminished by active SPNO or SNAP. Concomitantly, a new band referred to as RCh.4 appeared, the radioactivity of which increased as a function of incubation time and (*)NO donor concentration. RCh.4 material was also generated via direct iron/ascorbate reduction of 7alpha-OOH in the presence of (*)NO, consistent with 7alpha-nitrite (7alpha-ONO) identity. However, various other lines of evidence suggest that RCh.4 is not 7alpha-ONO, but rather 5alpha-hydroxycholesterol (5alpha-OH) generated by reduction of 5alpha-ONO arising from 7alpha-ONO rearrangement. 5alpha-OH was only detected when (*)NO was present in the reaction system, thus providing indirect evidence for the existence of nitrosated Ch intermediates arising from (*)NO chain-breaking activity.

Lipid peroxidation in photodynamically stressed mammalian cells: use of cholesterol hydroperoxides as mechanistic reporters

Photodynamic action of merocyanine 540, an antileukemic sensitizing dye, on murine L1210 cells results in the formation of lipid hydroperoxides and loss of cell viability. High-performance liquid chromatography with mercury cathode electrochemical detection was used for determining lipid oxidation products, including the following cholesterol-derived hydroperoxides: 5 alpha-OOH, 6 alpha-OOH, 6 beta-OOH, and unresolved 7 alpha, 7 beta-OOH. Among these species, 5 alpha-, 6 alpha-, and 6 beta-OOH (singlet oxygen adducts) were predominant in the early stages of photooxidation, whereas 7 alpha- and 7 beta-OOH (products of free radical reactions) became so after prolonged irradiation or during dark incubation after exposure to a light dose. These mechanistic changes were studied in a unique way by monitoring shifts in the peroxide ratio, i.e., 7-OOH/5 alpha-OOH, or 7-OOH/6-OOH. When cells (10(7)/ml) were exposed to a visible light fluence of 0.6 J/cm2 in the presence of 10 microM merocyanine 540, 7-OOH/5 alpha-OOH increased by approximately 100% after 2 h of dark incubation at 37 degrees C. The increase was much larger (approximately 250%) when cells were photooxidized after treatment with 1 microM ferric-8-hydroxyquinoline, a lipophilic iron donor, whereas no increase was observed when cells were pretreated with 100 microM desferrioxamine, an avid iron chelator/redox inhibitor. Correspondingly, postirradiation formation of thiobarbituric acid-reactive material was markedly enhanced by ferric-8-hydroxyquinoline and suppressed by desferrioxamine, as was the extent of cell killing. When added to cells after a light dose, chain-breaking antioxidants such as butylated hydroxytoluene and alpha-tocopherol strongly protected against cell killing and slowed the increase in 7-OOH/5 alpha-OOH ratio. It is apparent from these results that (1) the 7-OOH/5 alpha-OOH or 7-OOH/6-OOH ratio can be used as a highly sensitive index of singlet oxygen vs. free radical dominance in photodynamically stressed cells; and (2) that postirradiation chain peroxidation plays an important role in photodynamically initiated cell killing.

Sterol peroxidation by Pseudomonas fluorescens cholesterol oxidase

Cholesterol is oxidized by commercially available Pseudomonas fluorescens cholesterol oxidase to 6 beta-hydroperoxycholest-4-en-3-one as the initial product, with none of the expected produce, cholest-4-en-3-one, formed. The transformation indicates that P. fluorescens cholesterol oxidase also acts as a flavoprotein dioxygenase.

Disruption of cholesterol 7alpha-hydroxylase gene in mice. II. Bile acid deficiency is overcome by induction of oxysterol 7alpha-hydroxylase

Past experiments and current paradigms of cholesterol homeostasis suggest that cholesterol 7alpha-hydroxylase plays a crucial role in sterol metabolism by controlling the conversion of cholesterol into bile acids. Consistent with this conclusion, we show in the accompanying paper that mice deficient in cholesterol 7alpha-hydroxylase (Cyp7-/- mice) exhibit a complex phenotype consisting of abnormal lipid excretion, skin pathologies, and behavioral irregularities (Ishibashi, S., Schwarz, M., Frykman, P. K. , Herz, J., and Russell, D. W.(1996) J. Biol. Chem. 261, 18017-18023). Aspects of lipid metabolism in the Cyp7-/- mice are characterized here to deduce the physiological basis of this phenotype. Serum lipid, cholesterol, and lipoprotein contents are indistinguishable between wild-type and Cyp7-/- mice. Vitamin D3 and E levels are low to undetectable in knockout animals. Stool fat content is significantly elevated in newborn Cyp7-/- mice and gradually declines to wild-type levels at 28 days of age. Several species of 7alpha-hydroxylated bile acids are detected in the bile and stool of adult Cyp7-/- animals. A hepatic oxysterol 7alpha-hydroxylase enzyme activity that may account for the 7alpha-hydroxylated bile acids is induced between days 21 and 30 in both wild-type and deficient mice. An anomalous oily coat in the Cyp7-/- animals is due to the presence of excess monoglyceride esters in the fur. These data show that 7alpha-hydroxylase and the pathway of bile acid synthesis initiated by this enzyme are essential for proper absorption of dietary lipids and fat-soluble vitamins in newborn mice, but not for the maintenance of serum cholesterol and lipid levels. In older animals, an alternate pathway of bile acid synthesis involving an inducible oxysterol 7alpha-hydroxylase plays a crucial role in lipid and bile acid metabolism.

Time course of oxysterol formation during in vitro oxidation of low density lipoprotein

Cholesterol oxidation products (oxysterols) have been implicated in several aspects of atherogenesis; they affect key enzymes in cholesterol homeostasis, induce calcification in vascular cells and possess cytotoxic properties. Oxysterols are formed during oxidative modification of low density lipoprotein (LDL). Using a recently developed method based on isotope dilution-mass spectrometry, the kinetics of formation of oxysterols during oxidation of LDL by cupric ions or soybean lipoxygenase was studied. The same products, mainly 7- and 5-oxygenated cholesterol, were formed by the two oxidation methods. Virtually no side-chain oxidized oxysterols were formed. During the oxidations, preferentially esterified cholesterol was consumed and consumption of polyunsaturated fatty acids and formation of conjugated dienes preceded the appearance of oxysterols. Cholesterol 7-hydroperoxides potential cytotoxins, were present in LDL oxidized by copper or lipoxygenase.

Photodynamically generated 3-beta-hydroxy-5 alpha-cholest-6-ene-5- hydroperoxide: toxic reactivity in membranes and susceptibility to enzymatic detoxification

Singlet oxygen (1O2)-mediated photooxidation of cholesterol gives three hydroperoxide products: 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide (5 alpha-OOH), 3 beta-hydroxycholest-4-ene-6 alpha-hydroperoxide (6 alpha-OOH) and 3 beta-hydroxycholest-4-ene-6 beta-hydroperoxide (6 beta-OOH). These species have been compared with respect to photogeneration rate on the one hand and susceptibility to enzymatic reduction/detoxification on the other, using the erythrocyte ghost as a cholesterol-containing test membrane and chloroaluminum phthalocyanine tetrasulfonate (AlPcS4) as a 1O2 sensitizer. Peroxide analysis was accomplished by high-performance liquid chromatography with mercury cathode electrochemical detection (HPLC-EC[Hg]). The initial rate of 5 alpha-OOH accumulation in AlPcS4/light-treated ghosts was found to be about three times greater than that of 6 alpha-OOH or 6 beta-OOH. Membranes irradiated in the presence of ascorbate and ferric-8-hydroxyquinoline (Fe[HQ]2, a lipophilic iron complex) accumulated lesser amounts of 5 alpha-OOH, 6 alpha-OOH and 6 beta-OOH but relatively large amounts of another peroxide pair, 3 beta-hydroxycholest-5-ene-7 alpha- and 7 beta-hydroperoxide (7 alpha, 7 beta-OOH), suggestive of iron-mediated free radical peroxidation. When photoperoxidized membranes containing 5 alpha-OOH, 6 alpha,6 beta-OOH and 7 alpha,7 beta-OOH (arising from 5 alpha-OOH rearrangement) were incubated with glutathione (GSH) and phospholipid hydroperoxide glutathione peroxidase (PHGPX), all hydroperoxide species underwent HPLC-EC(Hg)-detectable reduction to alcohols, the relative first order rate constants being as follows: 1.0 (5 alpha-OOH), 2.0 (7 alpha,7 beta-OOH), 2.4 (6 alpha-OOH) and 3.2 (6 beta-OOH).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of plasma cholesterol oxidation products using gas- and high-performance liquid chromatography/mass spectrometry

The application of gas chromatography and high-pressure liquid chromatography/mass spectrometry techniques for analysis of plasma cholesterol oxidation products is described. Cholesterol oxides that are widely identified in biological samples were subjected to gas (GC) and high-pressure liquid chromatographic (HPLC) separations, and their detection and characterization by mass spectrometry (MS) were compared. Analysis of cholesterol oxides from plasma samples revealed distinct advantages for each method according to the specific cholesterol oxide in question. Whereas HPLC/MS analysis of cholesterol oxides provided less resolution and lower sensitivity as compared to GC/MS, a distinct advantage was evident for direct measurements of cholesterol-7-hydroperoxides and 7-ketocholesterol. These two cholesterol oxides are particularly sensitive to storage in solvents, derivatization procedures, and analytical conditions used for GC analysis, which are minimized or avoided using the HPLC/MS conditions described. Analysis of human and rabbit plasma samples identified cholest-5-ene-3 beta, 7 beta-diol (7 beta-hydroxycholesterol); 5,6 alpha-epoxy-5 alpha-cholestan-3 beta-ol (cholesterol-5 alpha, 6 alpha-epoxide); 5 alpha-cholestane-3 beta, 5,6 beta-triol (cholestanetriol); 3 beta-hydroxycholest-5-ene-7-one (7-ketocholesterol); and 5,6 beta-epoxy-5 beta-cholestan-3 beta-ol (cholesterol-5 beta,6 beta-epoxide) as commonly occurring components (trivial names indicated in parentheses). The latter two compounds were dramatically increased in hypercholesterolemic samples and were found in approximately equal amounts in the free cholesterol and cholesteryl ester fractions. Although most of the plasma cholesterol oxides are found in the dietary cholesterol, others are not, particularly cholesterol-5 beta,6 beta-epoxide, suggesting that at least some of these compounds are formed by in vivo oxidation of cholesterol. Despite the readily measurable levels of the above cholesterol oxides, as well as other less prominent oxides, there was no evidence of cholesterol-7-hydroperoxides associated with plasma free cholesterol. Although several of the plasma cholesterol oxides may derive from cholesterol-7-hydroperoxides, it appears that the latter are either unstable and decompose in plasma, are metabolized to other cholesterol oxidation products, or break down during their isolation.

7 alpha hydroxylation of 25-hydroxycholesterol in liver microsomes. Evidence that the enzyme involved is different from cholesterol 7 alpha-hydroxylase

Rat, pig and human liver microsomes were found to catalyze 7 alpha-hydroxylation of 25-hydroxycholesterol. In contrast to cholesterol 7 alpha-hydroxylase activity, the 7 alpha-hydroxylase activity towards 25-hydroxycholesterol in rat liver was not stimulated by cholestyramine treatment. After transfection with cDNA for human cholesterol 7 alpha-hydroxylase, COS cells showed a significant activity towards cholesterol but not towards 25-hydroxycholesterol. During purification of cholesterol 7 alpha-hydroxylase from pig liver microsomes, about 99% of the 7 alpha-hydroxylase activity towards 25-hydroxycholesterol and 27-hydroxycholesterol was clearly separated from 7 alpha-hydroxylase activity for cholesterol. The small amount of 25-hydroxycholesterol 7 alpha-hydroxylase activity retained in a partially purified preparation of cholesterol 7 alpha-hydroxylase was not inhibited by addition of cholesterol, indicating that the oxysterol binding site is different from the cholesterol binding site, presumely due to the presence of two different enzymes. It is concluded that different enzymes are involved in 7 alpha-hydroxylation of cholesterol and 7 alpha hydroxylation of side-chain-oxidized cholesterol in rat, pig and human liver. Inhibition experiments with a partially purified fraction of the oxysterol 7 alpha-hydroxylase from pig liver gave results consistent with the contention that the same enzyme is responsible for 7 alpha hydroxylation of both 25-hydroxycholesterol and 27-hydroxycholesterol. It has been suggested that cholesterol 7 alpha-hydroxylase can preferentially use oxysterols, in particular 25-hydroxycholesterol, as substrates and by this means inactivate important physiological regulators of cholesterol homeostasis. Such a mechanism would explain the unique property of the liver to resist down-regulation of the low-density-lipoprotein receptor [Dueland, S., Trawick, J.D., & Davies, R.A. (1993) J. Biol. Chem. 267, 22695-22698]. The present results do not support the contention that the important coupling between cholesterol 7 alpha-hydroxylase activity, the low-density-lipoprotein receptor activity and hydroxymethylglutaryl coenzyme A reductase activity in liver cells is due to inactivation of 25-hydroxycholesterol or 27-hydroxycholesterol by the action of cholesterol 7 alpha-hydroxylase.

Peroxide-dependent amino acid oxidation and chemiluminescence catalysed by magnesium-pyridoxal phosphate-glutamate complex

Magnesium-pyridoxal-5'-phosphate-glutamate (MPPG) has been shown to ameliorate atherosclerotic symptoms in rabbits. In vitro, MPPG in the presence of peroxides such as cholesterolhydroperoxide or cumene hydroperoxide and Mn2+ ions produces "excited states" measurable as chemiluminescence or ethylene release from 1-aminocyclopropane-1-carboxylic acid (ACC). The reactions are stimulated synergistically by unsaturated fatty acids. Pyridoxal phosphate exhibits similar properties, but can be differentiated from the activities of MPPG or the sum of the components present in MPPG.

Photoperoxidation of cholesterol in homogeneous solution, isolated membranes, and cells: comparison of the 5 alpha- and 6 beta-hydroperoxides as indicators of singlet oxygen intermediacy

Singlet oxygen (1O2) can react with cholesterol (Ch) to give three possible ene-addition hydroperoxides: 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide (5 alpha-OOH), 3 beta-hydroxycholest-4-ene-6 alpha-hydroperoxide (6 alpha-OOH), and 3 beta-hydroxycholest-4-ene-6 beta-hydroperoxide (6 beta-OOH). The rates of dye-sensitized photogeneration and also the fates of 5 alpha-OOH and 6 beta-OOH in membrane bilayers have been studied and compared. Irradiation of unilamellar [14C]Ch/phospholipid vesicles in the presence of aluminum phthalocyanine tetrasulfonate or merocyanine 540 resulted in formation of 5 alpha-OOH and 6 beta-OOH, as determined by high performance liquid chromatography with radiochemical or electrochemical detection. The initial rate of 6 beta-OOH formation was 30-35% that of 5 alpha-OOH in a variety of liposomal systems. However, after a lag, 5 alpha-OOH invariably decayed via allylic rearrangement to 7 alpha-OOH (also known to be a free radical product), whereas 6 beta-OOH accumulated in unabated fashion until Ch depletion became limiting. Photooxidation of Ch in an isolated natural membrane (erythrocyte ghost) or in L1210 leukemia cells gave similar results. When the reaction was carried out in pyridine or methanol, the rate of 6 beta-OOH formation relative to 5 alpha-OOH was reduced by approximately half, with essentially no isomerization of the latter to 7 alpha-OOH.(ABSTRACT TRUNCATED AT 250 WORDS)

Photosensitized oxidation of cholesterol in biological systems: reaction pathways, cytotoxic effects and defense mechanisms

Cholesterol resembles other unsaturated lipids in being susceptible to peroxidative degradation when exposed to a sensitizing agent, exciting light of suitable wavelength and molecular oxygen. Selected hydroperoxides of cholesterol can be used as relatively convenient and reliable indicators of primary photochemical mechanisms, allowing a distinction to be made between free radical-mediated and singlet oxygen-mediated reactions. When generated in cell membranes, hydroperoxides of cholesterol and other lipids can have deleterious effects on membrane structure and function. Such damage may be exacerbated if these photoproducts undergo one-electron reduction to oxyl radicals which in turn initiate chain peroxidation reactions. Cells can resist these effects by using a membrane-based glutathione peroxidase to catalyze the two-electron reduction and detoxification of lipid hydroperoxides. Recent advances in our understanding of cholesterol photo-oxidation from the standpoints of (a) mechanistic information, (b) cytotoxicity and (c) cytoprotection are discussed in this article.

Wavelength dependence of photoinduced peroxidation and cytotoxicity of human low density lipoproteins

The relative abilities of UV-A, B and C radiations to initiate lipid peroxidation and apolipoprotein (apo) B modification of human purified low density lipoproteins have been compared. Ultraviolet-B and C (at 310 and 254 nm, respectively) exhibited similar efficacy as shown by the increase in lipid peroxidation markers (conjugated dienes, thiobarbituric acid reactive substances and fluorescent lipid soluble products) and in oxysterols, as well as by the decrease of the contents of natural antioxidants (tocopherols and carotenes) and in polyunsaturated fatty acids. In contrast, UV-A (at 360 nm) was found poorly effective and only at very high radiation intensities. Under all the conditions used, apoB was not affected by the UV radiations as shown by the stability of amino acid composition (except tryptophan level) and of trinitrobenzenesulfonic acid reactive amino group content. Similarly, the low density lipoprotein size was not altered. By comparison, low density lipoproteins oxidized by transition metal presented strong alterations of apoB and major changes of the apparent low density lipoprotein size. Finally, low density lipoproteins irradiated by UV-B. or C exhibited a much higher cytotoxicity on cultured cells than those irradiated by UV-A. Under the conditions used in this paper, the cytotoxic effect of the irradiated low density lipoproteins was positively correlated with their content in lipid peroxidation products and inversely correlated with their tocopherol content.

Phthalocyanine-sensitized lipid peroxidation in cell membranes: use of cholesterol and azide as probes of primary photochemistry

Various phthalacyanine (Pc) derivatives of phototherapeutic interest have been shown to be efficient type II (singlet oxygen, 1O2) sensitizers in aqueous and non-aqueous solutions. However, primary Pc photochemistry in biological environments, e.g. cell membranes, has not been studied in a definitive manner. To address this question, we used endogenous cholesterol in the erythrocyte ghost as a mechanistic reporter lipid Membranes sensitized with chloroaluminum Pc tetrasulfonate (AlPcS) and exposed to white light at 10 degrees C underwent lipid peroxidation, as indicated by the accumulation of hydroperoxides and thiobarbituric acid reactivity. Specific analysis of cholesterol photo-products by thin layer chromatography and high performance liquid chromatography revealed the presence of 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide (5 alpha-OOH), with much smaller amounts of 3 beta-hydroxycholest-5-ene-7 alpha-hydroperoxide (7 alpha-OOH) and 5 alpha-cholest-6-en-3 beta, 5-diol and cholest-5-en-3 beta, 7 alpha-diol (5 alpha-OH and 7 alpha-OH). Identification of 5 alpha-OOH as a major photoproduct provides unambiguous evidence for large scale 1O2 intermediacy. Azide inhibited lipid peroxidation in a dose-dependent fashion, providing additional support for a type II mechanism. However, the 1O2 quenching constant from Stern-Volmer analysis was approximately 50 times lower than that determined for a non-membrane probe, lactate dehydrogenase. The latter value agreed with literature values. A probable explanation is that membrane-bound dye generates most of the 1O2 involved in lipid peroxidation. Although azide can intercept any 1O2 escaping into (or formed in) the medium, it has limited access to 1O2 generated on the membrane and reacting (or being quenched) near its site of origin.

Photosensitized lipid peroxidation and enzyme inactivation by membrane-bound merocyanine 540: reaction mechanisms in the absence and presence of ascorbate

The lipophilic photosensitizing dye merocyanine 540 (MC540) is being studied intensively as an antitumor and antiviral agent. Since plasma membranes are believed to be the principal cellular targets of MC540-mediated photodamage, we have studied membrane damage in a well characterized test system, the human erythrocyte ghost. When irradiated with white light, MC540-sensitized ghosts accumulated lipid hydroperoxides (LOOHs derived from phospholipids and cholesterol) at a rate dependent on initial dye concentration. Neither desferrioxamine nor butylated hydroxytoluene inhibited LOOH formation, suggesting that Type I (iron-mediated free radical) chemistry is not important. By contrast, azide inhibited the reaction in a dose-dependent fashion, implicating a Type II (singlet oxygen, 1O2) mechanism. Stern-Volmer analysis of the data gave a 1O2 quenching constant approximately 50 times lower than that determined for an extramembranous target, lactate dehydrogenase (the latter value agreeing with literature values). This suggests that 1O2 reacts primarily at its membrane sites of origin and that azide has limited access to these sites. Using [14C]cholesterol-labeled membranes and HPLC with radiodetection, we identified 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide as the major cholesterol photoproduct, thereby confirming 1O2 intermediacy. Irradiation of MC540-sensitized membranes in the presence of added iron and ascorbate resulted in a large burst of lipid peroxidation, as shown by thiobarbituric acid reactivity and appearance of 7-hydroperoxycholesterol and 7-hydroxycholesterol as major oxidation products. Amplification of MC540-initiated lipid peroxidation by iron/ascorbate (attributed to light-independent reduction of nascent photoperoxides, with ensuing free radical chain reactions) could prove useful in augmenting MC540's phototherapeutic effects.

Photodynamic lipid peroxidation in biological systems

Oxidative degradation of cell membrane lipids in the presence of molecular oxygen, a sensitizing agent and exciting light is termed photodynamic lipid peroxidation (photoperoxidation). Like other types of lipid peroxidation, photoperoxidation is detrimental to membrane structure and function, and could play a role in many of the toxic as well as therapeutic effects of photodynamic action. Recent advances in our understanding of photoperoxidation and its biomedical implications are reviewed in this article. Specific areas of interest include (a) reaction mechanisms; (b) methods of detection and quantitation; and (c) cellular defenses (enzymatic and non-enzymatic).

Singlet molecular oxygen quenching by saturated and unsaturated fatty-acids and by cholesterol

The rate constants of molecular singlet oxygen quenching by saturated and unsaturated fatty-acids and by cholesterol-membrane critical components - membrane critical components - have been measured by time resolved detection of the 1270 nm phosphorescence of singlet molecular oxygen [O2(1deltag)]. We have determined (i) an increment of 5.7 x 10(2)M(-1)s(-1) per -CH2- in C6D6 and CD3OD for saturated fatty acids between C4 and C20, (ii) an increment of 3 x 10(4)M(-1)s(-1) per non-conjugated cis-double bond for C18 unsaturated fatty acids, identical in C6D6 and DC3OD, (iii) a lower quenching rate constant by a factor of 2.7 for the trans-C16 and trans-C18 as compared to the corresponding cis-monounsaturated fatty acids, (iv) a rate constant of O2x(1deltag) quenching by cholesterol of 5.7 x 10(4)M(-1)s(-1) in benzene. These rate constants are compared to those obtained for other membrane cellular components.

Theoretical analysis of a site-specific chemiluminescence reaction and its application to quantitation of lipid hydroperoxides

A system was designed for chemiluminescent measurement of lipid hydroperoxides by their site-specific reaction in sodium dodecylsulfate micelles. Ferrous ion-induced decomposition of lipid hydroperoxides in the sodium dodecylsulfate micelles resulted in strong chemiluminescence of the Cypridina luciferin analog, 2-methyl-6-phenyl-3,7-dihydroimidazo[1,2-alpha]pyrazin-3-one (CLA). After addition of ferrous sulfate to the micelles containing lipid hydroperoxide and luciferin, the chemiluminescence intensity reached a maximum rapidly and then decreased. The sequence of this reaction was elucidated by theoretical analysis, which demonstrated that the maximum chemiluminescence intensity is proportional to the initial concentration of hydroperoxide. Good linear relationships were observed between the maximum counts of chemiluminescence and the amounts of hydroperoxides of linoleic acid, phosphatidylcholine, choresterol (5 alpha), cumene and tert-butyl and hydrogen peroxide. This chemiluminescence method was simple and sensitive enough to detect picomole levels of linoleic acid and phosphatidylcholine hydroperoxides.

Sterol hydroperoxide metabolism by Salmonella typhimurium

In order to rationalize multiphasic dose-response data evincing mutagenicity towards Salmonella typhimurium TA1537 for sterol hydroperoxides 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide and 3 beta-hydroxycholest-5-ene-7 alpha-hydroperoxide their metabolism by the bacterial test strain was investigated. The 5 alpha-hydroperoxide was isomerized to the 7 alpha-hydroperoxide and reduced to 5 alpha-cholest-6-ene-3 beta,5-diol; the 7 alpha-hydroperoxide was reduced to cholest-5-ene-3 beta,7 alpha-diol and transformed to 3 beta-hydroxycholest-5-en-7-one. The 3 beta,5 alpha-diol and 3 beta,7 alpha-diol were not interconverted nor was either transformed to the 7-ketone.