Cholesterol as a singlet oxygen detector in biological systems

In cells under oxidative attack, membrane Ch, through the formation of its signature hydroperoxide and diol products, can serve as a unique detector in situ, allowing discrimination between 1O2 and free radical intermediacy. Of the two techniques described for analyzing Ch oxidation products, TLC with color development suffices for preliminary, mainly qualitative product screening, whereas a high-performance approach such as HPLC-EC(Hg) is advised when maximum resolution and sensitivity of quantitation are necessary. By using these strategies, one can monitor the formation of 1O2, for example, in a biologically relevant milieu (membrane), thus avoiding the difficulties associated with external detection, e.g., by physical means. These approaches would be valuable for assessing reaction mechanisms for various oxidative agents of biomedical importance, including environmental phototoxins and the rapidly emerging family of phototherapeutic drugs. Although photodynamic stress has been emphasized, the methods described should have broad applicability in the elucidation of oxidative mechanisms.

Reactivity of phospholipid hydroperoxide glutathione peroxidase with membrane and lipoprotein lipid hydroperoxides

A comparative study has been carried out on the general reactivity of lipid hydroperoxides in liposomes, biological membranes and lipoproteins with two Se-dependent peroxidases: Glutathione Peroxidase (GPX) and Phospholipid Hydroperoxide Glutathione Peroxidase (PHGPX). While PHGPX reduces all hydroperoxides derived from phospholipids, cholesterol and cholesterol esters, GPX reduces only fatty acid hydroperoxides released after treatment of phospholipid hydroperoxides with phospholipase A2. These findings highlight the role of PHGPX in protecting biomembranes from peroxidative damage and add new insight into how cholesterol hydroperoxides are detossified in cells.

Photodynamic action of bilirubin on human erythrocyte membranes. Modification of polypeptide constituents

The photodynamic action of bilirubin on isolated human erythrocyte membranes (ghosts) has been studied. When incorporated into ghosts (pH 8.0,10 degrees) the bile pigment photosensitizes in blue light the peroxidation of unsaturated lipids, as evidenced by a positive color reaction with 2-thiobarbituric acid. Accompanying lipid peroxidation was the disappearance of most of the major membrane proteins (Coomassie Blue staining in sodium dodecyl sulfate-polyacrylamide gel electrophoresis) and appearance of polypeptide photoproducts of greater size (mol wt greater than 250,000). The association of membrane proteins (presumably by cross-linking) was insignificant when bilirubin-ghost suspensions were kept in the dark, or when ghosts were irradiated in the absence of bilirubin. Electrophoretic bands 1 and 2 (Fairbanks, G., Steck, T.L., and Wallach, D. F.H (1971), Biochemistry 10, 2606) diminished rapidly during the photoreaction, whereas band 3 and the three sialoglycoproteins disappeared at a much slower rate. Dispersal of membrane consituents by treatment with sodium dodecyl sulfate prior to irradiation resulted in relatively little peroxidation and no noticeable formation of high molecular weight polypeptide complexes. The possibility that malonaldehyde, a product of lipid peroxidation, is involved in cross-linking during irradiation was studied by incubating ghosts with exogenous malonaldehyde. Although the reagent did cross-link membrane proteins (electrophoretic bands 1, 2, 2.1 2.2, and 4.1 diminished most rapidly and high molecular weight bands appeared), the reaction could only be demonstrated with malonaldehyde concentrations several orders of magnitude greater than those detected in irradiation experiments. If malonaldehyde cross-linking occurs, it does not appeare to be the predominant mechanism of polypeptide association during irradiation of bilirubin-containing ghosts.

Lipid photooxidation in erythrocyte ghosts: sensitization of the membranes toward ascorbate- and superoxide-induced peroxidation and lysis

The damaging effects of ascorbate (AH-) and superoxide (O-2) on resealed erythrocyte ghosts containing predetermined levels of lipid hydroperoxides (LOOHs) have been studied. Continuous blue light irradiation of membranes in the presence of protoporphyrin resulted in steadily increasing LOOH levels and enhanced release of a trapped marker, glucose 6-phosphate (G6P), after a 3- to 4-h lag. Neither superoxide dismutase (SOD) nor catalase inhibited these effects, ruling out O-2 and H2O2 as reactive intermediates. A 1-h light dose produced partially photoperoxidized ghosts, which, in the dark at 37 degrees C, released G6P no faster than unirradiated controls (approximately 7%/h). When xanthine oxidase plus xanthine (XO/X) was introduced as a source of O-2 and H2O2, the irradiated membranes lysed rapidly (t1/2 approximately 2 h). EDTA or SOD inhibited the reaction, whereas catalase had little or no effect. Unirradiated ghosts were not lysed by XO/X unless the system was supplemented with Fe(III), in which case total protection was afforded by SOD or catalase. In all experiments there was an excellent correlation between postirradiation lipid peroxidation (thiobarbituric acid reactivity) and G6P release. Similar observations were made with AH-. For example, dark incubation of photooxidized ghosts in the presence of 0.5 mM AH- resulted in rapid lysis (t1/2 approximately 1 h), which was stimulated approximately twofold by 50 microM Fe(III) and was inhibited by EDTA. By comparison, unirradiated ghosts showed no net peroxidation or lysis after 3 h exposure to Fe(III)/AH-. Neither SOD nor catalase protected against AH--stimulated damage. AH--promoted lipid peroxidation was inhibited by butylated hydroxytoluene, a lipophilic antioxidant, but was unaffected by 2,5-dimethylfuran or ethanol, singlet oxygen, and hydroxyl radical traps, respectively. These results suggest that a mechanism exists by which photogenerated LOOHs undergo redox metal-mediated reduction to alkoxy radicals (LO.), which trigger a burst of membrane-disrupting lipid peroxidation.

Singlet oxygen intermediacy in the photodynamic action of membrane-bound hematoporphyrin derivative

The cell-damaging photochemistry of hematoporphyrin derivative (HPD) has been investigated using isolated erythrocyte membranes as a test system. Irradiation of membranes in the presence of the tumor-localizing fraction of HPD resulted in formation of singlet molecular oxygen (1O2) as measured by the phosphorescence at 1268 nm. The authentic product of 1O2 attack on cholesterol, 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide, was identified in this system. Relatively insignificant amounts of free radical-derived hydroperoxides were detected. These results suggest that 1O2 plays a major role in the HPD-sensitized photokilling of tumor cells in vivo.

High-performance liquid chromatography with mercury cathode electrochemical detection: application to lipid hydroperoxide analysis

Lipid hydroperoxide species can be analyzed with high sensitivity and specificity, using reversed-phase high-performance liquid chromatography with reductive mode electrochemical detection on a mercury drop cathode [HPLC-ED(Hg)]. The purpose of this study was to examine different variables in the operation of HPLC-ED(Hg) and to select optimal conditions for the analysis of several biologically relevant peroxides, including species derived from cholesterol, cholesteryl linoleate, oleate, linoleate, and two synthetic phosphatidylcholines. Parameters such as operating potential and mobile-phase solvent proportions, electrolyte composition, and ionic strength were evaluated for each peroxide class. Under optimal conditions, we have achieved baseline separation of four cholesterol hydroperoxide species, not only from one another, but also from phospholipid hydroperoxides; detection limits were < 0.3 pmol and < 30 pmol for the cholesterol and phospholipid hydroperoxides, respectively.

Phospholipase Action of Platelet-activating Factor Acetylhydrolase, but Not Paraoxonase-1, on Long Fatty Acyl Chain Phospholipid Hydroperoxides

Phospholipid hydroperoxide (PLOOH) degrading activity of high density lipoprotein (HDL)-derived paraoxonase-1 (PON1) was investigated, using peroxidized 1-palmitoyl-2-oleoyl phosphatidylcholine (PCOOH) as substrate and high performance thin layer chromatography for quantitative peroxide analysis. Incubation of PCOOH with PON1 resulted in decay of the latter and reciprocal buildup of oleic acid hydroperoxide (OAOOH) at rates unaffected by GSH or other reductants. A serine esterase inhibitor blocked this activity and a recombinant PON1 was devoid of it, raising the possibility that the activity represents platelet-activating factor acetylhydrolase (PAF-AH), an esterase that co-purifies with PON1 from HDL. This was verified by showing that a recombinant PAF-AH recapitulates the ability of natural PON1 to hydrolyze PCOOH and release OAOOH while having essentially no effect on parental PC. Furthermore, recombinant PAF-AH and natural PON1 were shown to have similar K(m) values for PCOOH hydrolysis. Finally, we found that recombinant PAF-AH, but not PON1, catalyzes PLOOH hydrolysis in peroxidized low density lipoprotein. We conclude from this study that PON1 is neither a PLOOH peroxidase nor hydrolase and that the phospholipase A(2)-like activity previously attributed to PON1 in natural enzyme preparations was actually due to novel PLOOH hydrolytic activity of contaminating PAF-AH.

Hyperresistance to cholesterol hydroperoxide-induced peroxidative injury and apoptotic death in a tumor cell line that overexpresses glutathione peroxidase isotype-4

The selenoenzyme phospholipid hydroperoxide glutathione peroxidase (PHGPX; GPX4) plays a key role in eukaryotic defense against potentially lethal peroxidative injury and also regulation of physiological peroxide tone. In this work we focused on the cytoprotective antiperoxidant effects of GPX4, using a breast tumor epithelial cell line that over-expresses the enzyme. Wild-type COH-BR1 cells, which exhibit little (if any) GPX4 activity, were transfected with a construct encoding the mitochondrion-targeted long (L) form of the enzyme. Several transfectant clones were selected which expressed relatively large amounts of GPX4, as determined by both Northern and Western analysis. Enzyme activity ranged from 15-fold to 190-fold greater than that of wild-type or null-transfected cells. The functional ramifications of GPX4 overexpression were tested by challenging cells with photochemically generated cholesterol hydroperoxides (ChOOHs) in liposomal form. Compared with vector controls, overexpressing clones were found to be substantially more resistant to ChOOH-induced killing, as determined by annexin-V (early apoptotic) and thiazolyl blue (mitochondrial dehydrogenase) reactivity. Concomitantly, the clones exhibited a striking hyper-resistance to free radical-mediated lipid peroxidation, as assessed by labeling cell membranes with [(14)C]cholesterol and measuring a family of radiolabeled oxidation products (ChOX). L-form GPX4's antiperoxidant and cytoprotective effects could reflect its ability to detoxify ChOOHs as they enter cells and/or cell-derived lipid hydroperoxides arising from ChOOH one-electron turnover.

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.

Glucose administration augments in vivo uptake and phototoxicity of the tumor-localizing fraction of hematoporphyrin derivative

Rhabdomyosarcoma tumors in rats made hyperglycemic by multiple injections of glucose exhibited a transient decrease in pH and an increased ability to accumulate derivatives of hematoporphyrin (HPD). Photoradiation of tumors in glucose/HPD-treated animals produced a greater cell kill than in galactose/HPD-treated controls. A therapeutic benefit of glucose administration in conjunction with HPD-phototherapy is suggested.

Accelerated migration and invasion of prostate cancer cells after a photodynamic therapy-like challenge: Role of nitric oxide

Employing an in vitro model for 5-aminolevulinic acid (ALA)-based photodynamic therapy (PDT), we recently reported that human prostate cancer PC3 cells rapidly and persistently overexpressed inducible nitric oxide synthase (iNOS) and nitric oxide (NO) after a moderate ALA/light challenge. The upregulated iNOS/NO was shown to play a key role in cell resistance to apoptotic photokilling and also in the dramatic growth spurt observed in surviving cells. In the present study, we found that PC3 cells surviving an ALA/light insult not only proliferated faster than non-stressed controls, but migrated and invaded faster as well, these effects being abrogated by an iNOS inhibitor or NO scavenger. Photostressed prostate DU145 cells exhibited similar behavior. Using in-gel zymography, we showed that PC3 extracellular matrix metalloproteinase-9 (MMP-9) was strongly activated 24 h after ALA/light treatment and that MMP-9 inhibitor TIMP-1 was downregulated, consistent with MMP-9 involvement in enhanced invasiveness. We also observed a photostress-induced upregulation of α6 and β1 integrins, implying their involvement as well. The MMP-9, TIMP-1, and integrin effects were strongly attenuated by iNOS inhibition, confirming NO's role in photostress-enhanced migration/invasion. This study reveals novel, potentially tumor-promoting, side-effects of prostate cancer PDT which may be averted through use of iNOS inhibitors as PDT adjuvants.

Photooxidation of cell membranes in the presence of hematoporphyrin derivative: reactivity of phospholipid and cholesterol hydroperoxides with glutathione peroxidase

The susceptibility of photodynamically-generated lipid hydroperoxides to reductive inactivation by glutathione peroxidase (GPX) has been investigated, using hematoporphyrin derivative as a photosensitizing agent and the human erythrocyte ghost as a target membrane. Photoperoxidized ghosts were reactive in a glutathione peroxidase/reductase (GPX/GRD)-coupled assay only after phospholipid hydrolysis by phospholipase A2 (PLA2). However, enzymatically determined lipid hydroperoxide values were consistently approx. 40% lower than iodometrically determined values throughout the course of photooxidation. Moreover, when irradiated ghosts were analyzed iodometrically during PLA2/GSH/GPX treatment, a residual 30-40% of non-reactive lipid hydroperoxide was observed. The possibility that cholesterol product(s) account for the non-reactive lipid hydroperoxide was examined by tracking cholesterol hydroperoxides in [14C]cholesterol-labeled ghosts. The sum of cholesterol hydroperoxides and GPX/GRD-detectable lipid hydroperoxides was found to agree closely with iodometrically determined lipid hydroperoxide throughout the course of irradiation. Thin-layer chromatography of total lipid extracts indicated that cholesterol hydroperoxide was unaffected by PLA2/GSH/GPX treatment, whereas most of the phospholipid peroxides were completely hydrolyzed and the released fatty acid peroxides were reduced to alcohols. It appears, therefore, that the GPX-resistant lipid hydroperoxides in photooxidized ghosts were derived primarily from cholesterol. Ascorbate plus Fe3+ produced a burst of free-radical lipid peroxidation in photooxidized, PLA2-treated ghosts. As expected for fatty acid hydroperoxide inactivation, the lipid peroxidation was inhibited by GSH/GPX, but only partially so, suggesting that cholesterol hydroperoxide-derived radicals play a major role in the reaction.

Hyperresistance to photosensitized lipid peroxidation and apoptotic killing in 5-aminolevulinate-treated tumor cells overexpressing mitochondrial GPX4

Antitumor photodynamic therapy (PDT) with administered 5-aminolevulinic acid (ALA) is based on metabolism of ALA to protoporphyrin IX (PpIX), which acts as a sensitizer of photo-oxidative damage leading to apoptotic or necrotic cell death. An initial goal of this study was to ascertain how the PpIX-sensitized death mechanism for a breast tumor line (COH-BR1 cells) might be influenced by the conditions of ALA exposure in vitro. Two different treatment protocols were developed for addressing this question: (i) continuous incubation with 1 mM ALA for 90 min; and, (ii) discontinuous incubation, i.e., 15 min with 1 mM ALA followed by 225 min without it. Following exposure to 2 J/cm2 of visible light, cell viability, death mechanism, and lipid hydroperoxide (LOOH) level were evaluated for each protocol using thiazolyl blue, Hoechst staining, and HPLC with electrochemical detection assays, respectively. PpIX was found to sensitize apoptosis when it existed mainly in mitochondria (protocol-1), but necrosis when it diffused to other sites, including plasma membrane (protocol-2). Experiments with a transfectant clone, 7G4, exhibiting approximately 85 times greater activity of the LOOH-detoxifying selenoenzyme GPX4 than parental cells, provided additional information about death mechanism. Located predominantly in mitochondria of 7G4 cells, GPX4 strongly inhibited both LOOH accumulation and apoptosis under protocol-1 conditions, but had no significant effect under protocol-2 conditions. These findings support the hypothesis that LOOHs produced by attack of photogenerated singlet oxygen on mitochondrial membrane lipids play an important early role in the apoptotic death cascade.

Photodynamic action of merocyanine 540 on erythrocyte membranes: structural perturbation of lipid and protein constituents

erocyanine 540 (MC540) is a membrane-directed photosensitizing dye with antileukemic and antiviral properties. In this study, biophysical and biochemical techniques have been used to examine MC540-sensitized photooxidative damage in the lipid and protein compartments of a test membrane, the human erythrocyte ghost. Irradiation of MC540-sensitized ghosts with white light resulted in oxidative damage to proteins, as manifested by (i) loss of sulfhydryl groups; (ii) intermolecular cross-linking of major polypeptides; and (iii) loss of Mg(2+)-ATPase and Na+,K(+)-ATPase activities. Photooxidation also produced a rapid and progressive increase in general protein motion, as measured by electron paramagnetic resonance spectrometry (EPR) with the sulfhydryl spin label MAL-6. In addition to these effects, ghosts exposed to MC540 and light underwent lipid peroxidation. EPR with two lipophilic spin probes, 5-doxylstearate and 16-doxylstearate, showed that lipid peroxidation is accompanied by a progressive decrease in bilayer fluidity (motional freedom). At a given dye concentration, structural perturbations of proteins were detected at much lower light fluences than those of lipids. When photoreactions were carried out in the presence of ascorbate and iron, there was a strong stimulation of lipid peroxidation (attributed to free radical chain reactions), with a concomitant greater decrease in lipid mobility. Thus, the deleterious effects of photoperoxidation on lipid structure and motional freedom were greatly exacerbated by ascorbate and iron. Membrane damage similar to that described here may play a role in the phototherapeutic activity of MC540.

Lipid hydroperoxide analysis by high-performance liquid chromatography with mercury cathode electrochemical detection

In addition to the applications described, HPLC-EC(Hg) can be used for determining LOOHs in lipoproteins and for monitoring LOOH detoxification in cells. As it continues to be developed and refined, this approach should prove to be valuable not only for ultrasensitive determination of lipid-derived peroxides, but protein- and nucleic acid-derived peroxided as well.

Hemin-enhanced resistance of human leukemia cells to oxidative killing: antisense determination of ferritin involvement

Human HL-60 cells exhibited a strong hyperresistance to the lethal effects of photodynamic activity (singlet oxygen) or glucose oxidase activity (hydrogen peroxide) 16-20 h after being exposed to hemin (ferriprotoporphyrin IX). Hyperresistance was accompanied by the overproduction of immunodetectable ferritin, predominantly the heavy (H) subunit, which exhibits ferroxidase activity. Cells that had been enriched in apoferritin via pinocytotic uptake showed similar hyperresistance to both types of oxidative challenge. On the other hand, preincubating cells with hemin in the presence of a phosphorothioate-linked antisense oligodeoxynucleotide against H-ferritin mRNA resulted in a strong diminution in both hyperresistance and H-ferritin induction. No effects were seen when a scrambled order oligodeoxynucleotide of the same base composition was used, confirming that the antisense oligomer had specifically inhibited H-ferritin translation. These results indicate that induced ferritin played a crucial role in the observed cytological responses. Enhanced oxidant resistance is attributed to the ability of this ferritin to rapidly sequester and incapacitate redox-active iron.

Cytoprotective signaling associated with nitric oxide upregulation in tumor cells subjected to photodynamic therapy-like oxidative stress

Photodynamic therapy (PDT) employs photoexcitation of a sensitizer to generate tumor-eradicating reactive oxygen species. We recently showed that irradiating breast cancer COH-BR1 cells after treating with 5-aminolevulinic acid (ALA, a pro-sensitizer) resulted in rapid upregulation of inducible nitric oxide (NO) synthase (iNOS). Apoptotic cell killing was strongly enhanced by an iNOS inhibitor (1400W), iNOS knockdown (kd), or a NO scavenger, suggesting that NO was acting cytoprotectively. Stress signaling associated with these effects was examined in this study. ALA/light-stressed COH-BR1 cells, and also breast adenocarcinoma MDA-MB-231 cells, mounted an iNOS/NO-dependent resistance to apoptosis that proved to be cGMP-independent. Immunocytochemistry and subcellular Western analysis of photostressed COH-BR1 cells revealed a cytosol-to-nucleus translocation of NF-κB which was negated by the NF-κB activation inhibitor Bay11. Bay11 also enhanced apoptosis and prevented iNOS induction, consistent with NF-κB involvement in the latter. JNK and p38 MAP kinase inhibitors suppressed apoptosis, implicating these kinases in death signaling. Post-irradiation extent and duration of JNK and p38 phosphorylation were dramatically elevated by 1400 W or iNOS-kd, suggesting that these activations were suppressed by NO. Regarding pro-survival stress signaling, rapid activation of Akt was unaffected by 1400 W, but prevented by Wortmannin, which also enhanced apoptosis. Thus, a link between upstream Akt activation and iNOS induction was apparent. Furthermore, p53 protein expression under photostress was elevated by iNOS-kd, whereas robust Survivin induction was abolished, consistent with p53 and Survivin being negatively and positively regulated by NO, respectively. Collectively, these findings enhance our understanding of cytoprotective signaling associated with photostress-induced NO and suggest iNOS inhibitor-based approaches for improving PDT efficacy.

Analysis of cholesterol and phospholipid hydroperoxides by high-performance liquid chromatography with mercury drop electrochemical detection

High-performance liquid chromatography (HPLC) with reductive mode electrochemical detection on a mercury drop has been employed for the separation and determination of lipid hydroperoxides. Under the conditions used, baseline separation is achieved for three cholesterol hydroperoxide (ChOOH) standards, not only from one another, but also from two different phosphatidylcholine hydroperoxide (PCOOH) standards. Applying this method to a test system, photodynamically treated murine leukemia cells, we have identified and quantified a major family of overlapping PCOOHs and three ChOOHs, two of which are characteristic singlet oxygen adducts. In a typical separation, the detection limit is < 0.5 pmol for ChOOHs and < 50 pmol for more slowly eluting PCOOHs. In this respect, mercury drop detection outperforms all previously described electrochemical detection methods for lipid hydroperoxides and compares favorably with other HPLC-based approaches. However, in terms of equipment cost, relative simplicity of operation, and fewer potential artifacts, this method has a clear advantage over all other existing high-sensitivity methods.

Ascorbate-enhanced lipid peroxidation in photooxidized cell membranes: cholesterol product analysis as a probe of reaction mechanism

Cholesterol was used as an in situ probe for studying mechanisms of lipid peroxidation in isolated erythrocyte membranes subjected to different prooxidant conditions. The membranes were labeled with [14C]cholesterol by exchange with prelabeled unilamellar liposomes and photosensitized with hematoporphyrin derivative. Irradiation with a dose of blue light resulted in thiobarbituric acid-detectable lipid peroxidation that was increased markedly by subsequent dark incubation with 0.5-1.0 mM ascorbate (AH-). Ascorbate-stimulated lipid peroxidation was inhibited by EDTA, desferrioxamine (DOX) and butylated hydroxytoluene (BHT), suggesting that the process is free radical in nature and catalyzed by membrane-bound iron. Thin layer chromatography and radiometric scanning of extracted lipids from photooxidized membranes revealed that the major oxidation product of cholesterol was the 5 alpha-hydroperoxide (5 alpha-OOH), a singlet oxygen adduct. Post-irradiation treatment with AH-/Fe(III) resulted in an almost-total disappearance of 5 alpha-OOH and the preponderance of free radical oxidation products, e.g. 7-ketocholesterol, the epimeric 7 alpha-/7 beta-hydroperoxides (7 alpha-/7 beta-OOH) and their respective alcohols (7 alpha-/7 beta-OH). EDTA, DOX and BHT inhibited the formation of these products, while catalase and superoxide dismutase had no effect. These results are consistent with a mechanism involving 1-electron reduction of photogenerated hydroperoxides to oxyl radicals, which trigger bursts of free radical lipid peroxidation. Though generated in this system, partially reduced oxygen species, viz. superoxide, hydrogen peroxide and hydroxyl radical, appear to be relatively unimportant in the autoxidation process.

Role of hydrogen peroxide in the cytotoxic effects of UVA/B radiation on mammalian cells

Effects of selenium (Se) deficiency on the sensitivity of murine leukemia L1210 cells to broad band UVA/B radiation (310-400 nm) have been investigated. Cells rendered glutathione peroxidase (GPX) deficient by shortterm (2-3 week) growth in 1%, serum/RPMI medium without added Se [L.Se(-) cells] were found to be much less resistant to clonally assessed UVA/B lethality than Se-supplemented controls [L.Se(+) cells]. By contrast, long-term ( > 20 week) Se-deprived [L'.Se(-)] cells whose catalase (CAT) activity was elevated > 100-fold were far more resistant to UVA/B than L.Se(+) cells. Similar trends were observed for cells irradiated in 1% serum/RPMI or Hank's medium. Whereas the CAT inhibitor 3-amino-1,2,4-triazole had no effect on L.Se(+) photosensitivity, it produced a large increase in L'.Se(-) photosensitivity. These findings are consistent with H2O2 intermediacy in photokilling and suggest that L1210 cells depend mainly on GPX for protection against this species but switch to overexpressed CAT after chronic Se deprivation. In agreement with this, steady-state H2O2 levels measured by H2O2 electrode during UVA/B exposure were higher in L.Se(-) than L.Se(+) suspensions but much lower (barely detectable) in L'.Se(-) suspensions. Cytotoxic effects of UVA/B and variations thereof resulting from Se manipulation could be mimicked by treating cells with glucose oxidase in the presence of D-glucose, providing further support for H2O2 involvement. Whether UVA/B-generated H2O2 is directly cytotoxic or gives rise to a more damaging species such as hydroxyl radical (HO) is presently unknown.

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.