Genetic polymorphism of the CYP1A1, CYP2E1, GSTM1 and GSTT1 genes and lung cancer susceptibility in a north Indian population

The present study investigates in a experimental system in vitro the relationship between the non-enzymatic (ascorbate-Fe2+) and enzymatic (NADPH) lipid peroxidation in rat liver microsomes and nuclei. Chemiluminescence was measured as cpm/mg protein during 180 min at 37 degrees C. Approximately 50-55% of the fatty acids located in rat liver microsomes and nuclei are polyunsaturated with a prevalence of C18:2 n6 and C20:4 n6. The values of total light emission during the non-enzymatic and enzymatic lipid peroxidation were highest in microsomes than in nuclei. A significant decrease of C20:4 n6 and C22:6 n3 in rat liver microsomes and nuclei was observed during the lipid ascorbate-Fe2+-dependent peroxidation, whereas a significant decrease of C20:4 n6 in rat liver microsomes was observed during enzymatic lipid peroxidation. Over the time course studies, analysis of chemiluminescence in microsomes and nuclei demonstrated that the lipid peroxidation in the presence of ascorbate-Fe2+ reach a maximum at 50 and 30 min, respectively, whereas in the presence of NADPH it reachs a maximum at 20 min in both organelles. In liver microsomes and nuclei the peroxidizability index (pi) which indicates the degree of vulnerability to degradation of a selected membrane showed statistically significant differences between control versus ascorbate-Fe2+ when microsomes or nuclei were compared. Our results indicate that non-enzymatic (ascorbate-Fe2+) and enzymatic (NADPH) lipid peroxidation are operative in rat liver microsomes and nuclei but the sensitivities of both organelles to lipid peroxidation evidenced by chemiluminescence was greater in the presence of ascorbate-Fe2+ when compared with NADPH.     

Melatonin and N-acetyl serotonin inhibit selectively enzymatic and non-enzymatic lipid peroxidation of rat liver microsomes

Melatonin (N-acetyl-5-methoxytryptamine) and its immediate precursor N-acetyl serotonin in the metabolism of tryptophan are free radical scavengers that have been found to protect against non-enzymatic lipid peroxidation in many experimental models. By contrast, little is known about the antioxidant ability of these indoleamines against NADPH enzymatic lipid peroxidation. The light emission produced by rat-liver microsomes, expressed as total cpm during 180 min of incubation at 37 degrees C, was two-fold greater in the presence of ascorbate (0.4mM) when compared with NADPH (0.2 mM). Maximal peaks of light emission produced by microsomes lipid peroxidized with ascorbic-Fe(2+) or NADPH and expressed as cpm were 354,208 (at 60 min) and 135,800 (at 15 min), respectively. During non-enzymatic lipid peroxidation a decrease of total chemiluminescence (inhibition of lipid peroxidation) was observed when increasing concentrations of melatonin were added to liver microsomes. The protective effect was concentration-dependent. The inhibition observed in light emission was coincident with the protection of the most PUFAs. Preincubation of microsomes with N-acetyl serotonin reduced these changes very dramatically. Thus, in the presence of both antioxidants (0.36, 0.75, 1.5 mM), light emission percent inhibition during non-enzymatic (ascorbate-Fe(2+)) lipid peroxidation of rat liver microsomes was for melatonin: 6.12, 16.20, 34.88 and for N-acetyl serotonin: 85.10, 88.48, 84.4 respectively. The incubation of rat liver microsomes in the presence of NADPH (0.36, 0.75, 1.5 mM) produce a sudden increase of chemiluminescence that gradually increased and reached a maximal value at about 15 min; however, N-acetyl serotonin reduced these changes very efficiently.     

The relationship between chemiluminescence and lipid peroxidation in rat hepatic microsomes.

Studies were carried out to determine the relationship between NADPH- and ascorbate-initiated chemiluminescence (CL) and lipid peroxidation (LP) in rat hepatic microsomes. NADPH-initiated CL and LP become maximal 15 min after addition of NADPH to the microsomes and ascorbate-initiated CL and LP become maximal 90 to 120 min following addition of ascorbate. There are four lines of evidence to indicate that both NADPH- and ascorbate-initiated chemiluminescence are related to lipid peroxidation. (i) The time courses for the increases in CL and in LP are identical. (ii) There is a linear relationship between total (integral) or maximal CL and LP. (iii) Drug substrates which inhibit LP also inhibit CL in a quantitatively similar manner. (iv) Inhibitors of lipid peroxidation, such as Co²⁺, Mn²⁺, Hg²⁺, para-chloromercuribenzenesulfonic acid, and EDTA, also inhibit chemiluminescence. The results of these experiments indicate that chemiluminescence initiated in hepatic microsomes by either NADPH or ascorbate is directly proportional to lipid peroxidation.     

Antioxidant mechanism of Mn(II) in phospholipid peroxidation.

Antioxidant action of Mn2+ on radical-mediated lipid peroxidation without added iron in microsomal lipid liposomes and on iron-supported lipid peroxidation in phospholipid liposomes or in microsomes was investigated. High concentrations of Mn2+ above 50 microM inhibited 2,2'-azobis (2-amidinopropane) (ABAP)-supported lipid peroxidation without added iron at the early stage, while upon prolonged incubation, malondialdehyde production was rather enhanced as compared with the control in the absence of Mn2+. However, in a lipid-soluble radical initiator, 2,2'-azobis (2,4-dimethyl-valeronitrile) (AMVN)-supported lipid peroxidation of methyl linoleate in methanol Mn2+ apparently did not scavenge lipid radicals and lipid peroxyl radicals, contrary to a previous report. At concentrations lower than 5 microM, Mn2+ competitively inhibited Fe(2+)-pyrophosphate-supported lipid peroxidation in liposomes consisting of phosphatidylcholine with arachidonic acid at the beta-position and phosphatidylserine dipalmitoyl, and reduced nicotinamide adenine dinucleotide phosphate (NADPH)-supported lipid peroxidation in the presence of iron complex in microsomes. Iron reduction responsible for lipid peroxidation in microsomes was not influenced by Mn2+.     

A high involvement of O2- possibly generated in inner membranes for iron-induced microsomal lipid peroxidation.

The lipid peroxidation of and the O2- generation by rat liver microsomes in the presence of NADPH or both NADPH and Fe3+ were determined by thiobarbituric acid-reacting substance formation and by chemiluminescence intensities with a cypridina luciferin analog, 2-methyl-6-(p-methoxyphenyl)-3, 7-dihydroimidazo[1,2-a]pyrazin-3-one(MCLA), as a chemiluminescence probe. Judging from the experiments with various inhibitors on the O2- generation and the lipid peroxidation, O2- generated, at intramembranous site, by cytochrome P-450 system is considered to be highly involved in the iron-induced lipid peroxidation.     

Lipid peroxidation of phosphatidylcholine liposomes depressed by the calcium channel blockers nifedipine and verapamil and by the antiarrhythmic-antihypoxic drug stobadine

Nifedipine, verapamil and stobadine were tested and compared with butylated hydroxytoluene (BHT) as possible free radical scavengers inhibiting lipid peroxidation in phosphatidylcholine liposomes. Liposomes were peroxidized by incubation in air at 50 degrees C. Verapamil less than nifedipine less than BHT less than stobadine depressed the lipid peroxidation as detected spectroscopically for conjugate diene and thiobarbituric acid product formation. Verapamil and stobadine were tested as OH radical scavengers in a Fenton-type reaction against spin trap 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO), as detected by ESR spectroscopy. The tested drugs competed with DMPO in trapping OH radicals, with stobadine being more effective than verapamil. ESR spectra of nifedipine in the incubated liposomes revealed that nifedipine could be involved in free radical reactions in the liposomes leading to nifedipine-stable radical(s) which were immobilized in the membrane. The obtained results suggest that some of the beneficial effects of the studied drugs can be mediated in disease by their ability to scavenge free radicals and by their protective effect on lipid peroxidation.     

The antioxidant action of ketoconazole and related azoles: comparison with tamoxifen and cholesterol

The azole antifungal drug ketoconazole was found to inhibit Fe(III)-ascorbate dependent lipid peroxidation using either rat liver microsomes or ox-brain phospholipid liposomes as the substrate. It also inhibited microsomal peroxidation induced by the Fe(III)-ADP/NADPH system. The related azoles, miconazole and clotrimazole, were much weaker inhibitors than ketoconazole. Ketoconazole was approximately equipotent with the triphenylethylene anticancer drug tamoxifen in the microsomal system and was almost as effective as 4-hydroxytamoxifen in the liposomal system. Ketoconazole introduced into phospholipid liposomes during their preparation inhibited Fe(III)-ascorbate induced lipid peroxidation to a greater extent than similarly introduced cholesterol, ergosterol or tamoxifen. Miconazole and clotrimazole were again poor inhibitors of lipid peroxidation in this system. These antioxidant effects of ketoconazole may be due to membrane stabilization in the systems used. The implications of our findings for the clinical applications of these drugs are discussed.     

Superoxide generation by NADPH-cytochrome P-450 reductase: the effect of iron chelators and the role of superoxide in microsomal lipid peroxidation

Superoxide generation, assessed as the rate of acetylated cytochrome c reduction inhibited by superoxide dismutase, by purified NADPH cytochrome P-450 reductase or intact rat liver microsomes was found to account for only a small fraction of their respective NADPH oxidase activities. DTPA-Fe3+ and EDTA-FE3+ greatly stimulated NADPH oxidation, acetylated cytochrome c reduction, and O(2) production by the reductase and intact microsomes. In contrast, all ferric chelates tested caused modest inhibition of acetylated cytochrome c reduction and O(2) generation by xanthine oxidase. Although both EDTA-Fe3+ and DTPA-Fe3+ were directly reduced by the reductase under anaerobic conditions, ADP-Fe3+ was not reduced by the reductase under aerobic or anaerobic conditions. Desferrioxamine-Fe3+ was unique among the chelates tested in that it was a relatively inert iron chelate in these assays, having only minor effects on NADPH oxidation and/or O(2) generation by the purified reductase, intact microsomes, or xanthine oxidase. Desferrioxamine inhibited microsomal lipid peroxidation promoted by ADP-Fe3+ in a concentration-dependent fashion, with complete inhibition occurring at a concentration equal to that of exogenously added ferric iron. The participation of O(2) generated by the reductase in NADPH-dependent lipid peroxidation was also investigated and compared with results obtained with a xanthine oxidase-dependent lipid peroxidation system. NADPH-dependent peroxidation of either phospholipid liposomes or rat liver microsomes in the presence of ADP-Fe3+ was demonstrated to be independent of O(2) generation by the reductase.     

Ferritin, a physiological iron donor for microsomal lipid peroxidation

In the process of lipid peroxidation of microsomes induced either by oxygen radicals generated by xanthine oxidase or by NADPH, ferritin is able to donate the necessary iron. The amount of ferritin necessary to catalyze the process of lipid peroxidation is in the physiological range. In contrast to the finding with phospholipid liposomes, catalase hardly stimulates the lipid peroxidation of microsomes.     

Mechanism of cobalt (II) ion inhibition of iron-supported phospholipid peroxidation

Co2+ inhibited nonenzymatic iron chelate-dependent lipid peroxidation in dispersed lipids, such as ascorbate-supported lipid peroxidation, but not iron-independent lipid peroxidation. Histidine partially abolished the Co2+ inhibition of the iron-dependent lipid peroxidation. The affinity of iron for phosphatidylcholine liposomes in Fe(2+)-PPi-supported systems was enhanced by the addition of an anionic lipid, phosphatidylserine, and Co2+ competitively inhibited the peroxidation, while the inhibiting ability of Co2+ as well as the peroxidizing ability of Fe(2+)-PPi on liposomes to which other phospholipids, phosphatidylethanolamine, or phosphatidylinositol had been added was reduced. Co2+ inhibited microsomal NADPH-supported lipid peroxidation monitored in terms of malondialdehyde production and the peroxidation monitored in terms of oxygen consumption. The inhibitory action of Co2+ was not associated with iron reduction or NADPH oxidation in microsomes, suggesting that Co2+ does not affect the microsomal electron transport system responsible for lipid peroxidation. Fe(2+)-PPi-supported peroxidation of microsomal lipid liposomes was markedly inhibited by Co2+.     

Comparison of the ability of ferric complexes to catalyze microsomal chemiluminescence, lipid peroxidation, and hydroxyl radical generation

The interaction of microsomes with iron and NADPH to generate active oxygen radicals was determined by assaying for low level chemiluminescence. The ability of several ferric complexes to catalyze light emission was compared to their effect on microsomal lipid peroxidation or hydroxyl radical generation. In the absence of added iron, microsomal light emission was very low; chemiluminescence could be enhanced by several cycles of freeze-thawing of the microsomes. The addition of ferric ammonium sulfate, ferric-citrate, or ferric-ADP produced an increase in chemiluminescence, whereas ferric-EDTA or -diethylenetriaminepentaacetic acid (detapac) were inhibitory. The same response to these ferric complexes was found when assaying for malondialdehyde as an index of microsomal lipid peroxidation. In contrast, hydroxyl radical generation, assessed as oxidation of chemical scavengers, was significantly enhanced in the presence of ferric-EDTA and -detapac and only weakly elevated by the other ferric complexes. Ferric-desferrioxamine was essentially inert in catalyzing any of these reactions. Chemiluminescence and lipid peroxidation were not affected by superoxide dismutase, catalase, or competitive hydroxyl radical scavengers whereas hydroxyl radical production was decreased by the latter two but not by superoxide dismutase. Chemiluminescence was decreased by the antioxidants propylgallate or glutathione and by inhibiting NADPH-cytochrome P-450 reductase with copper, but was not inhibited by metyrapone or carbon monoxide. The similar pattern exhibited by ferric complexes on microsomal light emission and lipid peroxidation, and the same response of both processes to radical scavenging agents, suggests a close association between chemiluminescence and lipid peroxidation, whereas both processes can be readily dissociated from free hydroxyl radical generation by microsomes.     

LIPID PEROXIDE FORMATION IN RAT BRAIN

Abstract— Lipid peroxide formation as measured by the thiobarbituric acid reaction was demonstrated in subcellular fractions of rat brain. The ascorbic acid induced nonenzymic lipid peroxidation was distributed in all the subcellular fractions with a maximum in microsomes. The NADPH dependent enzymic lipid peroxidation occurred mainly in microsomes and to a smaller extent in synaptosomes; NADH could replace NADPH for the enzymic lipid peroxidation under the assay conditions employed. Fe²⁺ but not Fe³⁺ stimulated the NADPH or NADH dependent lipid peroxide formation. The optimum conditions with respect to pH, ascorbic acid or NADPH concentration, time of incubation and protein concentration were studied. Heating the microsomes at 100^oCdid not influence the ascorbate-induced lipid peroxidation but completely abolished the NADPH linked peroxidation. Several heavy metal ions, surface active agents and EDTA were inhibitory to lipid peroxidation. The effect of thiol agents indicated that -SH groups were involved in the enzymic lipid peroxidation. Studies on subcellular fractions of developing rat brain showed an increasing trend in lipid peroxidation with the advancing age of the animal. No significant difference in lipid peroxidation was observed between brains from normal rats and those from rats affected by experimental allergic encephalomyelitis.     

A new and suitable reconstructed system for NADPH-dependent microsomal lipid peroxidation

In order to evaluate the O-2 participation in NADPH-dependent microsomal lipid peroxidation, we used reconstructed system which contained detergent-solubilized NADPH-dependent cytochrome P-450 reductase, cytochrome P-450, phospholipid liposomes, NADPH and Fe3+-ADP. Lipid peroxidation, monitored by the formation of thiobarbituric acid-reactive substance, was increased with increasing concentration of detergent-solubilized NADPH cytochrome P-450 reductase, cytochrome P-450 or Fe3+-ADP. Cytochrome P-450-dependent lipid peroxidation was parallel to O-2 generation monitored by chemiluminescence probe with 2-methyl-6-(p-methoxyphenol)-3,7-dihydroimidazo[1,2-a]pyrazin++ +-3-one. Lipid peroxidation was significantly inhibited by superoxide dismutase, but not by catalase or sodium benzoate. The reconstructed system herein described is considered to be very close to NADPH-dependent microsomal lipid peroxidation system.     

Effect of cisplatin, carboplatin and stobadine on lipid peroxidation of kidney homogenate and phosphatidylcholine liposomes.

The effects of the nephrotoxic, anticancer agents cisplatin (CDDP) and carboplatin (CBDCA), and the free radical scavenger, stobadine, were investigated on lipid peroxidation (LPO) of rat kidney homogenates and phosphatidylcholine (PC) liposomes. Kidney homogenates were incubated in air at 37 degrees C for 6-48 h and lipid peroxidation was detected spectroscopically as absorbance (533 nm) of the thiobarbituric acid-malondialdehyde (TBA-MDA) complex. CDDP (0.3-10 mmol.l-1) increased LPO of the homogenate. CBDCA decreased the TBA-MDA absorbance, yet was found to interfere with MDA, TBA and/or with the TBA-MDA complex. Thus when CBDCA is involved, the TBA-MDA method for detection of LPO is not suitable. Stobadine (0.1 mmol.l-1 and 1 mmol.l-1) inhibited LPO either in the control homogenate and in the homogenate where peroxidation was increased by CDDP. The effect of CDDP and CBDCA on peroxidation of PC liposomes was monitored as oxygen consumption using a Clark-type oxygen electrode. CDDP increased but CBDCA decreased the rate of oxygen consumption during the peroxidation of liposomes induced by FeSO4. The results suggest that the effects of CDDP and CBDCA on LPO may be linked with their nephrotoxicity.     

NADPH-dependent inhibition of lipid peroxidation in rat liver microsomes.

Microsomal NADPH-driven electron transport is known to initiate lipid peroxidation by activating oxygen in the presence of iron. This pro-oxidant effect can mask an antioxidant function of NADPH-driven electron transport in microsomes via vitamin E recycling from its phenoxyl radicals formed in the course of peroxidation. To test this hypothesis we studied the effects of NADPH on the endogenous vitamin E content and lipid peroxidation induced in liver microsomes by an oxidation system independent of iron: an azo-initiator of peroxyl radicals, 2,2'-azobis (2,4-dimethylvaleronitrile), (AMVN), in the presence of an iron chelator deferoxamine. We found that under conditions NADPH: (i) inhibited lipid peroxidation; (ii) this inhibitory effect was less pronounced in microsomes from vitamin E-deficient rats than in microsomes from normal rats; (iii) protected vitamin E from oxidative destruction; (iv) reduced chromanoxyl radicals of vitamin E homologue with a 6-carbon side-chain, chromanol-alpha-C-6. Thus NADPH-driven electron transport may function both to initiate and/or inhibit lipid peroxidation in microsomes depending on the availability of transition metal catalysts.     

Effect of the microsomal system on interconversions between hydroquinone, benzoquinone, oxygen activation, and lipid peroxidation

Our previous results indicated that cytochrome P450 destruction by benzene metabolites was caused mainly by benzoquinone (Soucek et al., Biochem. Pharmacol. 47 (1994) 2233-2242). The aim of this study was to investigate the interconversions between hydroquinone, semiquinone, and benzoquinone with regard to both spontaneous and enzymatic processes in order to test the above hypothesis. We have also studied the participation of hydroquinone and benzoquinone in OH radicals formation and lipid peroxidation as well as the role of ascorbate and transition metals. In buffered aqueous solution, hydroquinone was slowly oxidized to benzoquinone via a semiquinone radical. This conversion was slowed down by the addition of NADPH and completely stopped by microsomes in the presence of NADPH. Benzoquinone was reduced to semiquinone radical at a significantly higher rate and this conversion was stimulated by NADPH and more effectively by microsomes plus NADPH while semiquinone radical was quenched there. In microsomes with NADPH. both hydroquinone and benzoquinone stimulated the formation of OH radicals but inhibited peroxidation of lipids. Ascorbate at 0.5-5 mM concentration also produced significant generation of OH radicals in microsomes. Neither hydroquinone nor benzoquinone did change this ascorbate effect. On the contrary, 0.1-1.0 mM ascorbate stimulated peroxidation of lipids in microsomes whereas presence of hydroquinone or benzoquinone completely inhibited this deleterious effect of ascorbate. Iron-Fe2+ apparently played an important role in lipid peroxidation as shown by EDTA inhibition, but it did not influence OH radical production. In contrast, Fe3+ did not influence lipid peroxidation, but stimulated OH radical production. Thus, our results indicate that iron influenced the above processes depending on its oxidation state, but it did not influence hydroquinone/benzoquinone redox processes including the formation of semiquinone. It can be concluded that interconversions between hydroquinone and benzoquinone are influenced by NADPH and more effectively by the complete microsomal system. Ascorbate, well-known antioxidant produces OH radicals and peroxidation of lipids. On the other hand, both hydroquinone and benzoquinone appear to be very efficient inhibitors of lipid peroxidation.     

Antioxidant activity of dihydrolipoate against microsomal lipid peroxidation and its dependence on alpha-tocopherol

The antioxidant effect of dihydrolipoate and lipoate was examined in microsomal fractions obtained from normal and alpha-tocopherol-deficient animals after initiation of lipid peroxidation with an NADPH/iron/ADP system. Dihydrolipoate prolonged the lag phase before the onset of low-level chemiluminescence and before the rapid accumulation of thiobarbituric acid-reactive substances in normal but not in vitamin E-deficient microsomes. Lipoate did not show such an antioxidant effect. It is concluded that the dihydrolipoate-mediated protection against lipid peroxidation by prolonging the lag phase is dependent on alpha-tocopherol. Likewise, dihydrolipoate prolonged the lag phase before the onset of the rapid loss of vitamin E during lipid peroxidation. Dihydrolipoate, like other biological thiols such as GSH, also affects the peroxidative process after the lag period. The effects included a smaller slope of the chemiluminescence increase, a lower maximal level of chemiluminescence, a slower loss of alpha-tocopherol and a slower accumulation, but unchanged maximal levels, of thiobarbituric acid-reactive substances. The biological significance may be most prominent in the mitochondrial matrix space, where lipoamide-containing ketoacid dehydrogenases are located. A potential pharmacological use of this biological dithiol in conditions associated with oxidative stress could be based on the antioxidant activity of dihydrolipoate.     

An NADPH-induced change in lipid bilayer of rat liver microsomes as observed by spin-labeled phosphatidylcholine.

Lipid bilayer of rat liver microsomes was spin-labeled by incubating with liposomes of 1-acyl 2-(12-doxylstearoyl) glycero-3-phosphorylcholine. When NADPH was added to the labeled microsomes, there appeared a rapidly tumbling component of spin label in the EPR spectrum. NADH was less effective than NADPH. The appearance of the sharp signal was prevented under anaerobic conditions or in the presence of either carbon monoxide, phenyl isocyanide or cytochrome $\text{c}$. The appearance of the rapidly tumbling component in the EPR spectrum was found to be due to the release of spin moiety from the membrane into the aqueous phase. That the release was associated with superoxide anion formation or with lipid peroxidation is unlikely, since 1) superoxide dismutase had little effect, 2) addition of either α-tocopherol or EDTA did not inhibit the release. These observations suggest that electron transfer from NADPH to oxygen $\text{via}$ cytochrome P-450 system induces a physical perturbation in the lipid bilayer resulting in the release of its component into the aqueous phase.     

Alterations of the microsomal glucose-6-phosphatase system evoked by ferrous iron- and haloalkane free-radical-mediated lipid peroxidation

Alterations of catalytic activities of the microsomal glucose-6-phosphatase system were examined following either ferrous iron- or halothane (CF3CHBrCl) and carbon tetrachloride (CCl4) free-radical-mediated peroxidation of the microsomal membrane. Enzyme assays were performed in native and solubilized microsomes using either glucose 6-phosphate or mannose 6-phosphate as substrate. Lipid peroxidation was assessed by the amounts of malondialdehyde equivalents formed. Regardless of whether the experiments were performed in the presence of NADPH/Fe3+, NADPH/CF3CHBrCl, or NADPH/CCl4, with the onset of lipid peroxidation, mannose-6-phosphatase activity of the native microsomes increased immediately, while further alterations in catalytic activities were only detectable when lipid peroxidation had passed characteristic threshold values: above 2 nmol malondialdehyde/mg microsomal protein, glucose-6-phosphatase activity of the native microsomes was lost, and at 10 nmol malondialdehyde/mg microsomal protein, glucose-6-phosphatase and mannose-6-phosphatase activity of the solubilized microsomes started to decline. It is concluded that the latter alterations are due to an irreversible damage of the phosphohydrolase active site of the glucose-6-phosphatase system, while the changes observed at earlier stages of microsomal lipid peroxidation may also reflect alterations of the transporter components of the glucose-6-phosphatase system. Virtually no changes in the catalytic activities of the glucose-6-phosphatase system occurred under anaerobic conditions, indicating that CF3CHCl and CCl3 radicals are without direct damaging effect on the glucose-6-phosphatase system. Further, maximum effects of carbon tetrachloride and halothane on lipid peroxidation and enzyme activities were observed at an oxygen partial pressure (PO2) of 2 mmHg, providing additional evidence for the crucial role of low PO2 in the hepatotoxicity of both haloalkanes.     

Microsomal lipid peroxidation: mechanisms of initiation. The role of iron and iron chelators

The role of iron and iron chelators in the initiation of microsomal lipid peroxidation has been investigated. It is shown that an Fe3+ chelate in order to be able to initiate enzymically induced lipid peroxidation in rat liver microsomes has to fulfill three criteria: (a) reducibility by NADPH; (b) reactivity of the Fe2+ chelate with rat liver microsomes has to fulfill three criteria: (a) reducibility by NADPH; (b) reactivity of the Fe2+ chelate with O2; and (c) formation of a relatively stable perferryl radical. NADH can support lipid peroxidation in the presence of ADP-Fe3+ or oxalate-Fe3+ at rates comparable to those obtained with NADPH but requires 10 to 15 times higher concentrations of the Fe3+ chelates for maximal activity. The results are discussed in relation to earlier proposed mechanisms of microsomal lipid peroxidation.