Lipid peroxidation in isolated hepatocytes.

Intracellular lipid peroxidation was initiated by the addition of ADP-complexed ferric iron to isolated rat hepatocytes and the reaction monitored by the thiobarbituric acid method or by measurement of the formation of conjugated dienes. Both the production of malondialdehyde (thiobarbituric-acid-reacting substances) and of conjugated dienes was dependent, on the ADP-Fe-3+ concentration in a dose-related fashion. Malondialdehyde formation stopped spontaneously within 20 min after the initiation of the reaction and the plateau reached was also related to the ADP-Fe-3+ concentration. Control experiments revealed that more than 90% of the malondialdehyde accumulating during the incubation period could be ascribed to intracellular production. The cellular NADPH/NADP+ ratio was always high and only slightly decreased upon ADP-Fe-3+-induced lipid peroxidation which, however, was associated with a marked decrease in the cellular glutathione concentration. The rate of accumulation of malondialdehyde as well as the final level reached during ADP-Fe-3+-initiated lipid peroxidation was increased by the addition of chloral hydrate. This apparent stimulatory effect could, however, be ascribed to the inhibition of the mitochondrial oxidation of the malondialdehyde formed during cellular lipid peroxidation, thus allowing more malondialdehyde to accumulate during the process. ADP-Fe-3+-induced cellular lipid peroxidation was associated with a decrease in the concentration of glutathione. Also, lowering of the intracellular glutathione level by the addition of diethyl maleate or by simply preincubating the hepatocytes (up to 50 min) promoted the ADP-Fe-3+ malondialdehyde production and formation of conjugated dienes. Furthermore, when cellular glutathione concentration had been lowered by preincubation of the hepatocytes, significant malondialdehyde production could be observed even at ADP-Fe-3+ concentrations which were too low to induce measurable lipid peroxidation in fresh hepatocytes. It is thus concluded that glutathione has an important role in the cell defence against lipid peroxidation and suggested that the isolated hepatocytes provide a suitable experimental model system for the characterization of this and other possible cellular defence mechanisms and how they are affected by the nutritional status of the donor animal.     

The involvement of superoxide and iron ions in the NADPH-dependent lipid peroxidation in human placental mitochondria

Incubation of human term placental mitochondria with Fe2+ and a NADPH-generating system initiated high levels of lipid peroxidation, as measured by the production of malondialdehyde. Malondialdehyde formation was accompanied by a corresponding decrease of the unsaturated fatty acid content. This NADPH-dependent lipid peroxidation was strongly inhibited by superoxide dismutase and singlet oxygen scavengers, markedly stimulated by paraquat, but was not affected by hydroxyl radical scavengers. Catalase enhanced the production of malondialdehyde by placental mitochondria. The effects of catalase and hydroxyl radical scavengers suggest that the initiation of NADPH-dependent lipid peroxidation is not dependent upon the hydroxyl radical produced via an iron-catalyzed Fenton reaction. These studies provide evidence that hydrogen peroxide strongly inhibits NADPH-dependent mitochondrial lipid peroxidation. The inhibitory effect of superoxide dismutase and stimulatory effect of paraquat, which was abolished by the addition of superoxide dismutase, suggests that superoxide may promote NADPH-dependent lipid peroxidation in human placental mitochondria.     

Lipid peroxidation in Ehrlich ascites cell mitochondria is not determined by the polyunsaturated fatty acid content of the membrane

Lipid peroxidation intensity is compared in Ehrlich Ascites Cell and in liver mitochondria, prepared from tumor bearing mice. Malondialdehyde formation is negligible in intact ascites tumour mitochondria, but it is significantly increased in permeabilised mitochondria and in isolated mitochondrial membranes. We suggest that the resistance against oxidative stress is a consequence of efficient protective mechanisms operating in the intact tumour mitochondria and the low level of polyunsaturated fatty acids under these circumstances cannot be the rate limiting factor in lipid peroxidation. Succinate, an effective inhibitor of mitochondrial lipid peroxidation in liver, cannot determine malondialdehyde formation in ascites tumour mitochondria.     

Iron content and degree of lipid peroxidation in liver mitochondria isolated from iron-loaded rats

1.The content of non-heme iron and the degree of lipid peroxidation were measured in liver mitochondria isolated from rats injected with either Jectofer (an iron-sorbitol-citric acid complex) or iron-nitrilotriacetate. 2. The sedimentation profiles of the mitochondria from controls and iron-treated rats as revealed by analytical differential centrifugation, indicated single population of mitochondria with $\text{s}{}_{\mathrm{4,B}}$ values of 13200± 560 S and 14200±590 S for controls and iron-loaded animals, respectively. In contrast, the sedimentation profiles of the acid phosphatase activity and the non-heme iron revealed marked polydispersities with at least three populations of particles for both controls and iron-loaded animals. 3. The mitochondria and iron-rich lysosomes were separated by density-gradient centrifugation in an isotonic medium of Percoll and sucrose. With this technique, the amount of non-heme iron in a mitochondrial fraction by differential centrifugation decreased from 69±28 nmol/mg protein to 5.6±1.1 nmol/mg protein and from 19.3±5.6 nmol/mg protein to 3.3±0.6 nmol/mg protein for Jectofer and iron-nitrilotriacetate injected rats, respectively. For control rats the amount of mitochondrial non-heme iron was about 2.7 nmol/mg protein both before and following density gradient centrifugation. The extra amount of non-heme iron still present in the purified mitochondrial fraction from iron-loaded rats, as compared to controls, was further characterized by the reactivity towards bathophenanthroline sulfonate. The results suggest that the extra iron was due to a small amount of either ferritin or hemosiderin still contaminaning the mitochondrial fraction. The amount of mitochondrial heme iron was the same in iron-loaded rats and controls. 4. The degree of lipid peroxidation in the mitochondria was estimated from the amount of malondialdehyde. The thiobarbituric acid method used for the quantitation of malondialdehyde was modified so that it was insensitive to variable amounts of iron present in the samples. No difference in the degree of lipid peroxidation was observed between the mitochondria from iron-loaded rats and controls. 5. In contrast to recent proposals (Hanstein, E.G. et al. (1981) Biochim. Biophys. Acta 678, 293–299), the present study showed that the amounts of non-heme iron and the degrees of lipid peroxidation are the same in mitochondria isolated from iron-loaded and control animals.     

The inhibitory effect of succinate on radiation-enhanced mitochondrial lipid peroxidation

The degree of mitochondrial ADP/Fe/NADPH-induced lipid peroxidation was increased up to the fourth day after 9.0 Gy whole body gamma-irradiation. The lipid peroxidation inhibiting effect of succinate added to isolated mitochondria was diminished as a consequence of irradiation. The succinate, administered in vivo prior to irradiation, decreased the amount of malondialdehyde production and protected the succinate dehydrogenase enzyme against inactivation. The mean survival of succinate-pretreated animals was much longer than that of controls. The role of mitochondrial lipid peroxidation in the pathogenesis of radiation injury is discussed.     

The role of Fe2+-induced lipid peroxidation in the initiation of the mitochondrial permeability transition

Iron and iron complexes stimulate lipid peroxidation and formation of malondialdehyde (MDA). We have studied the effects of Fe2+ and ascorbate on mitochondrial permeability transition induced by phosphate and Ca2+. Iron is necessary for detectable MDA formation, but only Ca2+ and phosphate are necessary for the induction of membrane potential loss (Deltapsi) and Ca2+ release. Keeping the iron at a constant concentration and varying the Ca2+ level changed the mitochondrial Ca2+ retention times, but not the amount of MDA formation. The antioxidant butylated hydroxytoluene at low concentrations prevented MDA formation, but not mitochondrial Ca2+ release. Preincubation of mitochondria with Fe2+ decreased Ca2+ retention time in a concentration-dependent manner and facilitated Ca2+-stimulated MDA accumulation. Thus, Ca2+ phosphate-induced mitochondrial permeability transition (MPT) can be separated mechanistically from MDA accumulation. Lipid peroxidation products do not appear to participate in the initial phase of the permeability transition, but sensitize mitochondria toward MPT.     

Docosahexaenoic and arachidonic acid peroxidation: It's a within molecule cascade

Peroxidation is a well-known natural phenomenon associated with both health and disease. We compared the peroxidation kinetics of phosphatidylcholine (PC) molecules with different fatty acid compositions (i.e. 18:0, 18:1n-9, 18:2n-6, 20:4n-6 and 22:6n-3 at the sn-2 and 16:0 at sn-1 position) either as molecules free in solution or formed into liposomes. Fatty acid levels, oxygen consumption plus lipid hydroperoxide and malondialdehyde production were measured from the same incubations, at the same time during maximal elicitable peroxidation. PCs with highly peroxidizable fatty acids (i.e. 20:4n-6 and 22:6n-3) in the same incubation were found to be either fully peroxidized or intact. Rates of peroxidation of PCs with multiple bisallylic groups (i.e. 20:4n-6 and 22:6n-3) peroxidized at 2-3 times the rate per bisallylic bond than the same phospholipid with 18:2n-6. The results suggest that propagation of peroxidation (H-atom transfer) is firstly an intramolecular process that is several-fold faster than intermolecular peroxidation. PCs in solution peroxidized twice as fast as those in liposomes suggesting that only half of the phospholipids in liposomes were available to peroxidize i.e. the outer leaflet. Experiments on liposomes suggest that even after heavy peroxidation of the outer leaflet the inner leaflet is unaffected, indicating how cells may protect themselves from external peroxidation and maintain control over internal peroxidation. Intramolecular peroxidation may produce highly concentrated, localized sites of peroxidation product that together with internal control of peroxidation of the inner leaflet of membranes provide new insights into how cells control peroxidation at the membrane level.     

The influence of NADPH-dependent lipid peroxidation on the progesterone biosynthesis in human placental mitochondria.

In an in vitro system consisting of human term placental mitochondria and an NADPH-generating system plus Fe2+, significant lipid peroxidation was observed along with a concomitant inhibition of progesterone biosynthesis. This inhibition could be markedly blocked by Mn2+, superoxide dismutase and dimethylfuran, inhibitors of NADPH-dependent lipid peroxidation. In addition, it has been found that malondialdehyde formation is accompanied by a corresponding decrease in placental mitochondrial cytochrome P-450 content. Inhibitors of lipid peroxidation also prevent the loss of cytochrome P-450, further demonstrating a direct relationship between NADPH-dependent lipid peroxidation and degradation of cytochrome P-450 in cell-free systems. These measurements provide the first evidence that the inhibition of progesterone biosynthesis by a NADPH-dependent lipid peroxidation in placental mitochondria is a consequence of cytochrome P-450 degradation due to lipid peroxidation.     

A comparison of phospholipid degradation by oxidation and hydrolysis during the mitochondrial permeability transition

The peroxidation and hydrolysis of mitochondrial phospholipids has been examined under conditions which are referable to induction of the permeability transition by t-butylhydroperoxide. Over a 30-min time course, the peroxide causes formation of 0.3 nmol/mg protein of malondialdehyde. This value is little effected by Ca2+, Sr2+, or Mn2+ but is increased approximately fivefold by Fe2+. The latter cation, but not the others, results in malondialdehyde formation in the absence of added peroxide. Partially oxidized phosphatidylethanolamine is present in normal mitochondria and is increased by approximately 50% following t-butylhydroperoxide treatment; however, the amounts observed are in the range of 0.4-0.6 mol% of total phosphatidylethanolamine. The minor degradation by peroxidation is in contrast to approximately 2.5 mol% degradation which occurs by hydrolysis. This degree of hydrolysis is accompanied by mitochondrial swelling and Mg2+ release, while a comparable level of peroxidation (malondialdehyde formation) is not. It is concluded that induction of the permeability transition by t-butylhydroperoxide does not represent damage to the membrane lipid phase caused by peroxidation. It is possible, however, that peroxidation accelerates the accumulation of phospholipid hydrolysis products and is thereby a factor which favors the transition.     

Analysis of lipid peroxidation mechanisms in human spermatozoa

The mechanisms by which ferrous ion promoters induce malondialdehyde generation by human spermatozoa have been investigated in order to provide a rational basis for the quantification and interpretation of lipid peroxidation assays. Incubation of human spermatozoa with a ferrous ion promoter in the presence of thiobarbituric acid (TBA) led to the generation of the bone fide malondialdehyde-TBA adduct. The importance of iron in the stimulation of lipid peroxidation was emphasized by the ability of Desferal* and EDTA to suppress malondialdehyde generation. Paradoxically, when the concentration of EDTA relative to iron was equimolar or greater, the suppression of malondialdehyde formation was accompanied by the generation of hydroxyl radicals. These results suggested that the addition of promoter did not effect the first-chain initiation of lipid peroxidation but favored an alternative mechanism involving the catalytic decomposition of pre-existing lipid peroxides. This conclusion was reinforced by the inability of reagents that would limit the formation (superoxide dismutase and/or catalase) or availability (mannitol, formate) of hydroxyl radicals, to influence malondialdehyde generation. While hydroxyl radicals were not directly involved in Fe²⁺-promoted malondialdehyde generation, the existence of significant correlations between reactive oxygen species production and the outcome of the TBA assay, suggested that Fenton chemistry might be important in the initiation of peroxidative damage. It is proposed that the impeded propagation of peroxidation initiated by Fenton or Haber Weiss reactions would lead to the accumulation of lipid peroxides in the spermatozoa and it is these peroxides that are induced to decompose during the Fe²⁺-promoted TBA assay, stimulating a lipoperoxidative chain reaction and malondialdehyde formation. © 1993 Wiley-Liss, Inc.     

Reduced nicotinamide adenine dinucleotide phosphate-dependent lipid peroxidation by beef heart submitochondrial particles.

Electron transport particles (ETP) prepared from beef heart mitochondria formed malondialdehyde by NADPH-dependent lipid peroixidation in the presence of ferric ions and ADP or ATP. The reaction was inhibited by MnCl2, EDTA, or radical scavengers, but was not inhibited by p-hydroxymercuribenzoate (PHMB) or respiratory chain inhibitors. The oxidation of NADPH and oxygen consumption by ETP were activated by the addition of ferric ions and APT, and inhibited by inhibitors of lipid peroxidation. This peroxidation system was apparently different from those of liver microsomes and mitochondria as regards the effect of PHMB, optimal pH and the concentration of NADPH for half-maximal reaction velocity.     

Prevention of peroxidation of cardiolipin liposomes by quinol-based antioxidants

In mammalian mitochondria, cardiolipin molecules are the primary targets of oxidation by reactive oxygen species. The interaction of oxidized cardiolipin molecules with the constituents of the apoptotic cascade may lead to cell death. In the present study, we compared the effects of quinol-containing synthetic and natural amphiphilic antioxidants on cardiolipin peroxidation in a model system (liposomes of bovine cardiolipin). We found that both natural ubiquinol and synthetic antioxidants, even being introduced in micro- and submicromolar concentrations, fully protected the liposomal cardiolipin from peroxidation. The duration of their action, however, varied; it increased with the presence of either methoxy groups of ubiquinol or additional reduced redox groups (in the cases of rhodamine and berberine derivates). The concentration of ubiquinol in the mitochondrial membrane substantially exceeds the concentrations of antioxidants we used and would seem to fully prevent peroxidation of membrane cardiolipin. In fact, this does not happen: cardiolipin in mitochondria is oxidized, and this process can be blocked by amphiphilic cationic antioxidants (Y. N. Antonenko et al. (2008) Biochemistry (Moscow), 73, 1273–1287). We suppose that a fraction of mitochondrial cardiolipin could not be protected by natural ubiquinol; in vivo, peroxidation most likely threatens those cardiolipin molecules that, being bound within complexes of membrane proteins, are inaccessible to the bulky hydrophobic ubiquinol molecules diffusing in the lipid bilayer of the inner mitochondrial membrane. The ability to protect these occluded cardiolipin molecules from peroxidation may explain the beneficial therapeutic action of cationic antioxidants, which accumulate electrophoretically within mitochondria under the action of membrane potential.     

Succinate-dependent lipid peroxidation and its prevention by reduced ubiquinone in beef heart submitochondrial particles.

When succinate and ADP-Fe3+ chelate were added to beef heart submitochondrial particles pretreated with 2-thenoyltrifluoroacetone, an inhibitor of succinate dehydrogenase of the mitochondrial respiratory chain, the formation of malondialdehyde was observed. No formation was observed without the pretreatment. Oxaloacetate competitively inhibited the malondialdehyde formation with an apparent Ki of 3.4 microM. The malondialdehyde formation seemed to be initiated at the location between the p-hydroxymercuribenzoate-sensitive site and the 2-thenoyltrifluoroacetone-sensitive site of the succinate dehydrogenase because it was inhibited by the mercurial. Ubiquinone-10 was rapidly destroyed during the malondialdehyde-forming reaction when it was in the oxidized form, while the ubiquinone was not destroyed and the malondialdehyde formation was abolished when about 50% of the ubiquinone in the particles was in the reduced state. These observations suggest that the succinate-dependent peroxidation is strongly controlled by the redox state of ubiquinone.     

Ferrous ion-mediated cytochrome P-450 degradation and lipid peroxidation in adrenal cortex mitochondria.

The relationship between the degradation reaction of cytochrome P-450 and lipid peroxidation was studied utilizing bovine adrenal cortex mitochondria. The two reactions were found to be closely correlated in terms of their response to storage of the mitochondrial preparation, stimulation by Fe2+, inhibition by EDTA and their initiation by cumene hydroperoxide. Both reactions were also found not to be inhibited by catalase, superoxide dismutase, 1,4-diazabicyclo-(2,2,2)-octane and alcohols, indicating that H2O2, superoxide, singlet oxygen and hydroxyl radicals do not participate in these reactions. Yet, diphenylamine proved to be a powerful inhibitor for both reactions, suggesting the involvement of a radical species. Cumene hydroperoxide could induce these two reactions at below 0.1 mM concentrations in the presence of molecular oxygen. The chemiluminescence observed during the Fe2+-mediated lipid peroxidation reaction which was not inhibited by either superoxide dismutase or 1,4-diazabicyclo-(2,2,2)-octane, was biphasic: one was a rapid burst; and the other was a slowly increasing emission. The latter portion of the emission of light coincided with the formation of malondialdehyde. These results indicate that in adrenal cortex mitochondria the degradation of cytochrome P-450 is closely related to lipid peroxidation.     

Effect of succinate on mitochondrial lipid peroxidation. 2. The protective effect of succinate against functional and structural changes induced by lipid peroxidation

The damaging effects of ADP/Fe/NADPH-induced lipid peroxidation were studied on the enzymes and membranes of rat liver mitochondria. Succinate, an inhibitor of mitochondrial lipid peroxidation, prevented or delayed most of the damage caused by the peroxidation on different mitochondrial structures and functions. There were marked abnormalities on the electrophoretic pattern of mitochondrial proteins during the course of lipid peroxidation. The disappearance of particular polypeptide bands and the accumulation of high-molecular-weight aggregates could be observed. Succinate was found to delay these effects. As a consequence of lipid peroxidation the succinate oxidase activity of mitochondria was decreased. The succinate dehydrogenase enzyme and the component(s) of the respiratory chain were inactivated. Succinate prevented the inactivation of succinate dehydrogenase but did not protect the other components of terminal oxidation chain. From the matrix enzymes the glutamate dehydrogenase retained its full activity but the NADP-linked isocitrate dehydrogenase was inactivated. The mitochondrial membranes became permeable to large protein molecules. Succinate prevented the inactivation of isocitrate dehydrogenase and delayed the release of protein molecules from mitochondria.     

Lactoperoxidase-catalyzed lipid peroxidation of microsomal and artificial membranes

Lactoperoxidase, in the presence of H₂O₂, I^?, and rat liver microsomes, will peroxidize membrane lipids, as evidence by malondialdehyde formation. Fe³⁺ assists in the formation of malondialdehyde. Fe³⁺ can be added at the end of the reaction period as well as at the beginning with equal effectiveness, suggesting that it only acts to assist in the conversion of lipid peroxides, previously formed by lactoperoxidase, to malondialdehyde. The addition of EDTA to the microsomal reaction mixture results in a 40% decrease in malondialdehyde formation. The antioxidant butylated hydroxytoluene will completely block the formation of malondialdehyde. Malondialdehyde formation is not dependent upon the production of superoxide, singlet oxygen, or hydroxyl radicals. Peroxidation of membrane lipids by this system is equally effective in both intact microsomes and in liposomes, indicating that iodination of microsomal protein is not required for lipid peroxidation to occur.     

Effect of clofibrate treatment on lipid peroxidation in rat liver homogenate and subcellular fractions

1. A study was made of the effect of hypolipidemic drug clofibrate on the level of lipid peroxidation in homogenates and subcellular fractions of rat liver. The intensity of lipid peroxidation was measured using chemiluminescence technique and malondialdehyde formation. 2. It was shown that under the action of clofibrate the levels of Fe/ADP-ascorbate-, as well as t-butyl hydroperoxide (Bu'OOH)-induced lipid peroxidation were decreased in the whole and "post-nuclear" liver homogenates. Dilution of the homogenates prevented depressing effect of clofibrate on lipid peroxidation. 3. Clofibrate significantly decreased the level of the Bu'OOH-dependent lipid peroxidation, but did not affect the activity of the Fe/ADP-ascorbate-induced reaction in rat liver mitochondria and microsomes. 4. Peroxidative alteration of membrane lipids in vivo was evaluated by determining the extent of conjugated dienes formation (absorption at 233 nm). It was shown that clofibrate did not increase the level of ultraviolet absorption of lipids from rat liver subcellular fractions. 5. The data obtained indicate that cytosol from the clofibrate treated rat liver contains a factor(s) which prevents lipid peroxidation in the mitochondria and microsomes.     

Effect of chronic ethanol treatment on the t-butyl hydroperoxide-dependent lipid peroxidation in rat liver

1. The effect of chronic ethanol consumption on the level of the t-butyl hydroperoxide (Bu'OOH)-induced lipid peroxidation in rat liver homogenate and subcellular fractions was measured using chemiluminescence technique and malondialdehyde formation. 2. It was shown that under the action of ethanol the rate of lipid peroxidation was decreased in the whole and "postnuclear" liver homogenates. 3. Ethanol significantly decreased the intensity of lipid peroxidation in microsomes, but did not affect the Bu'OOH-dependent process in mitochondria. 4. The level of lipid peroxidation was reduced after incubation of the total particulate fraction (mitochondria plus microsomes) with the undialysed cytosol from ethanol-treated rat liver. Dialysis of the cytosol prevented depressive effect of ethanol treatment on lipid peroxidation. 5. Reduced glutathione (0.1-1.0 mM) was shown to decrease the rate of lipid peroxidation in rat liver microsomes, but did not affect its level in mitochondria. 6. Pyrazole injections to rats reduced and phenobarbital treatment increased the level of the Bu'OOH-dependent lipid peroxidation in liver microsomes. 7. The data obtained indicate that the Bu'OOH-dependent lipid peroxidation is not an appropriate marker of the ethanol-induced oxidative stress in rat liver cells.     

Ferrous ion-EDTA-stimulated phospholipid peroxidation. A reaction changing from alkoxyl-radical- to hydroxyl-radical-dependent initiation.

The stimulatory effect of ferrous salts on the peroxidation of phospholipids can be enhanced by EDTA when the concentration of Fe2+ in the reaction is greater than that of EDTA. Hydroxyl-radical scavengers do not inhibit peroxidation until the concentrations of Fe2+ and EDTA in the reaction are equal. Lipid peroxidation is then substantially initiated by hydroxyl radicals derived from a Fenton-type reaction requiring hydrogen peroxide. Superoxide radicals appear to play some role in the formation of initiating species.     

Mitochondrial lipid peroxidation is influenced by dietary factors in early colon carcinogenesis

Oxidative damage to mitochondrial proteins, lipids, and DNA seem to influence the promotion and progression of tumors. High-fat diets and diets high in iron decrease manganese superoxide dismutase activity, a mitochondrial antioxidant, in colon mucosa. Lipid peroxidation products are low in microsomal preparations from colonic mucosa even under peroxide-inducing conditions. However, damage specific to mitochondrial membranes is unknown. This study was designed to investigate dietary lipid and iron effects on fatty acid incorporation and lipid peroxide formation in mitochondrial membranes of colonic mucosa. Male Fischer rats were fed high-fat diets containing either corn oil or menhaden oil with an iron level of either 35 or 535 mg/kg diet. Animals were given two injections of the colon carcinogen, azoxymethane, or saline. Colon tissue was collected 1 and 6 weeks after injections. Mitochondrial and microsomal fractions were prepared for fatty acid analysis and quantitation of lipid peroxidation products. Results showed that lipid composition of both subcellular fractions were influenced by diet. Fatty acid composition of mitochondria differed from microsomes, but overall saturation remained constant. Peroxidation products in mitochondrial membranes were significantly greater than in microsomal membranes. Dietary treatment significantly affected mitochondrial peroxidation in carcinogen-treated animals. Therefore, mitochondria from colon mucosa are more susceptible to peroxidation than are microsomes, dietary factors influence the degree of peroxidation, and the resulting damage may be important in early colon carcinogenesis.