Phospholipid oxidation catalyzed by cytochrome c in liposomes

Lysine 72 substitutions differently affect lipid membrane permeabilizing and proapoptotic activities of horse heart cytochrome c

Peroxidase activity of cytochrome c (cyt c)/cardiolipin (CL) complex is supposed to be involved in the initiation of apoptosis via peroxidative induction of mitochondrial membrane permeabilization. As cyt c binding to CL-containing membranes is at least partially associated with electrostatic protein/lipid interaction, we screened single-point mutants of horse heart cyt c with various substitutions of lysine at position 72, considered to play a significant role in both the binding and peroxidase activity of the protein. Contrary to expectations, K72A, K72R and K72L substitutions exerted slight effects on both the cyt c binding to CL-containing liposomal membranes and the cyt c/H₂O₂-induced calcein leakage from liposomes, used here as a membrane permeabilization assay. Both the binding and permeabilization were decreased to various extents, but not significantly, in the case of K72E and K72N mutants. A drastic difference was found between the sequence of the permeabilizing activities of the cyt c variants and the previously described order of their proapoptotic activities (Chertkova et al., 2008).

Antioxidant activity evaluation by physiologically relevant assays based on haemoglobin peroxidase activity and cytochrome c-induced oxidation of liposomes

Two new protocols for exploring antioxidant-related chemical composition and reactivity are described: one based on a chronometric variation of a haemoglobin ascorbate peroxidase assay and one based on cytochrome c-induced oxidation of lecithin liposomes. Detailed accounts are given on their design, application, critical correlations with established methods and mechanisms. These assays are proposed to be physiologically relevant and bring new information regarding a real sample, both qualitative and quantitative. The well-known assays used for evaluation of antioxidant (re)activity are revisited and compared with these new methods. Extracts of the Hedera helix L. are examined as test case, with focus on seasonal variation and on leaf, fruit and flower with respect to chromatographic, spectroscopic and reactivity properties. According to the set of assays performed, winter are the most antioxidant, followed by summer leaves, and then by flowers and fruits.

Acetaminophen inhibits cytochrome c redox cycling induced lipid peroxidation

Cytochrome (cyt) c can uncouple from the respiratory chain following mitochondrial stress and catalyze lipid peroxidation. Accumulating evidence shows that this phenomenon impairs mitochondrial respiratory function and also initiates the apoptotic cascade. Therefore, under certain conditions a pharmacological approach that can inhibit cyt c catalyzed lipid peroxidation may be beneficial. We recently showed that acetaminophen (ApAP) at normal pharmacologic concentrations can prevent hemoprotein-catalyzed lipid peroxidation in vitro and in vivo by reducing ferryl heme to its ferric state. We report here, for the first time, that ApAP inhibits cytochrome c-catalyzed oxidation of unsaturated free fatty acids and also the mitochondrial phospholipid, cardiolipin. Using isolated mitochondria, we also showed that ApAP inhibits cardiolipin oxidation induced by the pro-apoptotic protein, tBid. We found that the IC(50) of the inhibition of cardiolipin oxidation by ApAP is similar in both intact isolated mitochondria and cardiolipin liposomes, suggesting that ApAP penetrates well into the mitochondria. Together with our previous results, the findings presented herein suggest that ApAP is a pleiotropic inhibitor of peroxidase catalyzed lipid peroxidation. Our study also provides a potentially novel pharmacological approach for inhibiting the cascade of events that can result from redox cycling of cyt c.

Tocopherols and tocotrienols in membranes: a critical review

The familiar role of tocols (tocopherols and tocotrienols) as lipid-soluble chain-terminating inhibitors of lipid peroxidation is currently in the midst of a reinterpretation. New biological activities have been described for tocols that apparently are not dependent on their well-established antioxidant behaviour. These activities could well be real, but there remain large gaps in our understanding of the behaviour of tocols in membranes, especially when it comes to the alpha-, beta-, gamma-, delta-chroman methylation patterns and the seemingly special nature of tocotrienols. It is inappropriate to make conclusions and develop models based on in vivo (or cell culture) results with reference to in vitro measurements of antioxidant activity. When present in biological membranes, tocols will experience a large variation in the local composition of phospholipids and the presence of neutral lipids such as cholesterol, both of which would be expected to change the efficiency of antioxidant action. It is likely that tocols are not homogeneously dispersed in a membrane, but it is still not known whether any specific combination of lipid head group and acyl chains are conferred special protection from peroxidation, nor do we currently appreciate the structural role that tocols play in membranes. Tocols may enhance curvature stress or counteract similar stresses generated by other lipids such as lysolipids. This review will outline what is known about the location and behaviour of tocols in phospholipid bilayers. We will draw mainly from the biophysical literature, but will attempt to extend the discussion to biologically relevant phenomena when appropriate. We hope that it will assist researchers when designing new experiments and when critically assessing the results, in turn providing a more thorough understanding of the biochemistry of tocols.

Mitochondrial Release of Pro-apoptotic Proteins

A key step in the initiation of apoptosis is the release from the mitochondrial intermembrane space of cytochrome c and other pro-apoptotic proteins such as Smac/DIABLO, Omi/HtrA2, apoptosis-inducing factor (AIF), and endonuclease G (EndoG). Discrepancies have arisen, however, as to whether all these proteins are released in different systems. Our results suggest that failure to observe cytochrome c release may be due to the use of different buffers because after permeabilization by caspase-8 cleaved human Bid (tBid), cytochrome c dissociation from mitochondria was highly dependent on ionic strength and required 50-80 mm KCl, NaCl, or LiCl. In addition, mitochondria isolated from apoptotic cells using low ionic strength buffer bound a greater proportion of endogenous cytochrome c. In contrast to cytochrome c, Smac/DIABLO and Omi/HtrA2 were released independent of ionic strength, and AIF and EndoG behaved as if they are exposed to the intermembrane space but tethered to or within the inner membrane. AIF and EndoG were also not released by active caspases, which suggests their involvement in apoptosis may be limited. In summary, whereas tBid permeabilizes the outer membrane to cytochrome c, Smac/DIABLO, and Omi/HtrA2, the release of cytochrome c during apoptosis will be underestimated unless sufficient ionic strength is maintained to overcome the electrostatic association of cytochrome c with membranes.

Formation of keto and hydroxy compounds of linoleic acid in submitochondrial particles of bovine heart

To observe lipid peroxidation of additive-free submitochondrial particles, we incubated submitochondrial particles in the absence of exogenous irons and t-butyl hydroperoxide. After the incubation, the phospholipids were hydrolyzed by phopholipase A2, and the fatty acid constituents were analyzed by high-performance liquid chromatography, gas chromatography-mass spectrometry, and liquid chromatography-mass spectrometry. Contrary to a commonly accepted theory, lipid peroxidation in the submitochondrial particles did not need the addition of NADH. In the phospholipid constituent fatty acids of the oxidized submitochondrial particles, derivatives of hydroperoxides of linoleic acid such as keto, hydroxy, trihydroxy, and hydroxyepoxy compounds were generated. Lipid peroxidation in the submitochondrial particles was not inhibited by the addition of catalase, superoxide dismutase, hydroxyl radical scavengers, or ethylenediaminetetraacetic acid, but was inhibited by the addition of KCN, antimycin-A, NADH, ubiquinol, deferoxamine mesylate, ascorbic acid, and alpha-tocopherol. The cardiolipin-cytochrome c lipid peroxidation system could mimic the lipid peroxidation of the submitochondrial particles, in terms of linoleic acid products and the inhibitory patterns of radical scavengers and electron transfer chain inhibitors. Thus, lipid peroxidation in the submitochondrial particles seems to be due to phospholipid-hemoprotein lipid peroxidation systems such as the cardiolipin-cytochrome c system.

Kinetics of radiation- and cytochrome c-induced modifications in liposomes analysed by FT-Raman spectroscopy

Fourier transform Raman spectroscopy on artificial lipid membranes was used to study radiation-induced peroxidation processes as a function of time after radiation exposure. The time dependent intensity changes of the Raman lines of various C=C bondings were compared to results obtained by measuring conjugated dienes and by the thiobarbituric acid test for malondialdehydes. The results show that mainly the cis C=C bonds of the lipid chains are involved and, therefore, indicate that gamma-radiation induces conformational changes in the lipid chain while the mobility of the lipid chains is reduced. New Raman bands can be assigned to aldehyde products induced at the end of the peroxidation process. The immediate decrease of the =CH vibration lines was directly correlated with the formation of conjugated C=C double bonds suggesting that these vibration lines are in contrast to the C=C lines solely Raman active, when isolated C=C bonds are present. Cytochrome c (ox.) incorporated into the bilayer of the artificial membranes induced autooxidation processes not influenced by gamma-radiation. It was observed that cytochrome c (ox.)-induced changes of the relative intensity of the C=C bonds differ from those induced by gamma-radiation. These results of cytochrome c together with the inhibitory effects of the antioxidant alpha-tocopherol suggest that the radical species involved in the cytochrome c induced process might be different from the free radicals involved in the gamma-radiation-induced process.

Formation of leukotoxin (9,10-epoxy-12-octadecenoic acid) during the autoxidation of phospholipids promoted by hemoproteins

Myoglobin (Mb) and cytochrome c (Cyt c) are known to promote lipid peroxidation when mixed with certain types of phospholipids. In the presence of phospholipids such as cardiolipin (CL), ferrous Mb and Cyt c were converted to ferric hemoproteins, and autoxidation of the phospholipids and the oxidation of free linoleic acid (LA) added to the reaction mixture were observed. When the reaction mixture comprising 0.01 mM Cyt c, 0.2 mM CL and 0.1 mM LA was incubated, 92.7% of LA was consumed, and the LA products included 2.49 microM 9,10-epoxy-12-octadecenoic acid (leukotoxin) and its isomer which are potent inhibitors of mitochondrial respiration and have toxic effects on cardiac function. Hemoglobin (Hb) could promote almost no lipid peroxidation in the presence of any kinds of phospholipids. The experiments using some scavengers of active oxygen species revealed that tocopherol and ascorbic acid could strongly reduced LA oxidation caused by Cyt c or Mb. As LTx production was also observed when LA was mixed with Fe2+, LTx may be a common product where non-enzymatic lipid peroxidation occurs.

Promotion of oxidative damage to arachidonic acid and alpha 1-antiproteinase by anti-inflammatory drugs in the presence of the haem proteins myoglobin and cytochrome C

A mixture of myoglobin and hydrogen peroxide (H2O2) causes peroxidation of arachidonic acid. This peroxidation is greatly accelerated by adding phenylbutazone, which is effective even in the absence of H2O2. A wide range of other drugs was examined for their ability to exert similar pro-oxidant effects. We found that meclofenamic acid and flufenamic acid stimulated myoglobin-dependent lipid peroxidation, but only in the presence of H2O2. Ascorbic acid inhibited peroxidation both in the presence and in the absence of these drugs. Phenylbutazone, meclofenamic acid and flufenamic acid could also cause damage to proteins (as measured by inactivation of alpha 1-antiproteinase) in the presence of myoglobin and H2O2. The mitochondrial protein cytochrome c can also stimulate lipid peroxidation in the presence of H2O2. Phenylbutazone and meclofenamic acid, but not flufenamic acid, enhanced the peroxidation, which was again inhibited by ascorbic acid. However, only phenylbutazone caused inactivation of alpha 1-antiproteinase in the presence of cytochrome c and H2O2. Since respiring mitochondria generate superoxide radicals and H2O2, catalysis of lipid peroxidation and of the formation of drug-derived radicals by cytochrome c could be a mechanism contributing to mitochondrial damage by drugs.

Indolinonic and quinolinic aminoxyls as protectants against oxidative stress

A study on thermally and peroxyl radical induced oxidation of linolenic acid micelles in the presence of different concentrations of aminoxyls was carried out in order to test their efficiency as antioxidants in lipid peroxidation. The extent of peroxidation was measured by the malondialdehyde (MDA) produced and by oxygen consumption evaluated using an oxygraph. The results obtained indicate that indolinonic and quinolinic aminoxyls synthesized by us could be used as effective antioxidants in biological systems.

Differences in the susceptibility of plant membrane lipids to peroxidation

Peroxidation of three membrane lipid preparations from plants was initiated using Fe-EDTA and ascorbate and quantified as the production of aldehydes and loss of esterified fatty acids. Using liposomes prepared from commercial soybean asolecithin, the degree of peroxidation was shown to be dependent on: the free radical dose, which was varied by the ascorbate concentration; the presence of tocopherol in the liposome; the configuration, of the liposome, multilamellar or unilamellar; and time after initiation. There were dramatic interactions among these factors which led to the conclusion that in comparing the susceptibility of different membrane preparations it is essential to examine the kinetics of the peroxidation reactions. The composition of the liposome was a major determinant of the degree of peroxidation and of the type of degradative reactions initiated by the oxygen free radicals. A fresh polar lipid extract from Typha pollen had very similar fatty acid composition to the soybean asolecithin, but was more resistant to peroxidation as shown by less aldehyde production and increased retention of unsaturated fatty acids after treatment. Similarly, microsomal membranes from the crowns of non-acclimated and cold acclimated winter wheat (Triticum aestivum L.) seedlings had a much higher linolenic acid content than soybean asolecithin but was much more resistant to peroxidation. In the winter wheat microsomes, the loss of esterified fatty acids was not selective for the unsaturated fatty acids; consequently, even with 40% degradation, the degree of unsaturation in the membrane did not decrease. These different reaction mechanisms which occur in plant membranes may explain why measurements of fatty acid unsaturation fail to detect peroxidative reactions during processes such as senescence, aging and environmental stress.

Interaction of cytochrome c with liposomes: covalent labeling of externally bound protein by the fluorescent probe, azidonaphthalenedisulfonic acid, enclosed in the inner aqueous compartment of unilamellar vesicles

The photoreactive fluorescent probe, 3-azidonaphthalene-2,7-disulfonic acid (ANDS) was encapsulated in the inner aqueous compartment of small unilamellar liposomes, prepared from egg phosphatidylcholine (PC) +/- 20 mol% dihexadecylphosphate (DHP). After adding cytochrome c externally to a suspension of these vesicles, the probe was activated by ultraviolet irradiation, and the protein was separated from the lipids. When negatively charged (egg PC/DHP) vesicles at low ionic strength were used, which form an electrostatic complex with cytochrome c, the protein was labeled by ANDS. This process depended on irradiation time, and was inhibited by increasing the ionic strength of the medium. Labeling was not observed with isoelectric (egg PC) vesicles. These observations suggest that electrostatic binding of cytochrome c to the bilayer is accompanied by intramembrane penetration to such a depth that the protein can communicate with the inner membrane-water interface.