On the lipid peroxidation of rat liver hepatocytes, the formation of fluorescent chromolipids and high molecular weight protein

Oxidative modification of human ceruloplasmin by peroxyl radicals

Ceruloplasmin (CP), the blue oxidase present in all vertebrates, is the major copper-containing protein of plasma. We investigated oxidative modification of human CP by peroxyl radicals generated in a solution containing 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH). When CP was incubated with AAPH, the aggregation of proteins was increased in a time- and dose-dependent manner. Incubation of CP with AAPH resulted in a loss of ferroxidase activity. Superoxide dismutase and catalase did not protect the aggregation of CP, whereas hydroxyl radical scavengers such as ethanol and mannitol protected the protein aggregation. The aggregation of proteins was significantly inhibited by the copper chelators, diethyldithiocarbamate and penicillamine. Exposure of CP to AAPH led to the release of copper ions from the enzyme and the generation of protein carbonyl derivatives. Subsequently, when the amino acid composition of CP reacted with AAPH was analyzed, cysteine, tryptophan, methionine, histidine, tyrosine, and lysine residues were particularly sensitive.

Mechanism of free radical-induced hemolysis of human erythrocytes: comparison of calculated rate constants for hemolysis with experimental rate constants

We previously developed a simple competitive reaction model between lipid peroxidation and protein oxidation in erythrocyte membranes that accounts for radical-induced hemolysis of human erythrocytes. In this study, we compared the rate constants calculated from the hemolysis curves of erythrocytes in the presence of radical initiators with those obtained from experiments using erythrocyte ghosts treated with radicals. 2,2'-Azobis(amidinopropane) dihydrochloride and 2,2'-azobis(2,4-dimethylvaleronitrile) were used as radical initiators. Plots of the logarithm of concentration of the radical initiator against the logarithm of the rate constant gave straight lines. The slope of the lines for the calculated lipid peroxidation was nearly equal with the experimental value. Similar results were obtained for oxidation of membrane proteins, except for band 3 oxidation. The values for the rate constants calculated from hemolysis curves seem to be accurate. The slope of the lines for the calculated rate constants for proteins was larger than the experimental value for band 3 oxidation, because band 3 oxidation is accompanied by aggregation or redistribution of band 3 proteins to form hemolytic holes. These results indicate that the competitive reaction model may be useful for analyzing radical-induced hemolysis.

Spontaneous chemiluminescence production, lipid peroxidation, and covalent binding in rat hepatocytes exposed to acetaminophen

Spontaneous chemiluminescence associated with the cell injury was observed in the isolated rat hepatocyte suspension during acetaminophen (APAP) metabolism, indicating the occurrence of oxidative stress. APAP apparently affected the hepatocytes in various manners. APAP, at low concentrations (1-2 mM), damaged the hepatocytes due to lipid peroxidation provoked during APAP metabolism, while at high concentrations (5-50 mM), APAP protected the hepatocytes due to a chemical antioxidant effect of the unmetabolized APAP that remained in the medium because of the saturation of APAP metabolism. The covalent binding of APAP to the hepatocytes increased with APAP concentration up to 50 mM without loss of cell viability. When an overdose of APAP was administered to rats, the APAP plasma concentration was around 1-3 mM, which corresponded to the concentration range where lipid peroxidation occurred in the isolated hepatocytes. Thus, it seems likely that lipid peroxidation contributes to the APAP-induced hepatotoxicity in the early stage of the toxic process.

Protective effects of beta-blockers against 2,2'-azobis(2-amidinopropane)-dihydrochloride-induced damage

The protective effects of beta-blockers against 2,2'-azobis(2- amidinopropane)-dihydrochloride (AAPH)-induced damage were investigated. With the exception of pindolol, none of the beta-blockers tested inhibited arachidonate peroxidation induced by AAPH in the absence of iron. In contrast, ADP-Fe(3+)- and NADPH-dependent microsomal lipid peroxidation was inhibited by all the beta-blockers tested, although the inhibitory effects of atenolol and metoprolol were very slight. Oxidation of tryptophan residues in bovine serum albumin (BSA) induced by AAPH was strongly inhibited by pindolol and propranolol but not by atenolol or metoprolol. All the beta-blockers tested, however, inhibited AAPH-induced carbonyl formation of BSA. Furthermore, all the beta-blockers tested also strongly inhibited the deoxyribose degradation induced by AAPH, suggesting that these agents act as hydroxyl radical scavengers to inhibit carbonyl formation. DNA strand scission was induced by AAPH in the absence or presence of O2. Only pindolol strongly inhibited the DNA damage in the absence of O2. In the presence of O2, however, all the beta-blockers tested effectively prevented the DNA damage. These results suggested that the hydroxyl radicals produced from AAPH damaged DNA and, that beta-blockers might act as hydroxyl radical scavengers to protect DNA against the AAPH-induced oxidative damage.

Fluorescent substances and high molecular weight protein aggregates formed in rat heart mitochondria upon doxorubicin-induced lipid peroxidation

A rat heart mitochondrial suspension was incubated with doxorubicin, FeCl3 and NADH. Fluorescent substances and high molecular weight protein aggregates were observed in the mitochondrial membranes upon the formation of thiobarbituric acid-reactive substances. Since both fluorescent substances and high molecular weight protein aggregates are retained in mitochondrial membranes, they can be of use in the clarification of the site of doxorubicin-induced lipid peroxidation.

Lipid peroxidation capacities in the myocardium of endurance-trained rats and mice in vitro

The endurance-training programme in Experiment 1 (Exp. 1) consisted of a total swimming time of 149-159 h per male Han Wistar rat and in Experiment 2 (Exp. 2) the male NMRI-mice run on a treadmill at a speed of 25 m min-1 1 h per day, 5 days a week for 3 weeks. One group of the rat hearts was perfused with 0.3 mM cumene hydroperoxide (CumOOH) while the others were fractioned (mitochondria, sarcolemma and sarcoplasmic reticulum) and these cell fractions and homogenates were used to determine the total concentration of peroxidative lipids and the susceptibility to lipid peroxidation. The perfusion with CumOOH caused the release of thiobarbituric acid reactive substances (TBARS) into the perfusate. The release of TBARS from the trained hearts was smaller than that of the control hearts (P < 0.01). The concentration of TBARS was also smaller in the myocardium of the right ventricle of the trained rats (P < 0.01). The concentration of reduced GSH remained at a higher level after the CumOOH perfusion suggesting a better redox state in the hearts of trained animals. The concentration of the lipids susceptible to lipid peroxidation was lower in the homogenates of the trained rat hearts (P < 0.05). However, this decrease could not be explained by any of the tissue fractions used when studied in rat hearts. In Exp. 2 the total concentration of lipids susceptible to peroxidation remained unchanged in the mice hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of adriamycin-induced lipid peroxidation

Lipid peroxidation is known to be a mechanism for Adriamycin-induced toxicity. In the present study, two methods which detect fluorescent substances and high molecular weight protein aggregates in peroxidized membranes were applied to Adriamycin-induced lipid peroxidation in liver microsomes. A rat liver microsomal suspension containing an NADPH-generating system was incubated with Adriamycin. Thiobarbituric acid reactive substances (TBA-RS), formed during this incubation, were transferred from the microsomes to the medium. Fluorescent substances determined by the fluorescence emitted from both the microsomes themselves and the chloroform/methanol extracts of the microsomes, were found to be formed during this incubation. High molecular weight protein aggregates determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, were also formed. Fluorescent substances and high molecular weight protein aggregates were found in microsomal membranes themselves and increased time dependently. These substances retained in membranes can be of great use to delineate the site of Adriamycin-induced lipid peroxidation in vitro and in vivo and to determine how this lipid peroxidation affects the membrane.

Fluorospectroscopic analysis of the fluorescent substances in peroxidized microsomes of rat liver

The fluorescent substances formed in rat liver microsomes in the course of lipid peroxidation were investigated by fluorescence techniques. The fluorescence emitted from peroxidizing microsomes continuously increased as lipid peroxidation progressed, while the steady-state fluorescence anisotropy increased and then reached a plateau. A similar increase was observed in the steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene in peroxidizing microsomes. The fluorescence from peroxidized microsomes consisted of at least three species having short, middle or long fluorescence lifetimes. The lifetimes and relative amplitudes of fluorescence were unaffected by the extent of lipid peroxidation. Both fluorescence of the chromolipids extracted and the proteins isolated from peroxidized microsomes had the same characteristics in fluorescence lifetimes as the fluorescence from whole peroxidized microsomes. Thus, these lipids and proteins appear to be the major biological substances responsible for the fluorescence emanating from whole peroxidized microsomes. Furthermore, fluorescent substances formed in microsomes seem to increase in quantity rather than change in quality as lipid peroxidation progresses.

Peroxidative modification of phospholipids in myocardial membranes

Rat heart myocardial membranes exposed to the free radical generating system, Fe2+/ascorbate, undergo lipid peroxidation as evidenced by the accumulation of thiobarbituric acid-reactive substances, loss of polyunsaturated fatty acids from phospholipids, and formation of conjugated dienes and fluorescent substances. In addition, the treated membranes exhibit a dramatic decrease in extractable phospholipids. This decrease is even more pronounced in individual phospholipid classes isolated by high-performance liquid chromatography. The decrease in lipid phosphorus under oxidant stress is accompanied by an increase in the phosphorus content of the aqueous phase after Folch extraction and by an even greater increase of phosphorus in the protein residue. In addition, increased amounts of saturated and monounsaturated fatty acyl groups are found in the protein residue of Fe2+/ascorbate-treated membranes. Extraction of the oxidant-treated membranes with acidic solvents does not enhance the recovery of phospholipids and neither does treatment with detergents, trypsin, and chymotrypsin prior to lipid extraction. However, treatment with the bacterial protease, Pronase, markedly enhances the recovery of phospholipids from the peroxidized membranes. These results indicate that membrane phospholipids undergoing free radical-induced peroxidation may form lipid-protein adducts, which renders them inextractable with lipid solvents.

Time dependent changes occurring in rat liver microsomes upon lipid peroxidation

Steady-state fluorescence anisotropy of diphenylhexatriene and n-(9-anthroyloxy)stearic acids (n = 2,12) in rat liver microsomes showed a marked increase in the early stages of enzymatically or non-enzymatically induced lipid peroxidation. The changes in fluorescence anisotropy occurred in parallel with the formation of thiobarbituric acid-reactive substances (TBA-RS). Parallel to these changes, the fluorescence emitted from peroxidized microsomes increased markedly in the early stages of lipid peroxidation. In contrast to the changes in the fluorescence anisotropy and in the formation of TBA-RS, the fluorescence showed a continuing increase over the three hr period of lipid peroxidation. Glucose-6-phosphatase was inactivated in the early stages of lipid peroxidation, whereas NADH-cytochrome b5 reductase underwent a slow deactivation over three hr. The apparently slow deactivation of the peripheral protein may be explained by the formation of fluorescent substances.

Fluorescence emitted from microsomal membranes by lipid peroxidation

The fluorescence emitted from rat liver microsomal membranes which had undergone enzymatic and nonenzymatic lipid peroxidation was detected directly. This fluorescence produced in peroxidized membranes increased progressively with peroxidation reaction time, and the fluorescent substances produced were retained in the membranes without being released into the aqueous phase. Extracts of the peroxidized membranes with organic solvents (chloroform/methanol) emitted fluorescence which was also dependent on the peroxidation reaction time. The generation profiles of fluorescence emitted from both the peroxidized membranes and their extracted membrane lipids differed essentially from that of thiobarbituric acid-reactive substances which reached a plateau at a relatively early stage of peroxidation reaction. These results indicate that lipid peroxidation induces stepwise chemical and physical changes in membranes and that the fluorescence from peroxidized membranes will be useful in studying such changes occurring in biological membranes.

Oxidative stress limits vitamin D metabolism by bovine proximal tubule cells in vitro

When bovine proximal tubule cells are placed in primary culture, they are subject to elevated oxidative stress which acts to limit the expression of mitochondrial vitamin D3 1 alpha- and 24-hydroxylase activities. This increased oxidative stress was demonstrated by increased production of cell and mitochondrial membrane lipid hyperperoxides (LOOH). This increased production was prevented by the addition of the antioxidants butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT). Cell and mitochondrial membrane LOOH increased from 1 to 2 pmol/mg protein on the day of plating to 70-90 pmol/mg protein after 6 days in culture. Pretreatment of cultures with BHA and BHT resulted in membrane LOOH of 15-20 pmol/mg protein after 6 days. Mitochondrial LOOH production was greater than total cell LOOH after 6 days. The increase in cellular oxidative stress was paralleled by decreases in both 1 alpha- and 24-hydroxylase activities toward 25-OH D3. Mitochondrial hydroxylase activities were inversely proportional to the increase in mitochondrial membrane LOOH production. Mitochondrial cytochrome P-450 content, determined spectrophotometrically, was decreased over time in culture. Mitochondrial cytochrome P-450 content determined by a specific polyclonal antibody in an enzyme-linked immunosorbant assay also decreased over time in culture. Specificity of polyclonal antibodies, raised against rat liver microsomal cytochrome P-450 RLM5, was demonstrated by the immunosequestration of both 1 alpha- and 24-hydroxylase activities from a partially purified preparation of renal mitochondrial cytochrome P-450. BHA showed the loss of 1 alpha- and 24-hydroxylase activities and mitochondrial P-450 content measured by all criteria. These experiments indicate that oxidative stress-mediated changes in hydroxylase activities are mediated directly by changes in hydroxylase content and not at distal sites. A partially purified preparation of bovine proximal tubule mitochondrial cytochrome P-450, with purified renal ferredoxin, ferredoxin reductase, and NADPH, expressed both 1 alpha- and 24-hydroxylase activities toward 25-OH D3. LOOH, derived from mitochondrial membranes of 5-day-old cultures, when added to this mixture, caused a dose-dependent decrease in both activities. These experiments suggested that an increase in mitochondrial LOOH production resulted in a loss of 1 alpha- and 24-hydroxylase activities. 1 alpha-Hydroxylase was more sensitive to the effects of LOOH treatment than 24-hydroxylase. At a ratio of LOOH:P-450 of 5:1 (molar), all 1 alpha-hydroxylase activity was lost but 50% of the 24-hydroxylase activity remained.(ABSTRACT TRUNCATED AT 400 WORDS)

Recovery of malondialdehyde in urine as a 2,4-dinitrophenylhydrazine derivative analyzed with high-performance liquid chromatography

Malondialdehyde (MDA) in urine was measured as a 2,4-dinitrophenylhydrazine (DNPH) derivative using high-performance liquid chromatography (HPLC) for the analysis. MDA standard coeluted with a peak obtained from rat urine after i.p. administration of MDA standard. This peak was also the only peak containing 14C after injection of a [14C]MDA standard, and was shown by mass spectrometry to contain 1-(2,4-dinitrophenyl)pyrazole, the derivative formed when MDA is treated with DNPH. Depending on the amount given (0.3-5.5 mumol), the recovery (after 24 h sampling period) in urine was 0.7-2.6%. This apparent non-linear kinetics may relate to several factors, such as dose-dependent metabolism. However, the peak urinary concentration approached the expected plasma concentration and reproducible recovery data were obtained, suggesting that MDA was passively excreted in a reasonably stable form. These data indicate that monitoring MDA excretion in urine can give useful information about lipid peroxidation in vivo.

The influence of the chain length of aldehydes on the fluorescence of chromolipids

Incubation of phosphatidylethanolamine containing liposomes with malondialdehyde and other aldehydes with different chain lengths results in the fluorescence of chromolipids. Relative to malondialdehyde, the fluorescence was greatly enhanced with increasing chain length upon incubation of 2-alkenals with phospholipids. Similar results were found using the total lipid extracted from erythrocyte ghosts. It seems that the hydrophobic character of the aldehydes is important for the amount of fluorescence detected in lipid bilayers.

Lipofuscin and lipofuscin-like substances

Lipofuscin is defined as being a yellowish brown, lipid-rich, heterogeneous, cytoplasmic granular pigment emitting an intense yellow autofluorescence when excited with ultraviolet light, which accumulates in various tissues of animals during their aging. It is believed that the pigments are derived from the reaction of some of reactive secondary products including malonaldehyde, formed during membranous lipid peroxidation, with amino groups of phospholipids and proteins, etc., and that these formations are accompanied by alteration of the membrane structure and inactivation of the enzymes. The fluorescence measurement of the pigments is widely used as a parameter of lipid peroxidation in vivo as well as in vitro. However, their origin, chemical structure, biological significance or fate has not as yet been fully elucidated. This article introduces and discusses the recent studies on these problems.

Detection of oxidized lipid-modified erythrocyte membrane proteins by radiolabeling with tritiated borohydride

Human erythrocyte ghosts treated with tert-butyl hydroperoxide or ADP-Fe3+ incorporated radioactivity on reduction with tritiated borohydride. The tritium incorporation closely correlated with membrane lipid oxidation as assessed by the formation of thiobarbituric acid-reactive substances and fluorescent substances. Treatment of ghosts with the inducers in the presence of butylated hydroxytoluene, thiourea, or desferrioxamine suppressed the tritium incorporation in the subsequent reduction. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the tritiated ghost proteins showed that the label was incorporated into the intermolecularly cross-linked and the uncross-linked proteins of bands 1, 2, 3, 4.1, 4.2, 5 and 6, and into the noncross-linked glycophorin A (PAS-1). Glycophorin A was hardly cross-linkable but modified during membrane lipid oxidation. Possible candidates for producing borohydride-reducible functions in the proteins are various mono- and bifunctional aldehydes, as well as those for producing fluorescence and cross-links. A part of thiobarbituric acid-reactive or fluorescent substances may be involved in borohydride reduction and tritium labeling.

Possible involvement of the lipid-peroxidation product 4-hydroxynonenal in the formation of fluorescent chromolipids

The effects of the lipid-peroxidation product 4-hydroxynonenal on the formation of fluorescent chromolipids from microsomes, mitochondria and phospholipids were studied. Incubation of freshly prepared rat liver microsomes or mitochondria with 4-hydroxynonenal results in a slow formation of a fluorophore with an excitation maximum at 360 nm and an emission maximum at 430 nm. The rate and extent of the development of the 430 nm fluorescence can be significantly enhanced by ADP-iron (Fe3+). With microsomes, yet not with mitochondria. NADPH has a catalytic effect similar to that of ADP-iron. Fluorescent chromolipids with maximum excitation and emission at 360/430 nm are also formed during the NADPH-linked ADP-iron-stimulated lipid peroxidation. Phosphatidylethanolamine and phosphatidylserine react with 4-hydroxynonenal revealing a fluorophore with the same spectral characteristics as that obtained in the microsomal and mitochondrial system. The findings suggest that the fluorescent chromolipids formed by lipid peroxidation are not derived from malonaldehyde, but are formed from 4-hydroxynonenal or similar reactive aldehydes via a NADPH and/or ADP-iron-catalysed reaction with phosphatidylethanolamine and phosphatidylserine contained in the membrane.

In vitro lipid oxidation modifies proteins and functional properties of sarcoplasmic reticulum

Changes in protein and fatty acid compositions of flounder sarcoplasmic reticulum during NADH plus ascorbate-dependent lipid peroxidation in vitro were related to the ability of the sarcoplasmic reticulum to sequester Ca+2. Progressive accumulation of high-molecular-weight protein components occurred concomitantly with loss of Ca+2-sequestering activity. Part of this polymerized protein may be the dimer or trimer of Ca+2, Mg+2-ATPase. Loss in Ca+2, Mg+2-ATPase protein could account for over 60% of the polymerized protein. Rate of loss of polyunsaturated fatty acids was C22:6 greater than C20:4 greater than C20:5 greater than C22:5. Loss of polyunsaturated fatty acids and accumulation of thiobarbituric acid-reactive substances occurred concomitantly with protein polymerization.

Chemiluminescent aerobic oxidation of protein adducts with glycolaldehyde catalyzed by horseradish peroxidase

Horseradish peroxidase (EC 1.11.1.7) is shown to catalyze the aerobic oxidation of lysozyme, bovine serum albumin, and protamine adducts with glycolaldehyde at physiological pH. This reaction is accompanied by light emission, which is attributed to the generation of triplet species. The intensity of chemiluminescence is enhanced by addition of chlorophyll alpha solubilized in Brij 35. A role of electronically excited species in deleterious and pathological processes associated with formation of Schiff-type adducts is suggested, with emphasis on the case of alcohol-induced liver injury.

Lipid peroxidation decreases the rotational mobility of cytochrome P-450 in rat liver microsomes

Phenobarbital-induced rat liver microsomes were subjected to NADPH- and Fe2+-catalyzed lipid peroxidation. The formation of approx. 95 nmol malondialdehyde/mg protein during 18 min peroxidation at 37 degrees C was observed. Membrane rigidity measured by means of the steady-state fluorescence anisotropy rs of diphenylhexatriene increased in parallel with the malondialdehyde formation. Both the amount of malondialdehyde and rs remained constant thereafter during incubation of the peroxidized membranes for 2 h. The aminopyrine demethylase activity decreased by about 60% upon lipid peroxidation for 18 min, whereas no significant loss of benzphetamine demethylase activity within the same time range was observed. A time-dependent formation of protein complexes of high molecular weight, comprising most of the microsomal polypeptides, upon lipid peroxidation was observed in SDS-polyacrylamide gel electrophoresis. The effect of microsomal lipid peroxidation on protein-protein interactions was examined by measuring the rotational mobility of intact cytochrome P-450. Rotational diffusion was measured by observing the decay of flash-induced absorption anisotropy r(t) of the P-450 X CO complex. Analysis was based on a 'rotation-about-membrane normal' model with the equation r(t) = r1exp(-t/phi 1) + r2exp(-t/phi 2). In control microsomes, two classes (rapid and slow) of rotating populations of cytochrome P-450 were observed with phi 1 approximately equal to 150 microseconds, fraction r1/(r1 + r2) approximately equal to 40% and phi 2 approximately equal to 2 ms, fraction r2/(r1 + r2) approximately equal to 60%. A relatively small decrease in the rotational mobility of P-450 was observed by a 18-min lipid peroxidation, while a subsequent incubation of peroxidized microsomes for 2 h at 37 degrees C resulted in a dramatic immobilization of P-450 by the increase of both r2/(r1 + r2) approximately equal to 75% and phi 2 approximately equal to 10-25 ms. The decrease in the P-450 mobility during 18-min lipid peroxidation would be due to the rigidification of the lipid bilayer. However, because the lipid fluidity remained unchanged thereafter, the significant immobilization of P-450 by the subsequent 2-h incubation is deduced to be due to formation of protein aggregates.

Functional disability of rat splenocytes provoked to lipid peroxidation by cumene hydroperoxide

Rat splenocytes were provoked to lipid peroxidation in a dose-dependent manner by cumene hydroperoxide. After exposure to cumene hydroperoxide, formation of high molecular weight protein, presumably through cross-linking of lower molecular weight protein, was stimulated in splenocytes as well as in erythrocyte ghosts. The mitogenic response to concanavalin A of splenocytes was remarkably depressed by addition of cumene hydroperoxide to cultures. This depression was due rather to failures of splenocytes in responding to concanavalin A than deactivation of concanavalin A molecules. It is notworthy that the viability of splenocytes was unaffected by cumene hydroperoxide under the culture conditions where the mitogenic response was depressed. The addition of alpha-tocopherol or thiourea could block the depression of mitogenic response by cumene hydroperoxide, indicating that the depressed response to concanavalin A was related to radical formation. Overall evidence suggests that the function of immunocompetent cells can be depressed through lipid peroxidation-associated mechanisms without suffering from lethal damage.

Effect of t-butyl hydroperoxide on liver microsomal membranes and microsomal calcium sequestration

In vitro exposure of hepatocytes or liver microsomes to t-butyl hydroperoxide resulted in a marked decrease of liver microsomal calcium pump activity. Decreased calcium pump activity was dependent upon both concentration and time. Liver microsomes could be protected from this effect by glutathione or dithiothreitol. In addition to decreased calcium pump activity, exposure of liver microsomes to t-butyl hydroperoxide produced a concentration-dependent aggregation of microsomal membrane protein as determined by polyacrylamide gel electrophoresis. Inhibition of microsomal calcium pump activity was observed when intact hepatocytes were incubated, in vitro, with t-butyl hydroperoxide. However, aggregation of microsomal membrane protein was not observed when hepatocytes were incubated with t-butyl hydroperoxide. The effects produced by exposure of liver microsomes to this compound do not appear to be a complete model of actions of the compound on intact cells.

Skin-tumour promoting activity of methyl ethyl ketone peroxide—a potent lipid-peroxidizing agent

The tumour-promoting activity of methyl ethyl ketone peroxide (MEKP) was tested on the skin of hairless mice using a two-stage initiation-promotion protocol. When ultraviolet radiation in the UVB region (280-320 nm) was used as tumour initiator, MEKP showed weak promoting activity. The promotional activity of MEKP was potentiated by diethyl maleate, which is known to deplete intracellular glutathione, suggesting that lipid peroxidation may be important in the tumour promotion.

The effect of (13 OOH) linoleic acid on human erythrocytes and on erythrocyte ghosts

Peroxidized linoleic acid (13 OOH) induces lipid peroxidation, measured as malondialdehyde in erythrocyte ghosts and intact erythrocytes. This process can be inhibited by desferal, thiourea and butylated hydroxytoluene. During the lipid peroxidation process fluorescent chromolipids are formed. The synthesis of these chromolipids can be blocked by desferal. Peroxidized linoleic acid gives cross-linking of the proteins present in the membranes leading to the formation of higher molecular weight proteins. Also in the lower molecular weight region of SDS-electrophoresis a protein band is formed. The cross-linking process of the membrane protein is most effectively blocked by thiourea, to some extent by butylated hydroxytoluene and hardly at all by desferal. Addition of peroxidized linoleic acid to intact erythrocytes leads to a drop in the glutathione level (analogous to cumene hydroperoxide). In the presence of glucose the glutathione level can be restored.

The effect of diethyldithiocarbamate on the lipid peroxidation of rat-liver microsomes and intact hepatocytes

The role of the oxygen radicals in lipid peroxidation, induced by ADP/Fe3+ or cumene hydroperoxide was investigated by administering diethyldithiocarbamate, an inhibitor of superoxide dismutase, to hepatocytes or rats. Intact rat-liver hepatocytes perform a delayed ADP/Fe3+-induced lipid peroxidation after pretreatment with diethyldithiocarbamate. The cumene hydroperoxide-induced lipid peroxidation is unchanged. Hepatocytes, isolated from a rat administered with diethyldithiocarbamate in vivo, exhibit the same pattern, a delayed iron-induced lipid peroxidation and an unchanged cumene hydroperoxide-induced lipid peroxidation. Liver microsomes isolated from liver of a rat administered with diethyldithiocarbamate do not perform lipid peroxidation with NADPH/ADP/Fe3+, but do undergo lipid peroxidation with cumene hydroperoxide. It can be concluded that besides the inhibition of superoxide dismutase, diethyldithiocarbamate inhibits directly the microsomal lipid peroxidation. Although this inhibition hampers the conclusion, evidence is obtained that superoxide dismutase is probably involved in the protection against lipid peroxidation of the mitochondria, but not of the microsomes.

Lipid peroxidation of human erythrocyte ghosts induced by organic hydroperoxides

Isolated human erythrocyte ghosts perform lipid peroxidation, measured as malondialdehyde, induced by cumene hydroperoxide and t-butyl hydroperoxide but not by H2O2. In contrast to Ames et al. (Ames, B.N., Cathcart, R., Schwiers, E. and Hochstein, P. (1981) Proc. Natl. Acad. Sci. 78, 6858-6862), no inhibition is found by uric acid, only an increase in lag-time of the malondialdehyde production. In parallel with the malondialdehyde production, fluorescent chromolipids are also formed. Both processes are blocked by the addition of desferal, a potent iron chelator. The malondialdehyde production is also inhibited by the OH radical scavenger, thiourea, and by the anti-oxidant, butylated hydroxytoluene. Treatment of erythrocyte ghosts with cumene hydroperoxide or t-butyl hydroperoxide leads to the genesis of high-molecular-weight protein, but not with H2O2. The appearance of high-molecular-weight proteins is accompanied by disappearance of protein bands, e.g., the alpha- and beta-spectrin band, the anion-exchanger and some other smaller bands. Furthermore, a protein band is formed in the lower-molecular-weight region. 4. The addition of desferal does not reveal any blockade of the high-molecular-weight protein genesis. In contrast, a marked diminution of high-molecular-weight proteins is observed by the addition of thiourea, accompanied by a protection of the protein bands which would otherwise disappear. Similar results are obtained with butylated hydroxytoluene. 5. It is concluded that under oxidative stress the process of high-molecular-weight protein genesis can occur independently of the lipid peroxidation process, measured as the revealing of malondialdehyde.