Mechanisms of action of malondialdehyde and 4-hydroxynonenal on xanthine oxidoreductase

Studies have been made on the possible involvement of malondialdehyde (MDA) and (E)-4-hydroxynon-2-enal (HNE), two terminal compounds of lipid peroxidation, in modifying xanthine oxidoreductase activity through interaction with the oxidase (XO) and/or dehydrogenase (XDH) forms. The effect of the two aldehydes on XO (reversible, XO(rev), and irreversible, XO(irr)) and XDH was studied using xanthine oxidase from milk and xanthine oxidoreductase partially purified from rat liver. The incubation of milk xanthine oxidase with these aldehydes resulted in the inactivation of the enzyme following pseudo-first-order kinetics: enzyme activity was completely abolished by MDA (0.5-4 mM), while residual activity (5% of the starting value) associated with an XO(irr) form was always observed when the enzyme was incubated in the presence of HNE (0.5-4 mM). The addition of glutathione to the incubation mixtures prevented enzyme inactivation by HNE. The study on the xanthine oxidoreductase partially purified from rat liver showed that MDA decreases the total enzyme activity, acting only with the XO forms. On the contrary HNE leaves the same level of total activity but causes the conversion of XDH into an XO(irr) form.     

Stability of microsomal mono-oxygenase during incubations for the liver microsomal assay with S9 fractions of mouse liver under various inductions

Aminopyrine-N-demethylase and p-nitroanisole-O-demethylase activities were determined in incubation mixtures for the liver microsomal assay at time zero and after 1 h of incubation in the conditions for the mutagenic assay. The experiments were performed with the S9 liver fraction of mice in the basal state and induced with sodium phenobarbital, β-naphthoflavone or both. Lipid peroxidation was also determined.

The experiments were repeated with female mice and also in the presence of styrene, as an example of a xenobiotic substance. The activity of pNAD was much more stable than that of APD in all the conditions tested. The pattern of stability, however, was similar for the two activities: more stable than controls with S9 fractions from β-NF-induced mice, less stable than controls in PB-induced mice, intermediate between controls and PB-induced mice in those with combined induction by PB + βNF. Styrene 50 mM in the incubation mixtures led to a marked inactivation of enzymic activity, similar in all cases and reaching about 90% in 1 h. S9 fractions from female mice gave enzymes slightly more stable in almost all cases. Lipid peroxidation was appreciably more elevated in basal than in induced animals.

It was concluded that, for a mutagenesis test on an unknown xenobiotic, S9 fractions from mice following PB and β-NF induction are to be preferred from the point of view of activation.     

Effect of chrysotile asbestos on cytochrome P-450-dependent monooxygenase and glutathione-S-transferase activities in rat lung

The in vitro and in vivo effect of a carcinogenic variety of asbestos, chrysotile, both on xenobiotic metabolizing enzymes such as benzo[a]pyrene hydroxylase, epoxide hydrolase as well as glutathione-S-transferase activities and microsomal lipid peroxidation in rat lung were examined. The in vitro incubation of chrysotile with microsomes significantly adsorbed heme proteins, cytochrome P-450 and P-448 with the concomitant decrease in the dependent monooxygenase activities. The prolonged incubation of this mineral fibre with microsomes also resulted in the release of heme. It also led to the depletion in the activities of epoxide hydrolase and glutathione-S-transferase. However, it induced lipid peroxidation. When these in vitro effects were validated in vivo, the exposure to early stages produced similar alterations as observed in in vitro studies. However, reverse pattern in the alterations was observed after 90 days of exposure except in the case of lipid peroxidation which remained induced.     

Study of the optimal temperature for the liver microsomal assay with mice S9 fractions

The effect of temperature on enzymatic activity and stability was studied with respect to the monooxygenase activities of aminopyrine-N-demethylase (APD) and p-nitroanisole O-demethylase (pNAD) under incubation conditions for the liver microsomal assay. The activities of S9 liver fractions of mice induced with sodium phenobarbital and beta-naphthoflavone were determined during a period of preincubation in a range of temperatures from 30 to 44 degrees C. The greatest value of the mean specific activity was found at 40-42 degrees C for both APD and pNAD. The rapid increase of lipid peroxidation after 1 h of incubation at temperatures higher than 42 degrees C can provide an explanation of the enhancement of the rate of inactivation. In order to determine whether biological response is affected by the modifications induced by temperature in the metabolic activating system, tester strain D7 of Saccharomyces cerevisiae was used to assay the genetic activity of the well known premutagenic agent cyclophosphamide by incubating the mixtures both at the traditional temperature of 37 degrees C and at 42 degrees C. We suggest that the use of more favourable conditions for LMA with respect to enzymatic activity, than the traditional ones could improve the reliability and the sensitivity of such tests.     

Mixtures of polycyclic aromatic compounds inhibit mutagenesis in the Salmonella/microsome assay by inhibition of metabolic activation

We observed that complex mixtures of aromatic compounds isolated from a coal-derived oil suppressed the mutagenic activity of the indirect mutagens benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, 2-aminofluorene, and 2-acetylaminofluorene as measured in the Salmonella/microsome mutagenicity assay, using strain TA98 and metabolic activation with Aroclor-induced rat-liver S9 or microsomes. The mixture also inhibited S9-dependent benzo[a]pyrene metabolism and covalent binding to DNA in a cell-free system. The mixture did not suppress the activity of either the direct acting mutagens 2-nitrofluorene and benzo[a]pyrene diol-epoxide, or of the indirect mutagen N-hydroxy-2-acetylaminofluorene which requires a microsomal deacetylase for metabolic activation. Spectrophotometric measurements showed that components of the mixture bound to microsomal cytochrome P-450. The mixture did not inhibit microsomal NADPH-cytochrome c (P-450) reductase. These observations show that the mixtures inhibited metabolic activation by the microsomal monooxygenase system, probably by binding of unidentified components to cytochrome P-450. The resulting inhibition of mutagenesis may have implications for risk estimates for the mixtures we examined as well as for other types of complex mixtures for which similar inhibitory effects have been observed.     

Hemoglobin degradation, lipid peroxidation, and inhibition of Na+/K(+)-ATPase in rat erythrocytes exposed to acrylonitrile

The effect of acrylonitrile (VCN) on erythrocyte lipid metabolism was investigated in vitro in metabolically active red cells from male Sprague-Dawley rats containing three types of hemoglobins: oxyhemoglobin, methemoglobin, and carbon monoxyhemoglobin. VCN at the concentration of 10 mM rapidly depleted erythrocyte glutathione (GSH) (75% of control) and induced lipid peroxidation (274% of control). Degradation of oxy- and methemoglobin was directly proportional to the extent of lipid peroxidation (r = 0.89). Addition of glucose to the incubation medium decreased hemoglobin degradation while it slightly increased VCN-induced lipid peroxidation. The highest amount of lipid peroxidation occurred in erythrocytes containing carbon monoxyhemoglobin and glucose. In the isolated red cell membranes incubated with 10 mM VCN, the lipid peroxidation was 400% of controls. VCN (25 mM) noncompetitively inhibited erythrocyte membrane Na+/K(+)-ATPase activity and the degree of inhibition was inversely proportional to the reaction temperature (r = -0.88). These findings indicate that the VCN induced hemoglobin degradation and lipid peroxidation are two extremes of a spectrum of oxidative damage in red cells leading to a change in physical state of membrane structure causing inhibition of adenosine triphosphate (ATPase) activity.     

Inhibition of iodoacetamide and t-butylhydroperoxide toxicity in LLC-PK1 cells by antioxidants: a role for lipid peroxidation in alkylation induced cytotoxicity

Previously we reported that thiol depletion and lipid peroxidation were associated with the cytotoxicity of nephrotoxic cysteine S-conjugates, a group of toxins which kill LLC-PK1 cells after metabolic activation and covalent binding. To determine if this is a general mechanism of cytotoxicity in these cells, we compared the effect of antioxidants, an iron chelator, and a thiol reducing agent on the toxicity of an alkylating agent, iodoacetamide (IDAM), and an organic peroxidant, t-butylhydroperoxide (TBHP). IDAM or TBHP toxicity was concentration (0.01 to 1.0 mM) and time (1 to 6 h) dependent. Both toxins caused lipid peroxidation which occurred prior to cell death as determined by leakage of lactate dehydrogenase (LDH). The alkylating agent IDAM bound to cellular macromolecules and depleted cellular non-protein thiols almost completely by 1 h, while LDH release occurred first at 2 to 3 h. The toxicity of IDAM and TBHP was inhibited by the antioxidants DPPD, BHA, BHQ, PGA, and BHT and the iron chelator deferoxamine. However, DPPD blocked TBHP- and IDAM-induced lipid peroxidation and toxicity without affecting binding and depletion of cellular nonprotein thiols. Furthermore, the thiol reducing agent dithiothreitol was able to block lipid peroxidation and toxicity. Therefore it is possible that with an alkylating agent, depletion of cellular nonprotein thiols cooperates with covalent binding and contributes to lipid peroxidation and cell death. There appear to be common elements in the toxicity of alkylating agents and organic peroxidants in LLC-PK1 cells.     

Effect of glucagon and phorbol myristate acetate on oxidative demethylation and lipid peroxidation in isolated hepatocytes.

Oxidative demethylation of aminopyrine and peroxidation of endogenous lipids induced by cumene hydroperoxide were studied in hepatocytes isolated from fed male rats. Glucagon and phorbol-12-myristate-13-acetate (PMA) inhibited both processes in the concentration-dependent manner. Pretreatment of hepatocytes with 1 microM glucagon decreased oxidative demethylation by 75% and had a much smaller effect on lipid peroxidation. Preincubation with 1 microM PMA inhibited both processes by 25-30%. Phosphorylation of three isoforms of cytochrome P-450 was observed in microsomes isolated from hepatocytes incubated in the presence of [32P]orthophosphate. After incubation with PMA the phosphorylation of all these proteins was increased by 60-100%, whereas glucagon increased the phosphorylation of only one isoform. Consequences of the phosphorylation of various isoforms of cytochrome P-450 for metabolic functions of the monooxygenase system are discussed.     

Lipid antioxidant properties of quercetin in vitro

The effect of quercetin on iron-catalyzed hepatic microsomal lipid peroxidation was investigated. Quercetin was shown to be a potent inhibitor of iron-induced lipid peroxidation with a I50 of 0.2 mM. The inhibitory effects of quercetin were dependent on incubation time, protein concentration and iron content in the incubation mixture. Since quercetin does not interact with malonyl-aldehyde it can be concluded that the inhibition of iron induced lipid peroxidation is due to lipid antioxidant property and this may serve as a model for the study by which "free" iron may initiate peroxidation in vivo.     

Oxidative inactivation of angiotensin-converting enzyme]

Hydrogen peroxide inactivates the purified human angiotensin-converting enzyme (ACE) in vitro; the inactivating effect of H2O2 is eliminated by an addition of catalase. The lung and kidney ACE are equally sensitive to the effect of hydrogen peroxide. After addition of oxidants (H2O2 alone or H2O2 + ascorbate or H2O2 + Fe2+ mixtures) to the membranes or homogenates of the lung, the inactivation of membrane-bound ACE is far less pronounced despite the large-scale accumulation of lipid peroxidation products. The marked inactivation of ACE in the membrane fraction (up to 55% of original activity) was observed during ACE incubation with a glucose:glucose oxidase:Fe2+ mixture. Presumably the oxidative potential of H2O2 in tissues in consumed, predominantly, for the oxidation of other components of the membrane (e.g., lipids) rather than for ACE inactivation.     

Evidence for the presence of distinct flavin-containing monooxygenases in human tissues

Catalytic activities and substrate specificity of flavin-containing monooxygenase were examined in human tissues. During incubation with imipramine, human hepatic microsomes efficiently carried out cytochrome P450-dependent reactions but not the formation of N-oxide, while in kidney imipramine N-oxide was the only metabolite formed during in vitro incubation. The production of imipramine N-oxide was essentially due to flavin-containing monooxygenase as shown by thermal inactivation. In contrast, thiobenzamide and dimethylaniline were actively transformed by both human liver and kidney flavin-containing monooxygenase. Neither the modification of pH nor the solubilization of microsomal membranes increased imipramine N-oxidation in human liver. Kinetic analysis indicated a poor affinity (about 7 mM) of human liver microsomes for imipramine versus 0.3 mM in kidney. Immunological studies were undertaken to support enzymatic data. Antibodies raised against rat liver flavin-monooxygenase reacted strongly with human kidney microsomes but extremely weakly with liver microsomes. The relative amount of immunochemically determined protein correlated well with imipramine N-oxidation activity. A dose-dependent inhibition of imipramine N-oxidation by anti-flavin-monooxygenase antibodies was observed in human kidney, as well as in rat kidney and liver. Taken together, the results can be interpreted by the possible existence in human tissues of distinct flavin-containing monooxygenases exhibiting a partial overlapping substrate specificity. The protein involved in imipramine N-oxidation is missing from human liver but actively carries out the reaction in kidney, while another protein catalyzes the oxidation of thiobenzamide and dimethylaniline in both tissues.     

Hemoglobin degradation lipid peroxidation,and inhibition of na+/k+-atpase in rat erythrocytes exposed to acrylonitrile

The effect of acrylonitrile (VCN) on erythrocyte lipid metabolism was investigated in vitro in metabolically active red cells from male Sprague-Dawley rats containing three types of hemoglobins: oxyhemoglobin, methemoglobin, and carbon monoxyhemoglobin. VCN at the concentration of 10 mM rapidly depleted erythrocyte glutathione (GSH) (75% of control) and induced lipid peroxidation (274% of control). Degradation of oxy- and methemoglobin was directly proportional to the extent of lipid peroxidation (r = 0.89). Addition of glucose to the incubation medium decreased hemoglobin degradation while it slightly increased VCN-induced lipid peroxidation. The highest amount of lipid peroxidation occurred in erythrocytes containing carbon monoxyhemoglobin and glucose. In the isolated red cell membranes incubated with 10 mM VCN, the lipid peroxidation was 400% of controls. VCN (25 mM) noncompetitively inhibited erythrocyte membrane Na⁺/K⁺-ATPase activity and the degree of inhibition was inversely proportional to the reaction temperature (r = ?0.88). These findings indicate that the VCN induced hemoglobin degradation and lipid peroxidation are two extremes of a spectrum of oxidative damage in red cells leading to a change in physical state of membrane structure causing inhibition of adenosine triphosphatase (ATPase) activity.     

Free radical inactivation of glucose-6-phosphate dehydrogenase in Saccharomyces cerevisiae in vitro

Partial purification and in vitro inactivation of glucose-6-phosphate dehydrogenase from the yeast Saccharomyces cerevisiae in the Fe2+/H2O2 oxidation system were conducted. At the protein concentration 1.5 mg/ml, the enzyme lost 50% of activity within 5 minutes of incubation in presence of 2 mM hydrogen peroxide and 3 mM ferrous sulphate. The inactivation extent depended on time and concentrations of FeSO4 and H2O2. EDTA, ADP and ATP at concentration 0.5 mM enhanced inactivation. At the same time, the presence of 0.5 mM NADPH, 1 mM glucose-6-phosphate, 10 mM mannitol, 30 mM dimethylsulphoxide or 20 mM urea diminished this process. In comparison with native enzyme, index S(0,5) of the partially inactivated enzyme for glucose-6-phosphate was 2.1-fold higher, but for NADP it was 1,6-fold lower. Maximal activity of the partially inactivated enzyme was 3-5-fold lower than that of native one.     

Activation of snail (Helix pomatia) nervous tissue tyrosine monooxygenase by calcium in vitro.

Addition of calcium chloride to soluble preparations of tyrosine monooxygenase from snail brain appears to produce an activation of the enzyme when assayed with subsaturating concentrations of the pteridine cofactor 6 MPH4 (2-amino-4-hydroxy-6-methyltetrahydropteridine). While some increase in the activity occurs with calcium chloride at a concentration of 0.01 mM, activation is increased by about 100% at 1mM and reaches a maximum at 5mM (144%) where it remains more or less constant up to 10mM. Barium chloride also produces an activating effect although it is much less pronounced while magnesium chloride is without effect. EGTA has no direct effect on the enzyme but antagonises the activation produced by calcium chloride. The activation of tyrosine monooxygenase by calcium is reflected in changes in the kinetic properties of the enzyme, decreasing the Km from 43 muM to 19 muM for tyrosine and from 670muM to 230muM for the pteridine cofactor. No change was observed with V values for either tyrosine or pteridine cofactor. It is suggested that calcium, which enters the nerve terminal during nerve stimulation, regulates the transmitter dopamine by activating the rate-limiting enzyme tyrosine monooxygenase.     

Inhibition of the microsomal N-hydroxylation of 2-amino-6-nitrotoluene by a metabolite of methimazole

Inhibition studies were used to investigate the identity of the microsomal enzyme(s) responsible for the NADPH-dependent N-hydroxylation of 2-amino-6-nitrotoluene. The N-hydroxylation reaction was inhibited by several cytochrome P-450 inhibitors as well as by methimazole, a substrate for flavin-containing monooxygenase. Heat inactivation of flavin-containing monooxygenase had no effect on the rate of the reaction but abolished the inhibition by methimazole. These results indicate that the flavin-containing monooxygenase-mediated metabolism of methimazole produced an inhibitor of the cytochrome P-450-catalyzed N-hydroxylation reaction. When glutathione was included in the incubation the inhibition by methimazole was abolished, presumably due to the reduction of oxygenated metabolites of methimazole. These results show that methimazole inhibition does not necessarily implicate flavin-containing monooxygenase in microsomal N-hydroxylation reactions.     

Glucose induces lipid peroxidation and inactivation of membrane-associated ion-transport enzymes in human erythrocytes in vivo and in vitro.

Erythrocytes of diabetic subjects (non-insulin dependent) were found to have eight- to ten-fold higher levels of endogenously formed thiobarbituric acid reactive malonyldialdehyde (MDA), thirteen-fold higher levels of phospholipid-MDA adduct, 15-20% reduced Na(+)-K(+)-ATPase activity with unchanged Ca+2-ATPase activity, as compared with the erythrocytes from normal healthy individuals. Incubation of normal erythrocytes with elevated concentrations (15-35 mM) of glucose, similar to that present in diabetic plasma, led to the increased lipid peroxidation, phospholipid-MDA adduct formation, reduction of Na(+)-K(+)-ATPase (25-50%) and Ca+2-ATPase (50%) activities. 2-doxy-glucose was 80% as effective as glucose in the lipid peroxidation and lipid adduct formation. However, other sugars, such as fructose, galactose, mannose, fucose, glucosamine and 3-O-methylmannoside, and sucrose, tested at a concentration of 35 mM, resulted in reduced (20-30%) lipid peroxidation without the formation of lipid-MDA adduct. Kinetic studies show that reductions in Na(+)-K(+)-ATPase and Ca+2-ATPase activities precede the lipid peroxidation as the enzyme inactivation occur within 30 min of incubation of erythrocytes with high concentration (15-35 mM) of glucose, while lipid peroxidation product, MDA appears at 4 hr and lipid-MDA adducts at 8 hr. The lipoxygenase pathway inhibitors, 5,8,11-eicosatriynoic acid and Baicalein (5,6,7-trihydroxyflavone), reduced the glucose-induced lipid peroxidation by 30% and MDA-lipid adduct formation by 26%. Indomethacin, a cyclooxygenase pathway inhibitor, had no discernible effect on the lipid peroxidation in erythrocytes. However, the inhibitors of lipid peroxidation, 3-phenylpyrazolidone, metyrapone, and the inhibitors of lipoxygenase pathways did not ablate the glucose-induced reduction of Na(+)-K(+)-ATPase and Ca+2-ATPase activities in erythrocytes. Erythrocytes produce 15-HETE (15-hydroxy-eicosatetraenoic acid), which is augmented by glucose. These results suggest that the formation of lipoxygenase metabolites potentiate the glucose-induced lipid peroxidation and that the inactivation of Na(+)-K(+)-ATPase and Ca+2-ATPase occurs as a result of non-covalent interaction of glucose with these enzymes.     

EDTA affects cytochrome P450-dependent biotransformation reactions during incubations for the liver microsomal assay

In order to optimize the condition of the liver microsomal assay (LMA), studies were carried out to determine the effects of EDTA on mixed-function oxidase activity and its stability under the exact incubation conditions for the LMA. Aminopyrine N-demethylase (APD) and p-nitroanisole O-demethylase (p-NAD) activities as well as lipid peroxidation development (LP) in S9 liver fractions from beta-naphthoflavone and sodium phenobarbital (beta-NF + PB)- or Aroclor 1254 (AC)-treated mice were examined during a period of preincubation with EDTA ranging from 1 to 40 mM. At 5 mM EDTA, we obtained a strong inhibition of the microsomal LP as well as the greatest value of the mean specific activity (Asp) for both APD and pNAD activities. In agreement with the biochemical data, the presence of 5 mM EDTA in the incubation mixtures for the LMA significantly increased the mitotic gene conversion, mitotic crossing-over and point-reverse mutation of the well-known premutagen cyclophosphamide (30 mM) on the diploid D7 strain of Saccharomyces cerevisiae as the outcome of a greater metabolic activity. We concluded that the systematic use of 5 mM EDTA in LMA mixtures could improve the reliability and sensitivity of such a test.     

The effect of galactose metabolic disorders on rat brain Na+,K+-ATPase activity

To evaluate the effect of galactose metabolic disorders on the brain Na+,K+-ATPase in suckling rats. Separate preincubations of various concentrations (1-16 mM) of the compounds galactose-1-phosphate (Gal-1-P) and galactitol (galtol) with whole brain homogenates at 37 degrees C for 1 h resulted in a dose dependent inhibition of the enzyme whereas the pure enzyme (from porcine cerebral cortex) was stimulated. Glucose-1-phosphate (Glu-1-P) or galactose (Gal) stimulated both rat brain Na+,K+-ATPase and pure enzyme. A mixture of Gal-1-P (2 mM), galtol (2 mM) and Gal (4 mM), concentrations commonly found in untreated patients with classical galactosemia, caused a 35% (p < 0.001) rat brain enzyme inhibition. Additionally, incubation of a mixture of galtol (2 mM) and Gal (1 mM), which is usually observed in galactokinase deficient patients, resulted in a 25% (p < 0.001) brain enzyme inactivation. It is suggested that: a) The indirect inhibition of the brain Na+,K+-ATPase by Gal-1-P should be due to the presence of the epimer Gal and phosphate and that the pure enzyme direct activation by Gal-1-P and Glu-1-P to the presence of phosphate only. b) The observed brain Na+,K+-ATPase inhibitions in the presence of toxic concentrations of Gal-1-P and/or galtol could modulate the neural excitability, the metabolic energy production and the catecholaminergic and serotoninergic system.     

Relative efficacies of indole antioxidants in reducing autoxidation and iron-induced lipid peroxidation in hamster testes

Increased iron stores are associated with free radical generation and carcinogenesis. Lipid peroxidation is involved in DNA damage, thus indirectly participating in the early steps of tumor initiation. Melatonin and structurally related indoles are effective in protecting against oxidative stress. The aim of the study was to compare the relative efficacies of melatonin, N-acetylserotonin (NAS), indole-3-propionic acid (IPA), and 5-hydroxy-indole-3-acetic acid (5HIAA) in altering basal and iron-induced lipid peroxidation in homogenates of hamster testes. To determine the effect of the indoles on the autoxidation of lipids, homogenates were incubated in the presence of each agent in concentrations of 0.0, 0.01, 0.05, 0.1, 0.25, 0.5, 0.75, 1.0, 2.0, 2.5, or 5.0 mM. To study their effects on induced lipid peroxidation, homogenates were incubated with FeSO(4) (30 microM + H(2)O(2) (0.1 mM) + each of the indoles in the same concentrations as above. The degree of lipid peroxidation was expressed as concentrations of malondialdehyde + 4-hydroxyalkenals (MDA + 4-HDA) per mg protein. The indoles decreased both basal and iron-related lipid peroxidation in a concentration-dependent manner. Melatonin reduced basal MDA + 4-HDA levels when used at the concentrations of 0.25 mM or higher, and prevented iron-induced lipid peroxidation at concentrations of 1.0, 2.0, 2.5, or 5.0 mM. The lowest effective concentrations of NAS required to lower basal and iron-related lipid peroxidation were 0.05 mM and 0.25 mM, respectively. IPA, only when used in the highest concentrations of 2.5 mM or 5 mM inhibited basal lipid peroxidation levels and it was ineffective on the levels of MDA + 4-HDA due to iron damage. 5HIAA reduced basal lipid peroxidation when used at concentrations of 0.25 mM or higher, and it prevented iron-induced lipid peroxidation only at the highest applied concentration (5 mM). In conclusion, melatonin and related indoles at pharmacological concentrations protect against both the autoxidation of lipids as well as induced peroxidation of lipids in testes. In doing so, these agents would be expected to reduce testicular cancer that is initiated by products of lipid peroxidation.     

4-Hydroxy-2,3-trans-nonenal stimulates microsomal lipid peroxidation by reducing the glutathione-dependent protection

Glutathione (GSH) protects liver microsomes against lipid peroxidation. This is probably due to the reduction of vitamin E radicals by GSH, a reaction catalyzed by a membrane-bound protein. Pretreatment of liver microsomes with 0.1 or 1mM 4-hydroxy-2,3-trans-nonenal (HNE), a major product of lipid peroxidation, reduces the GSH-dependent protection. GSH and vitamin E concentrations are not affected by this pretreatment. Pretreatment with 0.1 mM N-ethyl maleimide (NEM), a synthetic sulfhydryl reagent, resulted in a reduction similar to that with HNE of the GSH-dependent protection against lipid peroxidation. The reduction of the GSH-dependent protection by HNE and NEM is probably the result of inactivation of the membrane-bound protein by covalent binding to an essential SH group on the protein. If the GSH-dependent protection would proceed via the microsomal GSH transferase, pretreatment with NEM, which activates the microsomal GSH transferase, should enhance the GSH-dependent protection. Actually a decrease in the GSH-dependent protection is found. Apparently the GSH-dependent protection does not proceed via the microsomal GSH transferase. Also the microsomal phospholipase A2 is not involved, since addition of 0.1 mM mepacrine, an inhibitor of phospholipase A2, did not preclude the GSH-dependent protection. Once the process of lipid peroxidation, either in vivo or in vitro, has started, the protection of liver microsomes by GSH is less effective. This might be the result of formed HNE. In this way an endproduct of lipid peroxidation stimulates the process that generates this product.