A comparative evaluation of thiobarbituric acid methods for the determination of malondialdehyde in biological materials

A comparative evaluation was made of the conventional spectrophotometric procedure and three published high performance liquid chromatographic (HPLC) procedures for the determination of malondialdehyde (MDA) as the thiobarbituric acid (TBA) derivative when applied to liver, fish meal, serum, and urine. Except for urine, spectrophotometric analysis overestimated MDA content. Purification of the TBA-MDA complex obtained from liver and fish meal on reverse phase cartridges was found to entail a loss of complex bound to residual peptides in the trichloracetic acid (TCA) extract. Mincing as opposed to homogenizing liver samples led to a doubling of values for MDA content. Hexanal was a major TBA reactant, in addition to MDA, in all the samples. Acid hydrolysis and heat were necessary for the release of MDA bound to the amino groups of proteins and other amino compounds. Methods for free MDA have limited application to biological materials except short term in vitro preparations such as peroxidizing microsomes, in which free MDA accumulates. On the basis of these and other observations, a modified HPLC procedure for the determination of MDA as the TBA-MDA complex is proposed.

Glycosylated low density lipoprotein is more sensitive to oxidation: implications for the diabetic patient?

Oxidised low density lipoprotein (LDL) is considered to be atherogenic. This study examined the relationship between glycosylation and oxidation of LDL from 10 normocholesterolaemic Type 2 diabetic patients, 10 hypercholesterolaemic Type 2 diabetic patients, and 10 normocholesterolaemic non-diabetic subjects. LDL was isolated by sequential ultracentrifugation and susceptibility to oxidation assessed by measuring thiobarbituric reactive substances (TBARS) during a 4-h oxidation period. LDL glycosylation was measured by aminophenylborate gel chromatography. Results demonstrated an increased susceptibility to oxidation in LDL from both diabetic groups, the mean 3-h TBARS values being 35.2 +/- 2.1 and 36.4 +/- 2.6 nmol MDA/mg LDL protein for normocholesterolaemic and hypercholesterolaemic diabetic patients compared with 24.5 +/- 2.5 nmol MDA/mg LDL protein for control subjects. LDL glycosylation of 2.20% +/- 0.11% and 2.89% +/- 0.46% for normocholesterolaemic and hypercholesterolaemic diabetic LDL was significantly higher than that for the non-diabetic control subjects of 1.60% +/- 0.12% (P < 0.02). There was a significant positive correlation (P < 0.005) between LDL glycosylation and LDL oxidation. The esterified/free cholesterol ratio which correlated positively with oxidation (P < 0.01) was significantly higher in LDL from both diabetic groups compared with LDL from control subjects (P < 0.01). Thus the increased incidence of atherosclerosis in diabetes may be related to glycosylation of LDL through its increased susceptibility to oxidation.

Lipid peroxidation in sarcoplasmic reticulum membranes: effect on functional and biophysical properties

The fluorescent polyunsaturated parinaric acid (PnA) incorporated in sarcoplasmic reticulum membranes (SR) was used to probe the initial stages of membrane lipid peroxidation. The experimental set up of the PnA assay was investigated by means of several peroxidation initiators to ascertain peroxidation conditions. This assay in SR is particularly useful to evaluate the membrane susceptibility to peroxidation and to ascertain suitable conditions (concentration of initiators and cofactors) to challenge peroxidation in each preparation under study. On the basis of the PnA assay, Fe2+/ascorbate was selected among the different initiator systems to assess the effect of lipid peroxidation upon biochemical and biophysical parameters of SR membranes. Under mildly controlled conditions at 25 degrees C, the lipid degradative process, as detected by fatty acid analysis, decreases the Ca2+ uptake (up to about 50% of control) and reduces the Ca2+ pump efficiency (Ca2+/ATP ratio) up to about 58% of control, without inactivation the ATPase enzyme turnover. The effect of lipid peroxidation on the SR bilayer organization is dependent either on the extent of lipid peroxidation or on the depth of the bilayer as probed by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene and by intramolecular excimerization of 1,3-di(1-pyrenyl)propane. It is concluded that the effect of mild lipid peroxidation on Ca2+ pump activity is partially exerted through the alteration of physical properties in the lipid phase or lipid-protein interfaces.

Toxicity of oxidised low density lipoprotein towards mouse peritoneal macrophages in vitro

The addition of cupric sulphate-oxidised low density lipoprotein (oxLDL) to mouse peritoneal macrophage (MPM) cultures caused toxicity towards the cells, measured by tritiated adenine release. The degree of toxicity increased with increasing concentrations of oxLDL up to 18 h incubation with MPM, after which the toxicity appeared to be independent of the concentration of oxLDL used. Toxicity was significantly reduced when vitamin E in the form of D,L-alpha-tocopherol was added to the LDL before the artificial oxidation, but vitamin E had no effect on the toxicity when added alongside the oxLDL in the culture medium. However, if the MPM were pre-incubated for 24 h with 80 microM vitamin E, there was a significant reduction in the level of toxicity up to 6 h in culture with oxLDL. There was a strong correlation (r = 0.81) between the degree of oxidation of the LDL, measured as thiobarbituric-reactive substances, and the corresponding toxicity. This indicated that the more oxidised the LDL was, the more toxic it was to the macrophages. Native LDL also led to toxicity, but only after a time-lag of 20 h. Again, this toxicity was decreased by pre-incubation of the MPM with vitamin E, but not by addition of vitamin E at the same time as LDL. The findings suggest that oxLDL is capable of contributing to the onset of necrosis during atherogenesis and that the oxidative capacity of the macrophage foam cells themselves might contribute to the process.

Relationship of omega-3 fatty acid supplementation to plasma lipid peroxidation in predialysis patients with hypertriglyceridaemia

OBJECTIVES

Since it has been suggested that omega-3 polyunsaturated fatty acids (PUFA) may be an efficient treatment for dyslipoproteinaemia and renal function in uraemic patients, this study was carried out to evaluate the potential adverse effects of polyunsaturated acids on lipid peroxidation in patients with chronic renal failure.

PATIENTS AND METHODS

Five uraemic predialysis patients with various renal diseases presenting progressive renal failure and moderate hypertriglyceridaemia (1.93 +/- 0.60 g/L) in comparison with the reference values of our laboratory (0.88 +/- 0.20 g/L) were given 6 g of fish oil per day for 6 months in the form of Maxepa capsules containing a low dose of tocopherol acetate (10.5 mg/day). During the treatment we investigated the effect (expected to be beneficial) of n-3 PUFA on triglyceride plasma levels and renal function. We also investigated the potential side effect of such a treatment on lipid peroxidation.

RESULTS

A moderate decrease of triglycerides early after the introduction of the treatment (1.93 +/- 0.6 g/L vs 1.33 +/- 0.58 g/L; 0 vs 0.5 months) was observed. In all patients, lipid peroxides measured as thiobarbituric acid reactants increased early after the introduction of the n-3 PUFA therapy (2.8 + 0.8 vs 3.76 +/- 0.15 mumol/L; 0 vs 0.5 months). Organic hyperoxides were significantly different in these patients compared with laboratory values, but they did not increase significantly during the treatment.

CONCLUSION

It must be noted that polyunsaturated fatty acids should be used with caution in predialysis patients.

Inhibition of copper-mediated oxidation of LDL by rat serosal mast cells. A novel cellular protective mechanism involving proteolysis of the substrate under oxidative stress

Rat serosal mast cells, when stimulated to exocytose their cytoplasmic granules, effectively blocked the copper-mediated oxidation of low density lipoproteins (LDLs) in vitro. This effect depended on the proteolytic activity of the formed extracellular granule remnants, since specific inhibition of chymase, the neutral protease that they contain, blocked the protective effect of the mast cells. The mechanism of this chymase-mediated inhibition of LDL oxidation was found to be binding of the copper ions present in the incubation medium by peptides released from LDL on proteolytic degradation of their apolipoprotein B (apoB) component. This was verified by demonstrating that addition of such peptides to LDL--copper ion mixtures completely prevented oxidation of LDL and that this protective effect could be overcome by adding copper ions in excess. Furthermore, proteolytic degradation of the apoB of LDL, with concomitant release of copper-containing peptides, left the partially degraded apoB without the copper ions necessary for propagation of LDL oxidation. These observations provide the first evidence for cell-mediated inhibition of LDL oxidation.

Structural and functional properties of apolipoprotein B in chemically modified low density lipoproteins

The structural and compositional changes occurring during in vitro chemical modification of apolipoprotein B-100 (apo B), the apolipoprotein component of low density lipoproteins (LDL), were investigated in this study. The functional properties of chemically modified apo B and especially its potential to induce accumulation of cholesterol esters in macrophages were related to the structural changes of apo B. Acetylation, maleylation or malondialdehyde conjugation did not significantly affect the lipid composition of LDL. However, the unsaturated cholesteryl esters content, especially that of cholesteryl arachidonate was significantly decreased through Cu-oxidation. The number of reactive lysine residues in apo B was decreased by Cu-catalyzed LDL oxidation, acetylation, maleylation and by malondialdehyde conjugation. The number of free cysteines decreased from six in native apo B-100 to three in Cu-oxidized LDL. The tryptophan fluorescence intensity decreased most in malondialdehyde-conjugated LDL and in Cu-oxidized LDL, compared with acetylated and maleylated LDL. The secondary structure of native and chemically modified LDL was measured by attenuated total reflection infrared spectroscopy and by circular dichroism. No significant changes were observed in the secondary structure of any of the modified LDL. These data suggest that neither acetylation, malondialdehyde treatment or even Cu-oxidation substantially altered the secondary structure of apo B, in spite of significant modifications in the primary structure. Incubation of chemically modified LDL with J774 macrophages induced an accumulation of cellular cholesteryl esters and foam cell formation. The highest cholesterol accumulation was induced after malondialdehyde treatment of LDL. These data suggest that the cellular uptake and accumulation of modified LDL is not modulated by changes in the apo B structure. Rather it seems dependent upon the net charge of the apo B protein and probably involves the modification of critical lysine residues.

Low density lipoprotein oxidation by stimulated neutrophils and ferritin

Low density lipoprotein (LDL) oxidation mediated by phorbol myristate acetate (PMA)- and formylmethionylleucylphenylalanine (FMLP) -stimulated human neutrophils was enhanced by 70% in the presence of ferritin. Iron released from ferritin by the superoxide anion generated in the respiratory burst of stimulated neutrophils is shown to be involved in lipoprotein oxidation. Ascorbate (100 microM), superoxide dismutase (10 micrograms/ml) and uric acid (430 microM) showed inhibitory effects of 30% [corrected], 70% and 50% on LDL oxidation, respectively. Ceruloplasmin (2.7 microM) potentiated LDL oxidation by stimulated neutrophils and ferritin, both alone and in the presence of methionine. Methionine (1 mM) and catalase (30 micrograms/ml) increased LDL oxidation by stimulated neutrophils and ferritin. These data suggest that LDL oxidation by stimulated neutrophils and ferritin may be relevant in inflammation when both neutrophils and ferritin are increased.

Carvedilol, a new antihypertensive, prevents oxidation of human low density lipoprotein by macrophages and copper

Growing evidence indicates that oxidized low-density lipoprotein (LDL) may promote atherogenesis. Therefore, inhibition of LDL oxidation may impede this process. Carvedilol is a vasodilating, beta-adrenoceptor blocking agent. As a new antihypertensive drug, carvedilol is unique by virtue of its potent antioxidant activity. Therefore, we tested the ability of carvedilol to inhibit the oxidation of LDL by either macrophages or Cu2+. Carvedilol inhibited LDL oxidation by macrophages in a dose-dependent manner, with an IC50 value of 3.8 microM, as assessed by a thiobarbituric acid reactive substance (TBARS) assay. Under the same conditions, propranolol showed only a mild inhibitory effect (IC50 > 100 microM), while pindolol, atenolol and labetalol had almost no effect. Carvedilol, at 10 microM, almost completely inhibited the macrophage-induced increase in electrophoretic mobility of LDL, while other beta-blockers at 50-300 microM had no significant effect. Carvedilol inhibited superoxide release from mouse macrophages, which correlated well with its inhibition of LDL oxidation. Carvedilol also inhibited Cu(2+)-induced LDL oxidation with an IC50 value of 17 microM, while all other beta-blockers were inactive up to 300 microM. These observations suggest that carvedilol might not only be an effective antihypertensive drug, but might also be effective in prevention of atherosclerosis.

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)

The biochemical mechanisms of the excitotoxicity of kainic acid. Free radical formation

Kainic acid (KA) is a known potent neuroexcitotoxin, although the biochemical mechanism producing its underlying neurotoxic effect is not quite clear. Histopathological examination of gerbil brains 24 h after systemic injection of KA revealed severe neuronal lesions in different regions of the brain, especially the cerebellar and hippocampal areas. We have detected free radical formation in the brain 1 h after KA administration by using an in vivo spin trapping technique. We have also observed increased lipid peroxidation in the brain after KA-treatment by analyzing thiobarbituric acid reactive substances and conjugated diene formation. Diminished brain specific (Na+, K+)-ATPase activity was also found 2 h after KA injection and persisted to 24 h. It is possible that the free radical reaction is a primary cause of neuronal degeneration after KA administration.

Effects of niacin on bleomycin-induced increases in myeloperoxidase, prolyl hydroxylase, and superoxide dismutase activities and collagen accumulation in the lungs of hamsters

It has been shown that lung nicotinamide adenine dinucleotide (NAD) depletion accompanies bleomycin (BL)-induced lung fibrosis in the hamster and that treatment with niacin (NA), a precursor of NAD, was found to attenuate lung fibrosis caused by this agent. Niacin was used in the present study to investigate changes in some biochemical parameters and enzymes involved in the development of BL-induced lung fibrosis in the hamster. Niacin (500 mg/kg, IP), or an equivalent volume of saline (SA, IP), was given daily 2 days prior to intratracheal instillation of BL (7.5 U/5 mL/kg) or SA and everyday thereafter throughout the study. Hamsters were killed at 1, 4, 7, 10, and 14 days after the BL or SA instillation and their lungs processed for various biochemical assays. Hydroxyproline content and superoxide dismutase (SOD) activity in SABL treated animals were significantly (P < or = 0.05) elevated at 7 and 10 days, peaking at 14 days to 161 +/- 11% and 159 +/- 11% of the SASA treated animals, respectively. Although the hydroxyproline level of NABL treated animals was significantly elevated at 7 and 10 days and peaked at 14 days to 123 +/- 8% of the NASA control, these values were significantly lower than the SABL treated animals at the corresponding times. The lung SOD activity of NABL groups was significantly higher at 4 days but significantly lower at 10 and 14 days than the SABL groups at the corresponding times. Prolyl hydroxylase (PH) activity and total lung calcium in SABL treated groups were significantly elevated compared to SASA treated groups starting at 4 days, with PH peaking at 10 days to 163 +/- 13% and calcium peaking at 7 days to 148 +/- 8% of SASA treated groups. The NABL treated animals displayed a significant elevation in PH activity at 4 days only (132 +/- 15%), while the calcium content in this group was significantly increased at 4 and 14 days compared to NASA treated animals. However, the activity of PH in the NABL treated animals was significantly lower than the SABL treated animals at 7, 10, and 14 days. The calcium content of the NABL group was significantly lower than the SABL group at 7 and 10 days. The thiobarbituric acid reactive substance equivalents (TBARS) content and myeloperoxidase (MPO) activity were significantly elevated at all time points in SABL groups as compared to SASA groups, with peak elevation of TBARS to 160 +/- 9% at 4 days and MPO to 268 +/- 40% at 1 day.(ABSTRACT TRUNCATED AT 400 WORDS)

Aminoguanidine inhibits the modification of proteins by lipid peroxidation derived aldehydes: a possible antiatherogenic agent

The reaction of protein amino groups with lipid-peroxidation-derived aldehydes (LPDA) has been shown to play a key role in various pathological processes. Especially important is the reaction of LPDA with apolipoprotein B during oxidative modification of low density lipoprotein (LDL), which leads to its enhanced uptake by macrophages and, eventually, to atherogenesis. Since aminoguanidine, a drug which inhibits the advanced steps of glycation (probably by trapping reactive sugar-derived aldehydes), has been proposed as a therapeutic agent for the prevention of late diabetic complications, we have tested its ability to interfere with the modification of proteins by LPDA. LDL was incubated with cupric ions. Aminoguanidine at 5, 10 and 25 mM inhibited both the increase in electrophoretic mobility of LDL and the generation of thiobarbituric acid reactive substances (TBARS) (P < 0.001). It also inhibited the increase in electrophoretic mobility and 260-400 nm absorbance of bovine serum albumin incubated with malondialdehyde. These results suggest that aminoguanidine may have an antiatherogenic effect.