Simon P. Wolff

Ferrous ion oxidation in the presence of xylenol orange for detection of lipid hydroperoxide in low density lipoprotein

A simple and sensitive method for the direct measurement of lipid peroxides in lipoprotein and liposomes is described. The method is based on the principle of the rapid peroxide-mediated oxidation of Fe2+ to Fe3+ under acidic conditions. The latter, in the presence of xylenol orange, forms a Fe(3+)-xylenol orange complex which can be measured spectrophotometrically at 560 nm. Calibration with standard peroxides, such as hydrogen peroxide, linoleic hydroperoxide, t-butyl hydroperoxide, and cumene hydroperoxide gives a mean apparent extinction coefficient of 4.52 x 10(4) M-1 cm-1 consistent with a chain length of approximately 3 for ferrous ion oxidation by hydroperoxides. Endoperoxides are less reactive or unreactive in the assay. The assay has been validated in the study of lipid peroxidation of low density lipoprotein and phosphatidyl choline liposomes. By pretreatment with enzymes known to metabolize peroxides, we have shown that the assay measures lipid hydroperoxides specifically. Other methods for measuring peroxidation, such as the assessment of conjugated diene, thiobarbituric acid reactive substances and an iodometric assay have been compared with the ferrous oxidation-xylenol orange assay.

Protein glycation and oxidative stress in diabetes mellitus and ageing.

Hyperglycemia is increasingly regarded as the cause of the diabetic complications, in particular via the ability of glucose to glycate proteins and generate Maillard browning products which cross-link proteins and render them brown and fluorescent in vitro. Similar changes occur in vivo to long-lived proteins in diabetes mellitus as well as in ageing. The evidence supporting this route of glucose toxicity is discussed in the context of the ability of glucose to oxidize in vitro (catalyzed by trace amounts of transition metal) generating hydrogen peroxide, highly reactive oxidants, and protein-reactive ketoaldehyde compounds. It is suggested that protein browning in vivo may not result from the reactions of glucose with protein but from the transition metal-catalyzed reactions of other small autoxidisable substrates, such as ascorbate, with protein. Overall, studies of glycation and protein browning suggest a critical role for oxidative processes perhaps involving decompartmentalized transition metals and a variety of low molecular weight reducing agents in diabetes mellitus and ageing.

Autoxidative Glycosylation and Possible Involvement of Peroxides and Free Radicals in LDL Modification by Glucose

It has been postulated that the etiology of the complications of diabetes involves oxidative stress, perhaps as a result of hyperglycemia. Consistent with this hypothesis, it has been shown that glucose, under physiological conditions, produces oxidants that possess reactivity similar to the hydroxyl free radical. These oxidants hydroxylate benzoic acid, fragment protein, and induce peroxidation in phosphatidylcholine liposomes and low-density lipoprotein (LDL) when LDL is incubated with hyperglycemic levels of glucose in vitro. These reactions are accelerated by transition metals and inhibited by a metal-chelating agent. The atherosclerotic potential of LDL in diabetes mellitus is often discussed in terms of protein glycosylation, which may affect cellular interactions. Our studies demonstrate, however, that peroxidative reactions also accompany LDL glycosylation in vitro. Peroxidative modification of LDL has also been implicated in LDL atherogenicity. Our studies indicate that glycosylation and peroxidation occur concomitantly in LDL modified by glucose in vitro and may both contribute to the behavioral changes of this lipoprotein.

Lipid hydroperoxide measurement by oxidation of Fe2+ in the presence of xylenol orange. Comparison with the TBA assay and an iodometric method

Study of the role of hydroperoxides and lipid peroxidation in disease requires simple and sensitive methods for direct hydroperoxide measurement. We report on a technique for measuring hydroperoxide which relies upon the rapid hydroperoxide-mediated oxidation of Fe2+ under acidic conditions. Fe3+ forms a chromophore with xylenol orange which absorbs strongly at 560 nm, yielding an apparent E560 (for H2O2, butyl hydroperoxide and cumene hydroperoxide) of 4.3×104 M−1 cm−1. The assay was validated in a study of liposomal lipid peroxidation and shown to give results comparable with those obtained by an iodometric method or by measuring conjugated dienes. The assay involving thiobarbituric acid, by comparison, underestimates lipid peroxidation and does not measure hydroperoxideper se.

Hydrogen peroxide production during experimental protein glycation

The accumulation of hydrogen peroxide (H2O2) during incubations of protein with glucose (experimental glycation) has previously been too low for direct measurement although it is suggested to be the precursor of protein‐damaging hydroxylating agents. We have thus developed a simple H2O2‐measuring technique which relies upon the rapid peroxide‐mediated oxidation of Fe2+ to Fe3+ (catalysed by sorbitol) under acidic conditions followed by reaction of the latter cation with the dye, xylenol orange. We have used the method to demonstrate that incubation mixtures of protein and glucose generates nanomolar levels of hydrogen peroxide in the presence of protein under physiological conditions of pH and temperature.

Plasma 8‐epi‐PGF2α levels are elevated in individuals with non‐insulin dependent diabetes mellitus

This study reports plasma levels of a specific nonenzymatic peroxidation product of arachidonic acid, esterified 8‐epi‐PGF2α, from healthy‐ and NIDDM individuals as an index of oxidative stress in vivo. Plasma 8‐epi‐PGF2α was isolated by solid‐phase extraction on a C,8 followed by an NH2 cartridge and analyzed by GC‐MS/NICI as PFB‐ester/TMS‐ether derivative. We found that the average concentration of esterified 8‐epi‐PGF2α among NIDDM subjects (0.93 ± 0.07 nM, n = 39) was higher (P < 0.0001, Mann‐Whitney test) than in healthy individuals (0.28 ± 0.04 nM, n = 15). These data indicate that NIDDM is associated with increased plasma lipid peroxidation.

Elevated Levels of Authentic Plasma Hydroperoxides in NIDDM

Using a precise technique for measuring authentic plasma lipid hydroperoxides (ROOHs), we show that individuals with non-insulin-dependent diabetes mellitus (NIDDM) have higher levels of ROOH than do control subjects. ROOHs were measured by the ferrous oxidation with xylenol orange assay coupled with the selective ROOH reductant triphenylphosphine. Formation of the ferric xylenol orange complex was determined at 560 nm and calibrated against H2O2. For 22 individuals with NIDDM, a concentration of 9.04 ± 4.3 μmol/l (mean ± SD) ROOH was recorded. This concentration was higher (P < 0.0005 by separate-variance t test) than that of plasma ROOHs from control subjects (3.76 ± 2.48 μmol/l). There was no difference between concentrations of plasma malondialdehyde measured as thiobarbituric acid–reactive material (TBARM) in NIDDM or control subjects (1.00 ± 0.70 vs. 1.21 ± 0.62 μmol/l, respectively; P > 0.1). A trend to lower vitamin E levels in the NIDDM group (9.03 ± 3.31 vs. 10.31 ± 5.02 μg/ml in control subjects) failed to achieve significance at the 95% confidence level. Plasma ROOHs in the diabetic group did not correlate with total plasma cholesterol, triglyceride, fasting glucose, HbA1, vitamin E, or TBARM levels. These data indicate that measurement of authentic ROOHs shows NIDDM to be associated with oxidative stress, which may be unrelated to abnormalities in lipid metabolism and glycemic control.

Thioctic (lipoic) acid: a therapeutic metal-chelating antioxidant?

Thioctic (alpha-lipoic) acid (TA) is a drug used for the treatment of diabetic polyneuropathy in Germany. It has been proposed that TA acts as an antioxidant and interferes with the pathogenesis of diabetic polyneuropathy. We suggest that one component of its antioxidant activity requiring study is the direct transition metal-chelating activity of the drug. We found that TA had a profound dose-dependent inhibitory effect upon Cu(2+)-catalysed ascorbic acid oxidation (monitored by O2 uptake and spectrophotometrically at 265 nm) and also increased the partition of Cu2+ into n-octanol from an aqueous solution suggesting that TA forms a lipophilic complex with Cu2+. TA also inhibited Cu(2+)-catalysed liposomal peroxidation. Furthermore, TA inhibited intracellular H2O2 production in erythrocytes challenged with ascorbate, a process thought to be mediated by loosely chelated Cu2+ within the erythrocyte. These data, taken together, suggest that prior intracellular reduction of TA to dihydrolipoic acid is not an obligatory mechanism for an antioxidant effect of the drug, which may also operate via Cu(2+)-chelation. The R-enantiomer and racemic mixture of the drug (alpha-TA) generally seemed more effective than the S-enantiomer in these assays of metal chelation.

Oxidative Glycation and Free Radical Production: A Causal Mechanism of Diabetic Complications

Glucose may oxidise under physiological conditions and lead to the production of protein reactive ketoaldehydes, hydrogen peroxide and highly reactive oxidants. Glucose is thus able to modify proteins by the attachment of its oxidation derived aldehydes, leading to the development of novel protein fluorophores, as well as fragment protein via free radical mechanisms. The fragmentation of protein by glucose is inhibitable by metal chelators such as diethylenetriamine pentaacetic acid (DETAPAC) and free radical scavengers such as benzoic acid, and sorbitol. The enzymic antioxidant, catalase, also inhibits protein fragmentation. Protein glycation and protein oxidation are inextricably linked. Indeed, using boronate affinity chromatography to separate glycated from non-glycated material, we demonstrate that proteins which are glycated exhibit an enhanced tryptophan oxidation. Our observation that both glycation and oxidation occur simultaneously further supports the hypothesis that tissue damage associated with diabetes and ageing has an oxidative origin.

α-lipoic acid decreases oxidative stress even in diabetic patients with poor glycemic control and albuminuria

In the present cross-sectional study, the influence of alpha-lipoic acid on markers of oxidative stress, assessed by measurement of plasma lipid hydroperoxides (ROOHs), and on the balance between oxidative stress and antioxidant defence, determined by the ratio ROOH/(alpha-tocopherol/cholesterol), was examined in 107 patients with diabetes mellitus. Patients receiving alpha-lipoic acid (600 mg/day for > 3 months) had significant lower ROOHs and a lower ROOH/(alpha-tocopherol/cholesterol) ratio than those without alpha-lipoic acid treatment [ROOH: 4.76 +/- 2.49 vs. 7.16 +/- 3.22 mumol/l; p < .0001] and [ROOH/(alpha-tocopherol/cholesterol): 1.37 +/- 0.72 vs. 2.16 +/- 1.17; p < 0.0001]. In addition, the influence of glycemic control and albuminuria on ROOHs and on the ratio of ROOH/(alpha-tocopherol/cholesterol) was examined in the presence and absence of alpha-lipoic acid treatment. Patients were subdivided into three groups based on (1) their HbA1 levels (< 7.5, 7.5-9.5, and > 9.5%) and (2) their urinary albumin concentrations (< 20, 20-200, and > 200 mg/l). Neither poor glycemic control, nor the presence of micro- or macroalbuminuria prevented the antioxidant effect of alpha-lipoic acid. Using stepwise multiple regression analysis, alpha-lipoic acid was found to be the only factor significantly predicting low ROOHs and a low ratio of ROOH/(alpha-tocopherol/cholesterol). These data provide evidence that treatment with alpha-lipoic acid improves significantly the imbalance between increased oxidative stress and depleted antioxidant defence even in patients with poor glycemic control and albuminuria.