Thiobarbituric acid‐reactive malondialdehyde formation during superoxide‐dependent, iron‐catalyzed lipid peroxidation: Influence of peroxidation conditions

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Abstract
A systematic study of the influence of biological lipid peroxidation conditions on lipid hydroperoxide decomposition to thiobarbituric acid-reactive malondialdehyde is presented. A superoxide-dependent, iron-catalyzed peroxidation system was employed with xanthine oxidase plus hypoxanthine plus ferric iron-adenosine diphosphate complex as free radical generator. Purified cardiac membrane phospholipid (as liposomes) was the peroxidative target, and 15-hydroperoxy-eicosatetraenoic acid was used as a standard lipid hydroperoxide. Exposure of myocardial phospholipid to free radical generator at physiological pH (7.4) and temperature (37°C) was found to support not only phospholipid peroxidation, but also rapid lipid hydroperoxide breakdown and consequent malondialdehyde formation during peroxidation. Under lipid peroxidation conditions, oxidative injury to the phospholipid polyunsaturated fatty acids required superoxide radical and ferric iron-adenosine diphosphate complex, whereas 37°C temperature and trace iron were sufficient for lipid hydroperoxide decomposition to malondialdehyde. Harsh thiobarbituric acid-test conditions following peroxidation were not mandatory for either lipid hydroperoxide breakdown or thiobarbituric acid-reactive malondialdehyde formation. However, hydroperoxide decomposition that had begun in the peroxidation reaction could be completed during a subsequent thiobarbituric acid test in which no lipid autoxidation took place. Iron was more critical than heat in promoting the observed hydroperoxide decomposition to malondialdehyde during the lipid peroxidation reaction at 37°C and pH 7.4. These data demonstrate that the radical generator, at physiological pH and temperature, serves a dual role as both initiator of membrane phospholipid peroxidation and promotor of lipid peroxide breakdown and thiobarbituric acid-reactive malondialdehyde formation. Consequently, peroxidation reaction conditions can directly influence lipid hydroperoxide decomposition, malondialdehyde production and system thiobarbituric acid-reactivity. In vivo, decomposition of lipid peroxides to malondialdehyde during radical-mediated, metal-catalyzed membrane peroxidation may represent an integral component of oxidative tissue injury rather than a mere consequence of hydrolyzing the peroxidized biological sample in a thiobarbituric acid test.

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