Whole plasma oxidation assay as a measure of lipoprotein oxidizability

Lipoprotein oxidation induced in vitro in whole plasma is expected to be a more relevant model of the lipoprotein oxidation in the arterial wall than the in vitro oxidation of single isolated lipoproteins, e.g., low density lipoprotein (LDL). However, it is unclear, whether the oxidizability of whole plasma may serve as an adequate measure of the oxidizability of plasma lipoproteins. We measured the oxidizability of whole plasma diluted 150-fold as an absorbance increase at 234 nm known to reflect the level of conjugated dienes in the samples. Plasma oxidation was induced by Cu(II), 2,2'-azobis-(2-amidinopropane) hydrochloride (AAPH), lipoxygenase or myeloperoxidase+H2O2. Oxidizability of human plasma measured in the presence of Cu(II) was found to correlate with the oxidizability of LDL measured in the common Cu(II)-based LDL oxidation assay. The plasma oxidizability also correlated positively with plasma oxidizable fatty acid and negatively with plasma antioxidant content. Supplementation of human plasma with different antioxidants (albumin, urate, ascorbate, bilirubin, alpha-tocopherol and ubiquinol-10) in vitro decreased its oxidizability. Supplementation of Watanabe heritable hyperlipidaemic rabbits with different antioxidants (vitamin E, ubiquinone-10, probucol, carvedilol) in vivo lowered the oxidizability of rabbit plasma in comparison with rabbits fed standard diet. When plasma from hyperlipidaemic patients with or without coronary heart disease and from age-matched healthy controls was studied, the plasma oxidizability was found to be highest in the patients with coronary heart disease and lowest in the controls. Taken together, these data indicate that the plasma oxidation assay (i) provides information similar to that obtained using the common LDL oxidation assay, (ii) upgrades the latter, taking into account the effect of hydrophilic antioxidants on lipoprotein oxidation and characterizing the oxidizability of all plasma lipoproteins, and (iii) offers important practical advantages, such as fast and simple sample processing, low amount of plasma required and avoidance of artefactual oxidation during lipoprotein isolation. We propose the measurement of plasma oxidizability at 234 nm as an adequate practical index of the oxidizability of plasma lipoproteins.