Oxidative Susceptibility of Unfractionated Serum or Plasma: Response to Antioxidants in Vitro and to Antioxidant Supplementation

The susceptibility of plasma lipids to oxidation is thought to be a factor contributing to atherogenic risk. Various groups have studied the in vitro oxidizability of isolated LDL and examined the effects of conventional antioxidants. The drawbacks associated with the isolation of LDL for evaluation of in vitro oxidizability, however, have limited the application of this measurement in large-scale studies. We developed and evaluated an assay that can be used to directly assess the oxidative susceptibility of unfractionated serum or plasma lipids, obviating the need for isolation of lipoprotein fractions. Oxidative conditions were initiated in vitro with cuprous chloride and 2,2'-azobis(2-amidinopropane) hydrochloride. The effects of antioxidants added in vitro, and as an oral supplement, were monitored by conjugated diene formation. The addition of ascorbic acid (0-50 micromol/L) in vitro elicited a dose-dependent protective effect, increasing the lag time to oxidation (P < 0.001). In contrast, alpha-tocopherol demonstrated prooxidant behavior at increasing concentrations (0-50 micromol/L), although we observed a decrease in the maximum rate of oxidation. Our findings are supported by the results from plasma samples of participants in a randomized antioxidant (vitamins C and E) intervention study after acute ischemic stroke. The group receiving vitamins C and E for 14 days showed an increased lag time to plasma lipid oxidation in vitro compared with the nonsupplemented group (P < 0.05). The susceptibility of unfractionated plasma or serum lipids to oxidation in vitro offers an alternative to LDL for evaluating the efficacy of antioxidant regimens.