Oxidative modification and antioxidant protection of human low density lipoprotein at high and low oxygen partial pressures

Abstract

Oxidative modification of low density lipoprotein (LDL) in the subendothelial space of the arterial wall has been implicated as an initial process in atherosclerosis. In vitro studies of LDL oxidation are usually done at ambient oxygen partial pressure (pO2; approximately 160 torr, or 21% O2), which is considerably higher than arterial tissue pO2 (30-70 torr, and as low as 20 torr, or 2.5% O2, in atherosclerotic lesions). In addition, beta-carotene acts as an efficient free radical scavenger only at low pO2. Therefore, we investigated the effects of high (20%) and low (2%) pO2 on the kinetics of LDL oxidation, and the effectiveness of beta-carotene compared to other physiological antioxidants in preventing LDL oxidation. At low pO2, the rate of Cu(2+)-induced oxidative modification of LDL was lower than at high pO2. Furthermore, at high pO2 there was a distinct lag phase preceding the propagation phase of lipid peroxidation in Cu(2+)-exposed LDL, as measured by cholesteryl ester hydroperoxide formation; in contrast, there appeared to be no distinct lipid peroxidation lag phase in LDL incubated with Cu2+ at low pO2. Elevating alpha-tocopherol levels in LDL about 5-fold resulted in significant antioxidant protection: the lipid peroxidation lag phase at high pO2 increased by 45% (from 58 +/- 11 to 84 +/- 3 min, P < 0.05), and the initial rate (0-1 h) of lipid hydroperoxide formation at low pO2 was reduced by 52% (from 11.6 +/- 1.9 to 5.6 +/- 1.0 nmol/mg LDL protein/h, P < 0.01). In contrast, increasing LDL beta-carotene levels about 6-fold did not inhibit LDL oxidation at either pO2. Most remarkably, low concentrations of ascorbic acid (30 microM) drastically reduced LDL oxidation, regardless of pO2: the lipid peroxidation lag phase at high pO2 increased more than 7-fold (from 46 +/- 11 min to > 360 min, P < 0.001), and at low pO2 no lipid hydroperoxides could be detected for at least 6 h of incubation. These results show that at low physiological pO2, Cu(2+)-induced LDL oxidation occurs at a significantly lower rate than at ambient pO2. At both high and low pO2, beta-carotene cannot inhibit LDL oxidation, whereas alpha-tocopherol has a moderate protective effect, and low physiological concentrations of ascorbic acid very strongly suppress LDL oxidation.