The Effects of Vitamin C and Urate on the Oxidation Kinetics of Human Low-Density Lipoprotein 1

Abstract

Oxidative modification of human low-density lipoprotein (LDL) is believed to play an important role in atherogenesis. Vitamin C (ascorbate) and urate are major water-soluble plasma antioxidants in humans. Urate levels (300-395 μM) in human serum are considerably higher than ascorbate levels (30–50 μM). In this study, we compared the ability of urate to protect human LDL from in vitro oxidation with that of ascorbate. LDL was subjected to in vitro oxidation at 30°C with an O₂ saturated solution (0.15 M NaCl/0.25 mM EDTA) and 15 mM of a thermally labile water soluble azoinitiator (ABAP or azobis-2-amidinopropane HCl). In parallel experiments, 50 μM ascorbate or 50 μM urate were present in the oxidation buffer. The consumption of α-tocopherol, γ-tocopherol, urate or ascorbate and the formation of lipid hydroperoxides were measured as a function of time in the in vitro oxidation system. The rate of lipid hydroperoxide formation was found to be significantly increased after the LDL tocopherols (α-plus γ-tocopherol) were totally consumed, i.e., after the lag phase. Urate (50 μM) was more effective than ascorbate (50 μM) in extending the lag phase. Moreover, urate was consumed more slowly than ascorbate under identical oxidation conditions. Urate was not, however, as effective as ascorbate in preventing the formation of lipid hydroperoxides before the lag phase. An empirical mathematical model was also developed to describe the oxidation kinetics of LDL α-and γ-tocopherol in the presence or absence of urate or ascorbate.