THE EFFECTS OF VITAMIN-C AND URATE ON THE OXIDATION-KINETICS OF HUMANLOW-DENSITY-LIPOPROTEIN

Oxidative modification of human low-density lipoprotein (LDL) is belie ved to play an important role in atherogenesis. Vitamin C (ascorbate) and urate are major water-soluble plasma antioxidants in humans. Urate levels (300-395 mu M) in human serum are considerably higher than asc orbate levels (30-50 mu M). In this study, we compared the ability of urate to protect human LDL from in vitro oxidation with that of ascorb ate. LDL was subjected to in vitro oxidation at 30 degrees C with an O -2 saturated solution (0.15 M NaCl/0.25 mM EDTA) and 15 mM of a therma lly labile water soluble azo-initiator (ABAP or azobis-2-amidinopropan e HCI). In parallel experiments, 50 mu M ascorbate or 50 mu M urate we re present in the oxidation buffer. The consumption of alpha-tocophero l, gamma-tocopherol, urate or ascorbate and the formation of lipid hyd roperoxides were measured as a function of time in the in vitro oxidat ion system. The rate of lipid hydroperoxide formation was found to be significantly increased after the LDL tocopherols (alpha-plus gamma-to copherol) were totally consumed, i.e., after the lag phase. Urate (50 mu M) was more effective than ascorbate (50 mu M) in extending the lag phase. Moreover, urate was consumed more slowly than ascorbate under identical oxidation conditions. Urate was not, however, as effective a s 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 alpha- and gamma-tocopherol in the presence or absence of urate or ascorbate.