Microsomal cytochromes P450 catalyze the oxidation of low density lipoprotein

Low density lipoprotein (LDL) oxidation is a major contributor to foam cell formation during early atherogenesis. Several oxygenases have been implicated in the process of LDL oxidation in the arterial wall, where the environment is relatively low in antioxidants, but the exact mechanism for LDL oxidation in vivo is not known. In the present study we sought to determine the ability of cytochrome P450 2E1 (P450 2E1) and other P450s, located in the liver and in other tissues, to oxidize LDL. Upon incubation of LDL (0.1 mg of protein/ml) with purified, reconstituted rabbit P450 2E1 in the presence of NADPH and the NADPH-cytochrome P450 reductase, time- and P450 2E1 concentration-dependent LDL oxidation was observed, as analyzed by determining the formation of peroxides, thiobarbituric acid reactive substances (TBARS), and conjugated dienes. Within 1 h of initiating the reaction, almost maximal oxidation was observed. NADPH, and active P450 2E1 enzyme were required for LDL oxidation to occur. The rate of P450 2E1-induced LDL oxidation was also dependent on the lipoprotein concentration. P450 2E1 could also oxidize pure phospholipids and cholesteryl ester, the major lipids in LDL. In the presence of catalase or superoxide dismutase (SOD), LDL oxidation was completely blocked, suggesting that hydrogen peroxide and superoxide are involved in P450 2E1-induced LDL oxidation. The ability of P450 2E1 to oxidize LDL was not unique to this enzyme, and could be observed with some other purified, cytochromes P450 in the reconstituted system such as rat P450 2B1 and human P450 3A4. Finally, microsomal membranes obtained from rats that were induced to express high levels of P450s 2B1, 2E1, and 1A1/2 were able to oxidize LDL, whereas little oxidation was seen with microsomes that were induced to express 3A2. We thus conclude that LDL can be oxidized by some cytochrome P450s and, as some of these enzymes are present in liver and in arterial wall, they may have a physio/pathological relevance to LDL oxidation and atherogenesis.