Radical-initiated lipid peroxidation in low density lipoproteins: insights obtained from kinetic modeling

We present kinetic models of various complexity for radical-initiated lipid peroxidation in low density lipoproteins (LDL). The models, comprised of simultaneous differential equations programmed in Mathematica, were used to evaluate the concentration profiles of the reactants of interest. Single-phase reaction schemes describing lipid peroxidation and antioxidation according to the "conventional" and tocopherol-mediated peroxidation (TMP) model were simulated for conditions of low and high radical fluxes produced by thermolabile azo initiators. The results show that the particular dependencies of the rates of lipid peroxidation (Rp) on the rates of initiation (Ri) for the two reaction schemes were accurately predicted by the simulations. Both models qualitatively predicted inhibition of lipid peroxidation in the presence of alpha-tocopherol (alpha-TOH) under high radical flux conditions, suggesting that both can describe inhibited lipid peroxidation in solution under these conditions. TMP, but not the conventional model, could also predict the experimentally observed complex behavior of LDL lipid peroxidation induced with different concentrations of azo initiators. Specifically, TMP faithfully reproduced the observed kinetic chain length of lipid peroxidation of > > 1 at low and < < 1 at high concentration of the initiator (i.e., 0.2 and 10 mM, respectively for LDL at 1 mumol apoB-100/L) during the alpha-TOH-containing period of oxidation. It also demonstrated the experimentally observed nondependence of RpTMP on Ri. Kinetic analysis of radical generation and initiation of lipid peroxidation in an extended, two-compartment model of TMP showed that phase separation of bimolecular reactions in a suspension of LDL particles can lead to a approximately 400-fold increase in the rate of lipid hydroperoxide formation. The experimentally observed co-antioxidant action of water-soluble ascorbate and lipid-soluble ubiquinol-10 were verified using this model. A simple biophysical model constituting the reactions of TMP and incorporating the compartmental nature of an LDL suspension is proposed. Together, the results demonstrate that TMP is the only model that fits the experimental data describing the early stages of LDL lipid peroxidation under various oxidizing conditions. The implications of our findings are discussed in relation to atherogenesis and a recently proposed alternative model of LDL lipid peroxidation (Abuja and Esterbauer (1995) Chem. Res. Toxicol. 8, 753).