Cholesteryl ester hydroperoxide lability is a key feature of the oxidative susceptibility of small, dense LDL

Abundant evidence has been provided to substantiate the elevated cardiovascular risk associated with small, dense, low density lipoprotein (LDL) particles. The diminished resistance of dense LDL to oxidative stress in both normolipidemic and dyslipidemic subjects is established; nonetheless, the molecular basis of this phenomenon remains indeterminate. We have defined the primary molecular targets of lipid hydroperoxide formation in light, intermediate, and dense subclasses of LDL after copper-mediated oxidation and have compared the relative stabilities of the hydroperoxide derivatives of phospholipids and cholesteryl esters (CEs) as a function of the time course of oxidation. LDL subclasses (LDL1 through LDL5) were isolated from normolipidemic plasma by isopycnic density gradient ultracentrifugation, and their content of polyunsaturated molecular species of phosphatidylcholine (PC) and CE and of lipophilic antioxidants was quantified by reverse-phase high-performance liquid chromatography. The molar ratio of the particle content of polyunsaturated CE and PC species containing linoleate or arachidonate relative to alpha-tocopherol or beta-carotene did not differ significantly between LDL subspecies. Nonetheless, dense LDL contained significantly less polyunsaturated CE species (400 mol per particle) compared with LDL1 through LDL4 (range, approximate to 680 to 490 mol per particle). Although the formation of PC-derived hydroperoxides did not vary significantly between LDL subspecies as a function of the time course of copper-mediated oxidation, the abundance of the C18:2 and C20:4 CE hydroperoxides was uniquely deficient in dense LDL (23 and 0.6 mol per particle, respectively, in LDL5; 47 to 58 and 1.9 to 2.3 mol per particle, respectively, in other LDL subclasses) at propagation half-time. When expressed as a lability ratio (mol hydroperoxides formed relative to each 100 mol of substrate consumed) at half-time, the oxidative lability of CE hydroperoxides in dense LDL was significantly elevated (lability ratio <25:100) relative to that in lighter, larger LDL particle subclasses (lability ratio >40:100) throughout the oxidative time course. We conclude that the elevated lability of CE hydroperoxides in dense LDL underlies the diminished oxidative resistance of these particles. Moreover, this phenomenon appears to result not only from the significantly elevated PC to free cholesterol ratio (1.54:1) in dense LDL particles (1.15:1 to 1.25:1 for other LDL subclasses) but also from their unique structural features, including a distinct apoB100 conformation, which may facilitate covalent bond formation between oxidized CE and apoB100.