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

Abundant evidence has been provided to substantiate the elevated cardiovasc ular risk associated with small, dense, low density lipoprotein (LDL) parti cles. The diminished resistance of dense LDL to oxidative stress in both no rmolipidemic and dyslipidemic subjects is established; nonetheless, the mol ecular basis of this phenomenon remains indeterminate. We have defined the primary molecular targets of lipid hydroperoxide formation in light, interm ediate, and dense subclasses of LDL after copper-mediated oxidation and hav e compared the relative stabilities of the hydroperoxide derivatives of pho spholipids and cholesteryl esters (CEs) as a function of the time course of oxidation. LDL subclasses (LDL1 through LDL5) were isolated from normolipi demic plasma by isopycnic density gradient ultracentrifugation, and their c ontent of polyunsaturated molecular species of phosphatidylcholine (PC) and CE and of lipophilic antioxidants was quantified by reverse-phase high-per formance liquid chromatography. The molar ratio of the particle content of polyunsaturated CE and PC species containing linoleate or arachidonate rela tive to alpha-tocopherol or beta-carotene did not differ significantly betw een LDL subspecies. Nonetheless, dense LDL contained significantly less pol yunsaturated CE species (400 mol per particle) compared with LDL1 through L DL4 (range, approximate to 680 to 490 mol per particle). Although the forma tion of PC-derived hydroperoxides did not vary significantly between LDL su bspecies 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 hydroperoxid es formed relative to each 100 mol of substrate consumed) at half-time, the oxidative lability of CE hydroperoxides in dense LDL was significantly ele vated (lability ratio <25:100) relative to that in lighter, larger LDL part icle subclasses (lability ratio >40:100) throughout the oxidative time cour se. We conclude that the elevated lability of CE hydroperoxides in dense LD L underlies the diminished oxidative resistance of these particles. Moreove r, this phenomenon appears to result not only from the significantly elevat ed 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 feat ures, including a distinct apoB100 conformation, which may facilitate coval ent bond formation between oxidized CE and apoB100.