Upston J M, Neuzil J, Stocker R
Biochemistry Unit, Heart Research Institute, Camperdown, NSW, Australia.
J Lipid Res. 1996 Dec;37(12):2650-61.
Various lipoxygenases (LO) oxidize low density lipoprotein (LDL) in vitro and 15-LO has been implicated in the development of atherosclerosis in vivo. Direct oxidation of phospholipids (PL) and cholesteryl esters (CE) by LO has been proposed as a mechanism whereby these enzymes cause or contribute to LDL lipid peroxidation. Herein we show that the extent to which recombinant human 15-LO (rhLO) caused peroxidation of LDL's esterified core and surface lipids depended on, and directly related to, the alpha-tocopherol (alpha-TOH) content of the lipoprotein. Thus, CE and PL of in vivo alpha-TOH-depleted LDL, isolated from a patient with familial isolated vitamin E deficiency, were resistant to oxidation by rhLO, whereas those in alpha-TOH-containing LDL from the same patient receiving vitamin E supplements readily oxidized. The extent to which rhLO caused oxidation of CE and PL directly and linearly correlated with LDL's content of vitamin E, as demonstrated by studies with in vitro alpha-TOH-depleted lipoproteins. The geometric isomers of oxidized cholesteryl linoleate formed in LDL during oxidation initiated by rhLO, matched those obtained during non-enzymic, peroxyl radical-initiated oxidation of LDL whilst alpha-TOH was present. Ascorbate, an efficient co-antioxidant for alpha-TOH, completely prevented rhLO-initiated oxidation of LDL's CE, but did not inhibit rhLO-mediated oxidation of unesterified linoleate. These results are inconsistent with direct oxidation of LDL esterified lipids by rhLO. Isolated LDL contained free fatty acids (FFA), and its exposure to rhLO caused a rapid formation of linoleate hydroperoxide. To reconcile these data, we propose that during rhLO-initiated oxidation of LDL, enzymic oxidation of FFA preceeds the oxidation of CE and PL, which occurs largely via a tocopherol-dependent process.
多种脂氧合酶（LO）可在体外氧化低密度脂蛋白（LDL），且15-LO在体内动脉粥样硬化的发展过程中发挥作用。有人提出，LO对磷脂（PL）和胆固醇酯（CE）的直接氧化是这些酶导致或促成LDL脂质过氧化的一种机制。在此我们表明，重组人15-LO（rhLO）引起LDL酯化核心和表面脂质过氧化的程度取决于脂蛋白的α-生育酚（α-TOH）含量，并与之直接相关。因此，从患有家族性单纯维生素E缺乏症的患者体内分离出的体内α-TOH耗竭的LDL的CE和PL对rhLO氧化具有抗性，而同一患者在补充维生素E后其含α-TOH的LDL中的CE和PL则很容易被氧化。rhLO引起CE和PL氧化的程度与LDL中的维生素E含量直接呈线性相关，体外α-TOH耗竭的脂蛋白研究证明了这一点。在rhLO引发的氧化过程中，LDL中形成的氧化亚油酸胆固醇酯的几何异构体与在α-TOH存在时LDL的非酶促过氧自由基引发的氧化过程中获得的异构体相匹配。抗坏血酸是α-TOH的有效协同抗氧化剂，它完全阻止了rhLO引发的LDL的CE氧化，但不抑制rhLO介导的未酯化亚油酸的氧化。这些结果与rhLO对LDL酯化脂质的直接氧化不一致。分离出的LDL含有游离脂肪酸（FFA），其暴露于rhLO会导致亚油酸氢过氧化物的快速形成。为了协调这些数据，我们提出，在rhLO引发的LDL氧化过程中，FFA的酶促氧化先于CE和PL的氧化，而CE和PL的氧化主要通过依赖生育酚的过程发生。
