Retsky K L, Chen K, Zeind J, Frei B
The Evans Memorial Department of Medicine, Whitaker Cardiovascular Institute, Boston University School of Medicine, MA, USA.
Free Radic Biol Med. 1999 Jan;26(1-2):90-8. doi: 10.1016/s0891-5849(98)00151-8.
Oxidatively modified low-density lipoprotein (LDL) has numerous atherogenic properties, and antioxidants that can prevent LDL oxidation may act as antiatherogens. We have previously shown that vitamin C (L-ascorbic acid, AA) and its two-electron oxidation product dehydro-L-ascorbic acid (DHA) strongly inhibit copper (Cu)-induced LDL oxidation. These findings are unusual, as AA is known to act not only as an antioxidant, but also a pro-oxidant in the presence of transition metal ions in vitro, and DHA has no known reducing capacity. Here we report that human LDL (0.4 mg protein/ml) incubated with 40 microM Cu2+ binds 28.0 +/- 3.3 Cu ions per LDL particle (mean +/- SD, n = 10). Co-incubation of LDL with AA or DHA led to the time- and concentration-dependent release of up to 70% of bound Cu, which was associated with the inhibition of LDL oxidation. Incubation of LDL with Cu and AA or DHA also led to the time-dependent formation of 2-oxo-histidine, an oxidized derivative of histidine with a low affinity for Cu. Addition of free histidine prevented the formation of the LDL-Cu complexes and inhibited LDL oxidation, despite the fact that Cu remained redox-active. Interestingly, histidine was more effective than AA or DHA at limiting Cu binding to LDL, but at low concentrations AA and DHA were more effective than histidine at inhibiting LDL oxidation. These data suggest that there are at least two types of Cu binding sites on LDL: those that bind Cu in a redox-active form critical for initiation of LDL oxidation, and those that bind Cu in a redox-inactive form not contributing to LDL oxidation. The former sites may be primarily histidine residues of apolipoprotein B-100 that are oxidized to 2-oxo-histidine in the presence of Cu and AA or DHA, thus explaining, at least in part, the unusual inhibitory effect of vitamin C on Cu-induced LDL oxidation.
氧化修饰的低密度脂蛋白（LDL）具有多种致动脉粥样硬化特性，而能够防止LDL氧化的抗氧化剂可能具有抗动脉粥样硬化作用。我们之前已经表明，维生素C（L-抗坏血酸，AA）及其双电子氧化产物脱氢-L-抗坏血酸（DHA）能强烈抑制铜（Cu）诱导的LDL氧化。这些发现不同寻常，因为已知AA不仅作为抗氧化剂起作用，而且在体外存在过渡金属离子时还作为促氧化剂，并且DHA没有已知的还原能力。在此我们报告，与40微摩尔Cu2+一起孵育的人LDL（0.4毫克蛋白质/毫升）每个LDL颗粒结合28.0±3.3个Cu离子（平均值±标准差，n = 10）。LDL与AA或DHA共同孵育导致高达70%的结合Cu随时间和浓度依赖性释放，这与LDL氧化的抑制相关。LDL与Cu和AA或DHA一起孵育还导致2-氧代组氨酸随时间依赖性形成，2-氧代组氨酸是组氨酸的一种氧化衍生物，对Cu的亲和力较低。添加游离组氨酸可防止LDL-Cu复合物的形成并抑制LDL氧化，尽管Cu仍具有氧化还原活性。有趣的是，在限制Cu与LDL结合方面，组氨酸比AA或DHA更有效，但在低浓度时，AA和DHA在抑制LDL氧化方面比组氨酸更有效。这些数据表明LDL上至少存在两种类型的Cu结合位点：那些以对LDL氧化起始至关重要的氧化还原活性形式结合Cu的位点，以及那些以对LDL氧化无贡献的氧化还原无活性形式结合Cu的位点。前者位点可能主要是载脂蛋白B-100的组氨酸残基，在Cu和AA或DHA存在下被氧化为2-氧代组氨酸，从而至少部分解释了维生素C对Cu诱导的LDL氧化的异常抑制作用。
