Proudfoot J M, Puddey I B, Beilin L J, Stocker R, Croft K D
University of Western Australia Department of Medicine, Royal Perth Hospital.
Free Radic Biol Med. 1997;23(5):699-705. doi: 10.1016/s0891-5849(97)00021-x.
Oxidative modification of low-density lipoprotein (LDL) may be an important factor in atherogenesis. The susceptibility of LDL to oxidation is usually determined in isolation by exposing LDL to oxidative stress induced by Cu ions or a free radical initiator. In these cases oxidation is carried out in the absence of water-soluble vitamins or serum proteins that may be present at the site of oxidation in vivo. We have examined the Cu2+-induced oxidation of lipoproteins in diluted serum. When oxidizing isolated LDL, there is a decrease in lag time with increasing concentration of Cu2+ until a minimum "lag time" is reached at a Cu:LDL ratio of about 50:1. In serum, we have shown an initial decrease in "lag time" with increasing Cu concentration up to 12.5 microM. However, with higher Cu concentrations "lag time" to oxidation increases, contrary to expectation, until a maximum is reached at about 50 microM Cu. This dose response observed for Cu oxidation of diluted serum was highly reproducible in a number of individual subjects. When serum was gel-filtered to remove low molecular weight compounds, the resulting filtrate behaved the same as isolated LDL. Uric acid was found to be an important component of the low molecular weight fraction responsible for the paradoxical effect of Cu concentration on serum oxidation. The same paradoxical effect was found when isolated LDL was incubated with uric acid in the presence of human serum albumin (HSA) and Cu. The incubation of HSA with reducing agents such as uric acid or bilirubin in the presence of high Cu concentrations, produces a "peroxidase-like" activity, capable of breaking down hydrogen peroxide as well as lipid hydroperoxides. The decomposition of lipid peroxides is a likely explanation for the longer serum oxidation lag times seen at higher Cu concentrations. Our study highlights the possible importance of interactions between uric acid and serum proteins in the presence of high metal ion concentrations.
低密度脂蛋白（LDL）的氧化修饰可能是动脉粥样硬化形成的一个重要因素。LDL对氧化的敏感性通常是通过将LDL暴露于铜离子或自由基引发剂诱导的氧化应激中来单独测定的。在这些情况下，氧化是在不存在体内氧化部位可能存在的水溶性维生素或血清蛋白的情况下进行的。我们研究了稀释血清中铜离子诱导的脂蛋白氧化。当氧化分离的LDL时，随着铜离子浓度的增加，延迟时间会缩短，直到铜与LDL的比例约为50:1时达到最小“延迟时间”。在血清中，我们发现随着铜浓度增加至12.5微摩尔，“延迟时间”最初会缩短。然而，与预期相反，随着铜浓度升高，氧化的“延迟时间”会增加，直到在约50微摩尔铜时达到最大值。在许多个体受试者中，这种对稀释血清铜氧化观察到的剂量反应具有高度可重复性。当血清进行凝胶过滤以去除低分子量化合物时，所得滤液的行为与分离的LDL相同。发现尿酸是低分子量部分的重要成分，其导致了铜浓度对血清氧化的矛盾效应。当在人血清白蛋白（HSA）和铜存在的情况下将分离的LDL与尿酸一起孵育时，也发现了相同的矛盾效应。在高铜浓度存在下，HSA与尿酸或胆红素等还原剂孵育会产生一种“过氧化物酶样”活性，能够分解过氧化氢以及脂质氢过氧化物。脂质过氧化物的分解可能是在较高铜浓度下观察到血清氧化延迟时间更长的原因。我们的研究强调了在高金属离子浓度存在下尿酸与血清蛋白之间相互作用的潜在重要性。
