Department of Pathology, Case Western Reserve University, Cleveland, Ohio 44106 , USA.
J Biol Chem. 2009 Dec 11;284(50):34618-27. doi: 10.1074/jbc.M109.032094. Epub 2009 Oct 23.
Oxidative mechanisms during nuclear sclerosis of the lens are poorly understood, in particular metal-catalyzed oxidation. The lysyl oxidation product adipic semialdehyde (allysine, ALL) and its oxidized end-product 2-aminoadipic acid (2-AAA) were determined as a function of age and presence of diabetes. Surprisingly, whereas both ALL and 2-AAA increased with age and strongly correlated with cataract grade and protein absorbance at 350 nm, only ALL formation but not 2-AAA was increased by diabetes. To clarify the mechanism of oxidation, rabbit lenses were treated with hyperbaric oxygen (HBO) for 48 h, and proteins were analyzed by gas and liquid chromatography mass spectrometry for ALL, 2-AAA, and multiple glycation products. Upon exposure to HBO, rabbit lenses were swollen, and nuclei were yellow. Protein-bound ALL increased 8-fold in the nuclear protein fractions versus controls. A dramatic increase in methyl-glyoxal hydroimidazolone and carboxyethyl-lysine but no increase of 2-AAA occurred, suggesting more drastic conditions are needed to oxidize ALL into 2-AAA. Indeed the latter formed only upon depletion of glutathione and was catalyzed by H(2)O(2). Neither carboxymethyl-lysine nor glyoxal hydroimidazolone, two markers of glyco-/lipoxidation, nor markers of lenticular glycemia (fructose-lysine, glucospane) were elevated by HBO, excluding significant lipid peroxidation and glucose involvement. The findings strongly implicate dicarbonyl/metal catalyzed oxidation of lysine to allysine, whereby low GSH combined with ascorbate-derived H(2)O(2) likely contributes toward 2-AAA formation, since virtually no 2-AAA formed in the presence of methylglyoxal instead of ascorbate. An important translational conclusion is that chelating agents might help delay nuclear sclerosis.
晶状体核硬化过程中的氧化机制尚不清楚，尤其是金属催化氧化。赖氨酸氧化产物己二醛半缩醛（allysine，ALL）及其氧化终产物 2-氨基己二酸（2-AAA）的含量随年龄和糖尿病的存在而变化。令人惊讶的是，尽管 ALL 和 2-AAA 都随年龄增加，与白内障程度和 350nm 处蛋白质吸光度强烈相关，但只有糖尿病会增加 ALL 的形成，而不会增加 2-AAA。为了阐明氧化机制，用高压氧（HBO）处理兔晶状体 48 小时，并用气相和液相色谱质谱法分析 ALL、2-AAA 和多种糖基化产物。暴露于 HBO 后，兔晶状体肿胀，核呈黄色。与对照组相比，核蛋白部分的蛋白结合 ALL 增加了 8 倍。甲基乙二醛氢咪唑酮和羧乙基赖氨酸显著增加，但 2-AAA 没有增加，这表明需要更剧烈的条件才能将 ALL 氧化成 2-AAA。事实上，只有在谷胱甘肽耗竭的情况下，后者才会形成，并由 H2O2 催化。HBO 既没有引起羧甲基赖氨酸或乙二醛氢咪唑酮（糖基/脂质氧化的两个标志物），也没有引起晶状体葡萄糖（果糖-赖氨酸、葡糖胺）增加，这排除了脂质过氧化和葡萄糖的显著参与。这些发现强烈提示赖氨酸通过二羰基/金属催化氧化生成 allysine，其中低 GSH 与抗坏血酸衍生的 H2O2 结合可能有助于 2-AAA 的形成，因为实际上在存在甲基乙二醛而不是抗坏血酸的情况下几乎没有形成 2-AAA。一个重要的转化结论是，螯合剂可能有助于延缓核硬化。
