Kim So Ra, Nakanishi Koji, Itagaki Yasuhiro, Sparrow Janet R
Department of Ophthalmology, Columbia University, NY 10032, USA.
Exp Eye Res. 2006 May;82(5):828-39. doi: 10.1016/j.exer.2005.10.004. Epub 2005 Dec 20.
A2-PE is a pigment that forms as a byproduct of the visual cycle, its synthesis from all-trans-retinal and phosphatidylethanolamine occurring in photoreceptor outer segments. A2-PE is deposited in retinal pigment epithelial (RPE) cells secondary to phagocytosis of shed outer segment membrane and it undergoes hydrolysis to generate the RPE lipofuscin fluorophores, A2E, iso-A2E and other minor cis-isomers of A2E. We have demonstrated that A2-PE can initiate photochemical processes that involve the oxidation of A2-PE and that, by analogy with A2E are likely to include the formation of reactive moieties. We also show that potential sources of protection against the photooxidation of A2-PE are the lipid-soluble carotenoids zeaxanthin and lutein (xanthophylls), that constitute the yellow pigment of the macula. Irradiation of A2-PE in the presence of lutein or zeaxanthin suppressed A2-PE photooxidation and in experiments in which we compared the antioxidant capability of zeaxanthin and lutein to alpha-tocopherol, the carotenoids were more potent. Additionally, the effect with zeaxanthin was consistently more robust than with lutein and when alpha-tocopherol was combined with either carotenoid, the outcome was additive. Lutein, zeaxanthin and alpha-tocopherol were all efficient quenchers of singlet oxygen. We have also shown that lutein and zeaxanthin can protect against A2-PE/A2E photooxidation without appreciable consumption of the carotenoid by chemical reaction. This observation contrasts with the pronounced susceptibility of A2E and A2-PE to photooxidation and is of interest since lutein, zeaxanthin, A2E and A2-PE all have conjugated systems of carbon-carbon double bonds terminating in cyclohexenyl end-groups. The structural features responsible for the differences in quenching mechanisms are discussed. It has long been suspected that macular pigment protects the retina both by filtering high-energy blue light and by serving an antioxidant function. Evidence presented here suggests that the photochemical reactions against which lutein and zeaxanthin protect, may include those initiated by the A2-PE. Quantitative HPLC analysis revealed that in eyecups of C57BL/6J and BALB/cByJ mice, levels of A2-PE were several fold greater than the cleavage product, A2E. Taken together, these results may have implications with respect to the involvement of A2-PE formation in mechanisms underlying blue light-induced photoreceptor cell damage and may be significant to retinal degenerative disorders, such as those associated with ABCA4 mutations, wherein there is a propensity for increased A2-PE synthesis.
A2-PE是一种作为视觉循环副产物形成的色素，它由全反式视黄醛和磷脂酰乙醇胺在光感受器外段合成。A2-PE在吞噬脱落的外段膜后沉积于视网膜色素上皮（RPE）细胞中，并经历水解以产生RPE脂褐素荧光团、A2E、异A2E以及A2E的其他少量顺式异构体。我们已经证明A2-PE可以引发涉及A2-PE氧化的光化学过程，并且类似于A2E，可能包括活性部分的形成。我们还表明，针对A2-PE光氧化的潜在保护来源是脂溶性类胡萝卜素玉米黄质和叶黄素（叶黄素类），它们构成了黄斑的黄色色素。在叶黄素或玉米黄质存在下照射A2-PE可抑制A2-PE光氧化，并且在我们比较玉米黄质和叶黄素与α-生育酚抗氧化能力的实验中，类胡萝卜素更有效。此外，玉米黄质的效果始终比叶黄素更显著，并且当α-生育酚与任何一种类胡萝卜素联合使用时，效果是相加的。叶黄素、玉米黄质和α-生育酚都是单线态氧的有效猝灭剂。我们还表明，叶黄素和玉米黄质可以防止A2-PE/A2E光氧化，而类胡萝卜素不会因化学反应而有明显消耗。这一观察结果与A2E和A2-PE对光氧化的明显敏感性形成对比，并且很有趣，因为叶黄素、玉米黄质、A2E和A2-PE都具有以环己烯基端基结尾的碳-碳双键共轭体系。讨论了导致猝灭机制差异的结构特征。长期以来一直怀疑黄斑色素通过过滤高能蓝光和发挥抗氧化功能来保护视网膜。此处提供的证据表明，叶黄素和玉米黄质所保护的光化学反应可能包括由A2-PE引发的反应。定量HPLC分析显示，在C57BL/6J和BALB/cByJ小鼠的眼杯中，A2-PE的水平比裂解产物A2E高几倍。综上所述，这些结果可能与A2-PE形成参与蓝光诱导的光感受器细胞损伤机制有关，并且可能对视网膜退行性疾病具有重要意义，例如那些与ABCA4突变相关的疾病，其中A2-PE合成有增加的倾向。
