Photooxidation of A2-PE, a photoreceptor outer segment fluorophore, and protection by lutein and zeaxanthin.
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.
Kim
So Ra
SR
Department of Ophthalmology, Columbia University, NY 10032, USA.
Nakanishi
Koji
K
Itagaki
Yasuhiro
Y
Sparrow
Janet R
JR
D000818:Animals; D002851:Chromatography, High Pressure Liquid; D008027:Light; D014975:Lutein / Q000494:pharmacology* / Q000502:physiology; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D010084:Oxidation-Reduction / Q000187:drug effects; D011726:Pyridinium Compounds / Q000378:metabolism* / Q000528:radiation effects; D012176:Retinoids / Q000378:metabolism* / Q000528:radiation effects; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D024341:Xanthophylls / Q000494:pharmacology* / Q000502:physiology; D065146:Zeaxanthins
C112040:A2-E (N-retinylidene-N-retinylethanolamine); C511385:A2-PE compound; D011726:Pyridinium Compounds; D012176:Retinoids; D024341:Xanthophylls; D065146:Zeaxanthins; D014975:Lutein