ascorbic-acid and merocyanine-dye

The antioxidant action of carotenoids is believed to involve quenching of singlet oxygen and scavenging of reactive oxygen radicals. However, the exact mechanism by which carotenoids protect cells against oxidative damage, particularly in the presence of other antioxidants, remains to be elucidated. This study was carried out to examine the ability of exogenous zeaxanthin alone and in combination with vitamin E or C, to protect cultured human retinal pigment epithelium cells against oxidative stress. The survival of ARPE-19 cells, subjected to merocyanine 540-mediated photodynamic action, was determined by the MTT test and the content of lipid hydroperoxides in photosensitized cells was analyzed by HPLC with electrochemical detection. We found that zeaxanthin-supplemented cells, in the presence of either alpha-tocopherol or ascorbic acid, were significantly more resistant to photoinduced oxidative stress. Cells with added antioxidants exhibited increased viability and accumulated less lipid hydroperoxides than cells without the antioxidant supplementation. Such a synergistic action of zeaxanthin and vitamin E or C indicates the importance of the antioxidant interaction in efficient protection of cell membranes against oxidative damage induced by photosensitized reactions.

Topics: alpha-Tocopherol; Antioxidants; Ascorbic Acid; beta Carotene; Biological Assay; Cell Line; Cell Survival; Humans; Iron; Lipid Peroxidation; Oxidative Stress; Photochemistry; Photosensitizing Agents; Pigment Epithelium of Eye; Pyrimidinones; Singlet Oxygen; Tetrazolium Salts; Thiazoles; Xanthophylls; Zeaxanthins

The lipophilic dye merocyanine 540 (MC540) can photosensitize potentially lethal cell membrane damage as well as its own degradation (bleaching). Photobleaching in a test membrane, the human erythrocyte ghost has been examined. White light irradiation of MC540-sensitized ghosts resulted in lipid hydroperoxide (LOOH) formation, low-level thiobarbituric acid (TBA) reactivity, and dye bleaching (A568 decay). When the reaction was carried out in the presence of ascorbate (AH-), and added Fe3+, there was a large enhancement of TBA reactivity (indicative of free radical-mediated lipid peroxidation) and concomitant increase in the rate of photobleaching. Rapid bleaching also occurred when MC540 was incubated in the dark with ghosts that had been photoperoxidized with another dye (a phthalocyanine) and then exposed to AH-. The extent of bleaching in this system was found to be proportional to the starting level of LOOH. Like the wave of free radical lipid peroxidation that accompanied it, dye bleaching in AH(-)-treated, preperoxidized ghosts was stimulated by supplemental Fe3+, inhibited by desferrioxamine or butylated hydroxytoluene (BHT), but unaffected by catalase or superoxide dismutase. From this and related evidence, we deduce that: (1) in the absence of Fe3+/AH-, photoperoxidation and photobleaching occur independently and are nonradical, singlet oxygen-mediated processes; and (2) in the presence of Fe3+/AH-, 1-electron reduction of photogenerated LOOHs results in a surge of lipid peroxidation that amplifies dye loss via free radical processes. MC540 bleaching might be exploited as a relatively simple and sensitive indicator of lipid autoxidation in isolated membranes and cells.

Topics: Antioxidants; Ascorbic Acid; Erythrocyte Membrane; Ferric Compounds; Free Radicals; Humans; Lipid Peroxidation; Photochemistry; Photosensitizing Agents; Pyrimidinones; Thiobarbituric Acid Reactive Substances

The lipophilic photosensitizing dye merocyanine 540 (MC540) is being studied intensively as an antitumor and antiviral agent. Since plasma membranes are believed to be the principal cellular targets of MC540-mediated photodamage, we have studied membrane damage in a well characterized test system, the human erythrocyte ghost. When irradiated with white light, MC540-sensitized ghosts accumulated lipid hydroperoxides (LOOHs derived from phospholipids and cholesterol) at a rate dependent on initial dye concentration. Neither desferrioxamine nor butylated hydroxytoluene inhibited LOOH formation, suggesting that Type I (iron-mediated free radical) chemistry is not important. By contrast, azide inhibited the reaction in a dose-dependent fashion, implicating a Type II (singlet oxygen, 1O2) mechanism. Stern-Volmer analysis of the data gave a 1O2 quenching constant approximately 50 times lower than that determined for an extramembranous target, lactate dehydrogenase (the latter value agreeing with literature values). This suggests that 1O2 reacts primarily at its membrane sites of origin and that azide has limited access to these sites. Using [14C]cholesterol-labeled membranes and HPLC with radiodetection, we identified 3 beta-hydroxy-5 alpha-cholest-6-ene-5-hydroperoxide as the major cholesterol photoproduct, thereby confirming 1O2 intermediacy. Irradiation of MC540-sensitized membranes in the presence of added iron and ascorbate resulted in a large burst of lipid peroxidation, as shown by thiobarbituric acid reactivity and appearance of 7-hydroperoxycholesterol and 7-hydroxycholesterol as major oxidation products. Amplification of MC540-initiated lipid peroxidation by iron/ascorbate (attributed to light-independent reduction of nascent photoperoxides, with ensuing free radical chain reactions) could prove useful in augmenting MC540's phototherapeutic effects.

Topics: Ascorbic Acid; Cholesterol; Erythrocyte Membrane; Free Radicals; Humans; In Vitro Techniques; L-Lactate Dehydrogenase; Lipid Peroxidation; Photochemistry; Pyrimidinones; Radiation-Sensitizing Agents