The lipophilic
dye merocyanine 540 (MC540) localizes primarily in the plasma membrane (PM) of
tumor cells, where it can sensitize lethal photoperoxidative damage of potential therapeutic importance. We postulated (i) that chain peroxidation triggered by
iron-catalyzed turnover of nascent hydroperoxides (LOOHs) generated by
singlet oxygen ((1)O(2)) attack on PM
lipids contributes significantly to overall cytolethality, and (ii) that
nitric oxide (NO), a known scavenger of organic
free radicals, would suppress this and, thus, act cytoprotectively. In accordance, irradiation of MC540-sensitized L1210 cells produced 5alpha-OOH, a definitive (1)O(2) adduct of PM
cholesterol, which decayed during subsequent dark incubation with appearance of other signature
peroxides, viz.
free-radical-derived 7alpha/beta-OOH. Whereas chemical donor (SPNO or SNAP)-derived NO had little or no effect on post-irradiation 5alpha-OOH disappearance, it dose-dependently inhibited 7alpha/beta-OOH accumulation, consistent with interception of chain-carrying radicals arising from one-electron reduction of primary LOOHs. Using [(14)C]
cholesterol as an L1210 PM probe, we detected additional after-light products of chain peroxidation, including diols (7alpha-OH, 7beta-OH) and 5,6-epoxides, the yields of which were enhanced by
iron supplementation, but strongly suppressed by NO. Correspondingly, photoinitiated cell killing was significantly inhibited by NO introduced either immediately before or after light exposure. These findings indicate that prooxidant LOOH turnover plays an important role in photokilling and that NO, by intercepting propagating radicals, can significantly enhance cellular resistance.