Surprising inability of singlet oxygen-generated 6-hydroperoxycholesterol to induce damaging free radical lipid peroxidation in cell membranes

摘要

Singlet oxygen attack on cholesterol (Ch), a prominent monounsaturated lipid of mammalian cell plasma membranes, gives rise to three hydroperoxide (ChOOH) isomers, 5α-OOH, 6α-OOH and 6β-OOH, the latter two in lower yield than 5α-OOH, and 6α-OOH in lowest yield. A third possible positional isomer, 7α-OOH and 7β-OOH, is produced by free radical attack. In the presence of iron and ascorbate (Fe/AH), 5α-OOH or 6β-OOH in phosphatidylcholine/Ch/ChOOH (20:15:1 by mol) liposomes was reduced to its corresponding alcohol, the rate constant being approximately the same for both ChOOHs. Using [~(14)C]Ch as an in situ probe, we found that liposomal 5α-OOH readily set off free radical-mediated (chain) peroxidation reactions when exposed to Fe/AH, whereas 6β-OOH under the same conditions did not. Moreover, liposomal 5α-OOH triggered robust chain peroxidation in [~(14)C]Ch-labeled L1210 cells, leading to cell death, whereas 6β-OOH was essentially inert in this regard. Thus, 5α-OOH and 6β-OOH undergo iron-catalyzed reductive turnover, but only the former can provoke toxic membrane damage. These novel findings have important implications for UVA-induced photodamage in Ch-rich tissues like skin and eye, where ~1O_2 often plays a major role.