PROOXIDANT and ANTIOXIDANT EFFECTS OF ASCORBATE ON PHOTOSENSITIZED PEROXIDATION OF LIPIDS IN ERYTHROCYTE MEMBRANES

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Abstract
Abstract— Continuous blue light irradiation of resealed erythrocyte ghosts at 37°C in the presence of uroporphyrin or protoporphyrin results in 1O2‐mediated (azide inhibitable) lipid peroxidation and membrane lysis. Lipid peroxidation was assessed by thiobarbituric acid reactivity and by quantitation of total hydroperoxides, while lysis was measured in terms of trappedglucose–6‐P release. Low concentrations of ascorbate, AH‐ (e.g. 0.5 mM). present at the start of irradiation, significantly enhanced the rates of lysis and peroxidation, whereas relatively high concentrations of AH‐ (e.g. 15 mM) inhibited both processes. By way of contrast. AH‐ produced only a dose‐dependent inhibition of the photoinactivation of lysozyme, added as an extramembranous target. No significant AH‐induced lipid peroxidation was observed in dark or light controls, plus porphyrin or minus porphyrin, respectively. Stimulation of peroxidation and lysis by low levels of AH‐ was enhanced by added Fe(III), abolished by EDTA. but unaffected by catalase or superoxide dismutase. A plausible explanation for these results is as follows. At low concentrations of AH‐ prooxidant activity is favored. Redox metal‐mediated breakdown of photoperoxides occurs, which tends to amplify lipid peroxidation. Neither O2‐ nor H2O2 appears to be involved. At significantly high concentrations, AH‐ acts predominantly as an antioxidant by intercepting 1O2 and/or sensitizer triplet, or by scavenging free radical intermediates of lipid peroxidation.