Photodynamic action of merocyanine 540 on leukemia cells: iron-stimulated lipid peroxidation and cell killing.

Merocyanine 540 (MC540) is a lipophilic photosensitizing dye of biomedical interest in connection with its ability to preferentially inactivate leukemia cells in bone marrow grafts and enveloped viruses in blood products. Evidence that iron plays a role in dye-mediated photokilling is presented in this report. When sensitized with MC540 and irradiated with visible light, cultured murine leukemia L1210 cells underwent lipid peroxidation (accumulation of iodometrically detectable lipid hydroperoxides) and photokilling (loss of clonogenic capacity). Selenium-deficient [Se(-)] cells, which expressed minimal selenoperoxidase activity, were found to be more sensitive to photoperoxidation and photokilling than selenium-replete [Se(+)] controls. Since redox active iron in the presence of electron donors has been shown to exacerbate photoperoxidative damage in isolated membrane systems, it was of interest to examine the possible role of iron in MC540/light-induced cytotoxicity. Involvement of iron was established by showing (i) that desferrioxamine (a high-affinity chelator and redox inhibitor of Fe3+) acted protectively on Se(-) and Se(+) cells and (ii) that treating these cells with sublethal concentrations of the lipophilic chelate ferric 8-hydroxyquinoline [Fe(HQ)2] made them much more sensitive to photokiling and thiobarbituric acid-detectable lipid peroxidation. Lehal damage induced by t-butyl hydroperoxide was also amplified by Fe(HQ)2. Fe(HQ)2-enhanced photoperoxidation and photokilling were suppressed by alpha-tocopherol, suggesting that iron-catalyzed free radical reactions were involved. A mechanism based on iron-mediated one-electron reduction of nascent photoperoxides is proposed. We believe that under the conditions used, toxic one-electron chemistry overwhelms two-electron detoxification catalyzed by GSH-dependent selenoperoxidase(s).