Plasma membrane as a site of redox activation of daunomycin in intact human erythrocytes. Quantitative evaluation of the hydrogen peroxide produced by the membrane with respect to the cytosol

Extracellular activation of fluorinated aziridinylbenzoquinone in HT29 cells EPR studies

The plasma membrane of HT29 human colon carcinoma cells was characterized by EPR spectroscopy as the site for redox activation of 3,6-difluoro-2,5-bis(aziridinyl)-1,4-benzoquinone (F-DZQ). Supplementation of HT29 cells with F-DZQ yielded an EPR signal ascribed to the semiquinone species; the hyperfine splitting constants of the 11-line spectrum were 1.4 and 1.35 G for aN and aF, respectively. The intensity of the EPR signal was inhibited competitively by potassium ferricyanide, a compound which has no access to the intracellular milieu and used to evaluate transmembrane NADH-ferricyanide reductase activity. The extracellular localization of the signal was confirmed by using chromium trioxalate, a membrane-impermeant spin-broadening agent, which abolished in a concentration-dependent manner the semiquinone signal originating from the metabolism of F-DZQ by HT29 cells. The intensity of the semiquinone signal was decreased by agents which block sulfhydryl groups upon alkylation, fluorodinitrobenzene and p-chloromercuribenzoate, presumably acting on plasma membrane dehydrogenases. Other flavin dehydrogenase inhibitors, such as allopurinol, deprenyl or clorgyline, and D-arginine or NG-methyl-L-arginine did not affect the EPR signal. Conversely, the intensity of the semiquinone signal was increased upon supplementation of HT29 cells with glucose and insulin, which may enhance the intracellular levels of electron donors for the transplasma membrane dehydrogenase activity. The extracellular semiquinone signal was abolished by superoxide dismutase by a mechanism implying displacement of the equilibrium of the autoxidation reaction. Formation of oxygen-centered radicals during this redox activity was evaluated by EPR in conjunction with the spin trap 4-POBN. A composite signal consisting of the spin adducts of methyl, hydroxyl and superoxide radicals was observed (the former arising from hydroxyl radical attack on the quinone solvent, dimethylsulfoxide). The formation of these spin adducts was abolished by superoxide dismutase and their detection became impossible in the presence of the line broadening agent chromiun trioxalate, thus indicating their extracellular formation and localization, respectively. The occurrence of a redox site at the plasma membrane of HT29 cells for the activation of this halogenated aziridinylbenzoquinone is discussed in terms of its significance for intracellular processes and a build-up of oxyradicals in the extracellular milieu.

Reactive oxygen species do not cause arsine-induced hemoglobin damage

Previous work suggested that arsine- (AsH3-) induced hemoglobin (HbO2) damage may lead to hemolysis (Hatlelid et al., 1996). The purpose of the work presented here was to determine whether reactive oxygen species are formed by AsH3 in solution, in hemoglobin solutions, or in intact red blood cells, and, if so, to determine whether these species are responsible for the observed hemoglobin damage. Hydrogen peroxide (H2O2) was detected in aqueous solutions containing AsH3 and HbO2 or AsH3 alone but not in intact red blood cells or lysates. Additionally, high-activity catalase (19,200 U/ml) or glutathione peroxidase (68 U/ml) added to solutions of HbO2 and AsH3 had only a minor protective effect against AsH3-induced damage. Further, the differences between the visible spectra of AsH3-treated HbO2 and H2O2-treated HbO2 indicate that two different degradative processes occur. The presence of superoxide anion (O2-) was measured by O2(-)-dependent reduction of nitro blue tetrazolium (NBT). The results were negative for O2-. Exogenous superoxide dismutase (100 micrograms/ml) did not affect AsH3-induced HbO2 spectral changes, nor did the hydroxyl radical scavengers, mannitol, and DMSO (20 mM each). The general antioxidants ascorbate (< or = 10 mM) and glutathione (< or = 1 mM) also had no effect. These results indicate that the superoxide anion and the hydroxyl radical (OH) are not involved in the mechanism of AsH3-induced HbO2 damage. The results also indicate that although AsH3 contributes to H2O2 production in vitro, cellular defenses are adequate to detoxify the amount formed. An alternative mechanism by which an arsenic species is the hemolytic agent is proposed.

Enhancement of daunomycin toxicity by the differentiation inducer hexamethylene bisacetamide in erythroleukemia cells

Cytotoxic effects of daunomycin were investigated upon differentiation of Friend erythroleukemia cells induced with hexamethylene bisacetamide, a process during which a 20-fold increase in the hemoglobin content occurred. Daunomycin proved to be more toxic to differentiated Friend cells than to their undifferentiated counterparts. No changes in the daunomycin uptake rates of the two cell types were detectable. Externally added catalase and desferrioxamine mesylate protected against the additional cytotoxicity of daunomycin in differentiated cells, pointing to hydrogen peroxide and iron ions as mediators of the toxic effect. Daunomycin-dependent, cyanide-insensitive oxygen consumption of control and induced cells did not differ significantly, and the rate of formation of the daunomycin semiquinone radical electron paramagnetic resonance signal was similar in both cell types, indicating that the difference in toxicity was not due to increased drug activation by plasma membrane enzymes. Differentiated cells had a lowered catalase content; the cellular iron content was shown to increase by 2.8-fold upon cell differentiation, with hemoglobin-bound iron being about 50% of the total. Altogether, the results suggest increased intracellular hydrogen peroxide generation mediated by hemoglobin, combined with a decrease in catalase activity and an increase in accessible iron, as responsible for the higher sensitivity to daunomycin shown by differentiated Friend cells. This represents the first experimental system where the increase in anthracycline cytotoxicity upon cell differentiation can be attributed to redox activation and the formation of reactive oxygen species.

Comparative toxicity of alkyl-1,4-naphthoquinones in rats: relationship to free radical production in vitro

2-Methyl-1,4-naphthoquinone causes haemolysis in vivo. This toxic effect is believed to result from oxidative damage to erythrocytes by "active oxygen" species formed via one-electron reduction of the naphthoquinone by oxyhaemoglobin. In the present investigation, seven 2-alkyl-1,4-naphtoquinones have been studied with regard to their haemolytic activity in rats, their ability to cause oxidative damage in erythrocytes in vitro, and their reactivity toward oxyhaemoglobin. A close correlation was observed between the in vivo and in vitro parameters, suggesting that the proposed mechanism of toxicity of 2-methyl-1,4-naphthoquinone is correct and is also applicable to other alkylnaphthoquinones.