Inhibition of skeletal sarcoplasmic reticulum Ca2+-ATPase activity by deferoxamine nitroxide free radical

Deferoxamine is an inhibitor of iron-dependent free radical reactions. Despite the antioxidant roles, prolonged clinical use of the chelator is far from benign, and paradoxically, deferoxamine has been shown to promote lipid peroxidation. The possible toxicity of the drug's metabolites, such as deferoxamine nitroxide free radical, deserves attention. We, therefore, tested the hypothesis that deferoxamine nitroxide radicals produced as a result of enzymatic one-electron oxidation of deferoxamine by horseradish peroxidase in the presence of H2O2 are capable of inactivating Ca2+-ATPase of skeletal sarcoplasmic reticulum microsomes as a model system with which to explore the effect of the radical on a biological membrane. Ca2+-ATPase activity of sarcoplasmic reticulum was depressed by exposure to Fenton's reagent (H2O2/FeSO4); the observed effect was significantly enhanced by deferoxamine. We found that the Fenton reaction produced hydroxyl radical, as determined by electron spin resonance spectroscopy. The formation of hydroxyl radical was completely inhibited by deferoxamine; instead, under the same experimental conditions (in the presence of sarcoplasmic reticulum vesicles with or without FeSO4 but without spin trap 5, 5-dimethyl-1-pyrroline N-oxide), the spectral shape and hyperfine coupling constants of electron spin resonance signals confirmed to be long-lived deferoxamine radical were obtained. Furthermore, exposure of sarcoplasmic reticulum vesicles to deferoxamine radical formed by horseradish peroxidase via reaction with H2O2 caused an inhibition of the Ca2+-ATPase activity. The findings show that the sarcoplasmic reticulum vesicles can act as peroxidases and suggest that deferoxamine enhances the decreased Ca2+-ATPase activity afforded by H2O2/FeSO4 due to formation of its metabolites, possibly deferoxamine nitroxide free radical.