Cytochrome c/H2O2-mediated one electron oxidation of carcinogenic N-fluorenylacetohydroxamic acids to nitroxyl free radicals

  • Ritter C
  • Malejka-Giganti D
  • Polnaszek C
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Abstract

The oxidation of carcinogenic hydroxamic acids, N-hydroxy-N-2-fluorenylacetamide (N-OH-2-FAA) and N-hydroxy-N-S-fluovenylacetamide (N-OH-3-FAA) catalyzed by horseradish peroxidase (HRP) or cytochrome c in the presence of H2O2was investigated. HRP/H2O2was a more efficient system in oxidation of both hydroxamic acids and the standard substrate, guaiacol, then cytochrome c/H2O2. Peroxidative activity of cytochrome c was shown after incubation with Triton X-100 and H2O2for 20 min at room temperature in 0.05 M phosphate buffer (pH 7.5) or in 0.1 M sodium acetate (pH6.0) without Triton X-100. Both hydroxamic acids were oxidized to nitroxyl free radicals as shown by electron spin resonance (ESR) spectroscopy. These radicals dismutated to equimolar amounts of 2- or 3-nitrosofluorene and acetate esters of the corresponding hydroxamic acids as shown by thin layer chromatography and spectrophotometric analysis of the products. In addition, large amounts of the N-fluorenylamides were generated in the reactions with cytochrome c/H2O2system. Of the products, only 2- or 3-nitrosofluorene per se or when generated from the oxidation of the hydroxamic acids, interacted with lecithin (1 mg/ml) to yield ESR signals of the immobilized nitroxyl free radicals. In contrast to HRP/H2O2system, in which the initial velocity of the radical formation was too fast to measure and the maximal concentrations of the nitroxyl free radicals of both hydroxamic acids were similar, in the cytochrome c/H2O2system the nitroxyl free radical of N-OH-2-FAA formed at a 6-fold faster rate and accumulated at a 2-fold higher concentration than the radical of N-OH-3-FAA. In both enzyme systems, the persistence of the signal and the length of time before it had decreased to one half its maximum were several-fold longer for the nitroxyl free radical of N-OH-3-FAA than for that of N-OH-2-FAA. These data showed that these nitroxyl free radicals differed in their kinetic properties. One electron oxidation of N-OH-3-FAA by HRP/H2O2system and of both isomeric hydroxamic acids by cytochrome c/H2O2system are reported for the first time in this work and may be considered an activation reaction in earcinogenesis by these compounds. © 1983.

Author-supplied keywords

  • Carcinogen oxidation-Hydroxamic acids-Nitroxyl radicals-Cytochrome c

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Authors

  • C. L. Ritter

  • D. Malejka-Giganti

  • C. F. Polnaszek

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