No abstract

In the presence of chloroperoxidase, indole was oxidized by H2O2 to give oxindole as the major product. Under most conditions oxindole was the only product formed, and under optimal conditions the conversion was quantitative. This reaction displayed maximal activity at pH 4.6, although appreciable activity was observed throughout the entire pH range investigated, namely pH 2.5-6.0. Enzyme saturation by indole could not be demonstrated, up to the limit of indole solubility in the buffer. The oxidation kinetics were first-order with respect to indole up to 8 mM, which was the highest concentration of indole that could be investigated. On the other hand, 2-methylindole was not affected by H2O2 and chloroperoxidase, but was a strong inhibitor of indole oxidation. The isomer 1-methylindole was a poor substrate for chloroperoxidase oxidation, and a weak inhibitor of indole oxidation. These results suggest the possibility that chloroperoxidase oxidation of the carbon atom adjacent to the nitrogen atom in part results from hydrogen-bonding of the substrate N-H group to the enzyme active site.

Chloride peroxidase catalyses both the ring halogenation and N-oxidation reactions of 4-chloroaniline by H2O2 and either KCl or KBr. In the absence of any halide salt only the N-oxidation reaction was observed, with the resulting conversion of 4-chloroaniline into 4-chloronitrosobenzene. The N-oxidation reaction proceeded even more rapidly in the presence of Cl- or Br-, in spite of the fact that ring halogenation was also a rapid reaction. The enhancement of N-oxidation was highly dependent on the pH of the media and displayed an optimum in the region of pH 3.5-4.0. No rate enhancement was observed above pH 5.5. KF partially inhibited the rate of N-oxidation in a pH-dependent manner. On the basis of calculated catalytic-centre activity the N-oxidation reaction was the major reaction at pH 3.5 or higher, whereas the ring-halogenation reaction became the major reaction below pH 3.5. In the presence of high concentrations of 4-chloroaniline relative to H2O2 the reaction intermediate, 4-chlorophenylhydroxylamine, was detected for the first time in a chloride peroxidase-catalysed reaction with this arylamine substrate. These findings were interpreted on the basis of current knowledge concerning the mechanism of action of chloride peroxidase.

No abstract

Following stimulation with phorbol myristate acetate, human granulocytes were found to incorporate a series of arylamines into cellular nucleic acid. No such binding occurred if the granulocytes were not induced to undergo the respiratory burst. The relative amount of covalent binding to cellular DNA and RNA was found to depend strongly on the chemical structure of the arylamine. 2-Aminofluorene gave the highest ratio of DNA/RNA binding, while 4-nitroaniline showed a very low ratio of DNA/RNA binding. 4-Nitroaniline may bind only to RNA, since the degree of binding to DNA was at the level of detectability. Two other substrates, 4-chloroaniline and 4-methylaniline, gave intermediary ratios of DNA/RNA binding. Studies on the possible role of the granulocyte enzyme myeloperoxidase in the activation and binding of these arylamines were conducted in vitro and also through the use of azide, an inhibitor of myeloperoxidase activity in cells. The results indicate that myeloperoxidase probably plays only a limited role in causing the covalent binding of arylamines to nucleic acid in human granulocytes. It is probable that other reactive oxygen species, which are not dependent upon myeloperoxidase for their production, are necessary for the bioactivation of some arylamines, especially for substrates such as 4-nitroaniline. A free-radical mechanism for arylamine bioactivation, and its potential role in arylamine toxicity, was presented in the context of the current scientific literature.