Nitrosoarenes compounds

Figure 1. Redox interconversions of arylamine and nitroaromatic compounds via intermediate arylhydroxylamine and nitrosoarene compounds.

Figure 2. Conversion of nitroaromatic to nitrosoarene compounds by a two-electron addition reaction.

A compilation of reduction potentials for a series of substituted nitrosobenzenes is often cited (69). Unfortunately, these determinations were conducted at pH 1 in aqueous acetone, and while they illustrate the general tendency of electron-withdrawing substituents to increase the reduction potential of nitrosoarene compounds, their quantitative values should be viewed with skepticism for reactions occurring at physiological pH. 

The reduction of the hydroxylamine group involves an energy barrier. The generally sluggish reduction of an hydroxylamine group compared to reduction of its nitroso analog is reflected by the ease of reduction of many nitrosoarene compounds to the corresponding arylhydroxylamines by reduced pyridine and flavin coenzymes (1,2, 65). The lack of... 

Oxidative conditions favor the nitroso side of the equilibrium between arylhydroxylamine and nitrosoarene compounds. This property is illustrated by our observation that dilute solutions of mono-substituted arylhydroxylamines in seawater underwent spontaneous and nearly quantitative conversions to the respective nitrosoarene derivative (Corbett, unpublished). The rates of these oxidations are surprisingly fast, as demonstrated by the apparent first-order rate constant of 0.71 min for 4-methylphenylhydroxylamine in sterile-filtered seawater at 25°C. Stemson... 

In studies with chloroperoxidase (CPX), a peroxidative enzyme that is amazingly similar in certain properties to cytochrome P-450, we found the nitrosoarene metabolite to be the terminal product of arylamine oxidation (Fig. 6) (23, 31). CPX has been used to prepare nitrosoarene chemicals on a micro scale (17) because few chemical techniques are available for the direct conversion of arylamines to nitrosoarene compounds (28). It is probable that this enzymatic oxidation produces an intermediary hydroxylamine compound which, under the reaction conditions, is rapidly converted to the nitroso level. An apparent kinetic block in the oxidation of nitrosoarene to nitroaromatic compounds allows for the fairly selective production of the former by mild oxidants, particularly for those arylamines with electron-withdrawing substituents. 

A notable exception to the rule of thumb that the nitrosoarene compound is favored under oxidative conditions is the observation of arylhydroxylamine rather than nitrosoarene metabolite in microsomal oxidase studies of arylamine metabolism (43). In vitro studies of N-oxidation generally contain the biological reductant NADPH, which can, by a mechanism independent of cytochrome P-450 catalysis, reduce any nitroso product back to the hydroxylamine state. Our studies on CPX-catalyzed oxidation of arylamines by H2O2 did not require a reductant such as NADPH, and thus, may better reflect the actual enzymatic or equilibrium product of oxidative reactions, including microsomal oxidations. Microsomal oxidations in which substrates such as cumene hydroperoxide are used in place of O2/NADPH should yield the nitrosoarene rather than arylhydroxylamine metabolite. In the case of 4-chloroaniline, the nitroso metabolite rather than the hydroxylamine metabolite was produced however, the major product was the nitroaromatic compound, indicative of further oxidation (55). This raises the hypothetical question as to whether... 

In general, arylhydroxylamine compounds are much more readily oxidized to the nitrosoarene compounds than are the corresponding arylamine compounds. All reasonable chemical mechanisms for these oxidations suggest the existence of major mechanistic differences between arylamine and arylhydroxylamine oxidation since the former requires N-0 bond formation, while the latter is a dehydrogenation (31). The enzymatic conversion of arylhydroxylamine to nitrosoarene compounds is generally much more facile than the conversion of arylamine compounds to arylhydroxylamine compounds which suggests that the latter metabolic intermediate may be rapidly oxidized to the nitrosoarene compound before it can escape the immediate area of the enzyme active site. No study has yet determined the relative kinetic parameters for arylamine and arylhydroxylamine oxidations by an enzyme, such as CPX, capable of oxidizing both. 

The valence-bond structures of arylhydroxylamine and nitrosoarene compounds are generalized in Figure 8. Each has structural features that endow it with both nucleophilic... 

Figure 8. The valence bond structures of arylhydroxyiamine and nitrosoarene compounds indicating the potential for both compounds to react as either nucleophiles or electrophiles.

The electrophilic properties of arylhydroxylamine and nitrosoarene compounds arise from very different bond-forming and bond-breaking reactions and lack a common electrophilic intermediate. The differences in electrophilic intermediates between arylhydroxylamine and nitrosoarene compounds are sufficient to exert a strong effect on the preference of target nucleophile. In turn, the arylhydroxylamine and nitrosoarene compounds are expected to produce somewhat different products, including protein binding by two chemically distinct electrophilic mechanisms. 

Figure 20. Major products resulting from the cometabolism of nitrosoarene compounds by a-ketoglutarate dehydrogenase.

In research soon to be published, we have found that one or more of the potential electrophilic species generated from nitrosoarene compounds by both pyruvate dehydrogenase and a-KGD can effect extensive covalent adduction of nitrosoarene compounds to DNA. The arylhydroxylamine-O-esters are likely candidates for these phenomena. 

SOME ORGANIC AND BIOCHEMICAL REACTIONS OF NITROSOARENE COMPOUNDS... 

The principal reactions of nitrosoarene compounds under physiological conditions are those in which the nitroso group behaves as an electrophile. Unlike the electrophilic reactions of arylhydroxylamine compounds, these reactions occur spontaneously without the need for further activation. On the other hand, the level of reactivity of the nitroso group is significantly less than that of the electrophilic species derived from arylhydroxylamine compounds. 

Figure 25. Proposed mechanism for the reaction of nitrosoarene compounds with sulfhydryl groups. (GIu-SH is reduced glutathione Glu-S-S-Glu is oxidized glutathione).

A, becomes acetyl-CoA. This reaction is often depicted as an electron-pair displacement, but two sequential electron transfer processes might be possible. The question arose as to what might happen if the nucleophilic intermediates of thiamine catalysis were to react with nitrosoarene compounds since a theoretical assessment suggested that the analogous arylhy-droxamic acid would be produced (8). 

Figure 28. Modification of the nucleophile mechanism for the reaction of nitrosoarene compounds with thiamine-dependent enzymes. Structure A is the putative reaction intermediate at the enzyme active-site. B is active acetate when R=H.

Figure 31. Electrophilic and nucleophilic roles of the nitroso functional group in reactions of nitrosoarene compounds with a-oxoacids.

The bioreduction of nitroaromatic compounds can proceed with the formation of the intermediary nitroso and hydroxylamine metabolites. That the fates of these intermediates in a microbial milieu are potentially numerous should be evident from this presentation of their chemical properties. Much of this information originated from research on the mechanisms by which arylhydroxylamine and nitrosoarene compounds cause toxicity in higher organisms. Covalent binding of these intermediates to cellular macromolecules can occur either directly in the case of nitrosoarene compounds and proteins, or indirectly in the case... 

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