Enzymology of Nitrone Formation

Diarylnitrone (31) formation from N-substituted, diaromatic imines has been recognized to require the presence of NADPH/O2, and has been proposed to proceed via the intermediacy of an oxaziridine3 possibly arising from reaction of the parent imines with the putative P-450 [FeO]3+ species in analogy to the oxidation of olefins118. Ring cleavage of the oxaziridine then yields nitrone or amide (equation 10). 

It seems noteworthy that, apart from the P-450 system, other hemoproteins also appear to be active in the production of nitrones from secondary hydroxylamines. Thus, in the presence of O2, hemoglobin brings about diarylnitrone (31) formation from N-benzylphenylhydroxylamine, presumably by a peroxidative mechanism139. 

In addition to the pathways described above, nitrones can derive from condensation of a primary hydroxylamine with an aldehyde or ketone140. Under simulated biological conditions, Beckett and coworkers were able to demonstrate that primary hydroxylamines metabolically formed from amphetamine, mexiletine or norfenfluramine readily combined with ketones produced by metabolic deamination of the primary amines141. Reactions can be formulated according to equation 13, where R1 = aryl, alkyl or aralkyl, and R2 = alkyl or H. 

Moreover, nitrones can be generated from N-hydroxy products during extraction into an organic solvent, such as diethyl ether. The latter contains acetaldehyde as an impurity even after careful distillation. Indeed, nitrones arising as artifacts from the reaction of primary hydroxylamines, metabolically derived from methamphetamine and chlorpromazine, with acetaldehyde upon sample treatment with ether have been identified23,39. Acetone is frequently used as one of the components of the solvent systems applied for TLC analysis. 

FIGURE 2. Difference spectrum produced during NADPH/02-dependent rat liver microsomal metabolism of. Y-meth Idle ] phen l 2 propylamine N-oxide. The time interval between each scan is about 40 s. (Data taken from Ref. 136, with permission) 

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