Chemical oxidation of N,N-dimethyl-p-anisidine (DMA) with bromine at pH 5-8 gave p-N,N-dimethylaminophenoxy radical irrespective of the DMA/Br2 ratio (1 to 100), whereas the ethyl hydroperoxide-supported oxidation of DMA catalysed by lactoperoxidase gave the phenoxy radical or the cation radical of DMA depending upon the concentration of DMA. 2. The amount of p-benzoquinone produced by the chemical oxidation increased steeply with an increase in pH above 6.0, whereas that produced by the lactoperoxidase-ethyl hydroperoxide-bromide system exhibited a pH optimum centred around pH 6.0. When the concentration of DMA was increased 10-fold, the enzymic formation of p-benzoquinone greatly decreased, whereas that of formaldehyde increased. 3. The rate of formation of the oxidized bromine species by the lactoperoxidase-ethyl hydroperoxide-bromide system showed a similar pH-profile to the formation of p-benzoquinone. 4. The oxidized bromine species is considered to be the predominant oxidizing agent in the lactoperoxidase-ethyl hydroperoxide-bromide system at pH 6.0 and below. The decrease in the amount of p-benzoquinone formed, and the increase in the amount of formaldehyde formed, by the lactoperoxidase-ethyl hydroperoxide-bromide system with increasing concentration of DMA, were interpreted in terms of competition between bromide and DMA for the reaction with compound I of lactoperoxidase.
