Activation of N,N-Dimethylaminoazobenzene catalyzed by peroxidase

Abstract

The mechanism for the metabolic activatiou of N,N-dimethylaminoazobenzene is unknown. Although cytochrome P-450 dependent mixed function oxygenases are important in the activation of arylamines, there are target tissues for arylamines which do not contain these oxygenases. These tissues do contain peroxidases. Therefore a one electron oxidation mechanism was investigated by studying a Horseradish Peroxidase/hydrogen peroxide catalyzed metabolism of aminoazobenzenes [i.e; N-methyl-4-aminoazobenzene (MAB), N,N-dimethyl-4-aminoazobenzene (DAB), aminoazobenzene (AB)], and by following the binding of the activated products to exogenous DNA. In order to further explore the mechanism, the effect of biological and phenolic antioxidants on metabolism and binding of the activated products of [¹⁴C] MAB was studied. The oxidation products were analyzed by spectrophotometry, high pressure liquid chromatography and thin layer chromatography. The major product obtained from both MAB and DAB at pH 7.4 was a dimer of MAB, i.e; N-methyl,N(MAB)-4-aminoazobenzene. The product obtained at pH 5.0 was 4'-(MAB)-N-methyl-4-aminoazobenzene. It is suggested that DAB is first N-demethylaced, and then follows the same metabolic pathway as does MAB. Evidence suggests that AB is also oxidized to a dimer with N-N linkage at pH 7.4 and C-N linkage at pH 5.0. -- The binding of the activated products to calf thymus DNA was observed spectrophotometrically. [¹⁴C] MAB was used to determine the quantitative binding by liquid scintillation counting. DNA is attacked by a free radical of MAB, and also by other radicals formed in chain propagating reactions with the MAB radical. The maximum binding of [¹⁴C] MAB to homo polyriboguanylic acid among the four homo polyribonucleotides suggests the preferential binding of the activated products occurs at the guanine residues in DNA. -- The phenolic and biological antioxidants inhibit the DNA binding either by inhibiting the total oxidation of MAB or by converting the reactive metabolites to detoxication products.