Ferritin as a biocatalyst of the oxidation of aromatic amines

The kinetics of tetramethylbenzidine (TMB)(3) oxidation in the presence of horse spleen ferritin (FERR) were studied in TMB-FERR-H2O2 (1) and TMB-FERR-O-2 (2) systems at pH 4.2 and 6.0. When the TMB concentration was below 6 mM, the effect of the initial TMB and H2O2 concentrations on the initial rate of TMB oxidation in system (1) is described by the Michaelis-Menten equation. The maximal efficiency of FERR during TMB oxidation at pH 4.2 expressed as the ratio k(cat)/K-m was 2.8 . 10(3) M(-1). sec(-1). In system (1), TMB oxidation was significantly inhibited by mannitol, urea, ethanol, and sodium fluoride. Competitive inhibition by urea was characterized by K-i = 12.5 mM. In system (2) at TMB concentrations exceeding 6 mM, the amine oxidation is autocatalytic. In both systems the initial rate of TMB oxidation significantly increases with decreasing pH from 9.3 to 4.2. In system (1) the reaction of Fe3+ with H2O2 is initiating, and in the system (2) the reaction of Fe3+ with the amine is initiating. In both cases TMB oxidation is a catalytic chain reaction that includes ion-radical intermediate stages. HO. radicals and Fe3+ ions are the main oxidizing particles in system (2). The interaction of TMB with ferritin can activate lipid peroxidation in animals injected with aromatic amines.