Initiation of radicals in biochemical systems: ferritin-hydroperoxides-aromatic amines

The rates of initiation of free radicals were determined in systems containing horse spleen ferritin, H2O2, tert-butyl hydroperoxide (TBHP) or cumene hydroperoxide (CHP) in acetate buffer (pH 4.2) or phosphate buffer (pH 6.0) with 10-15% dimethylformamide (DMF). Benzidine (BD), benzidine sulfate (BDS), o-tolidine (TL), 3,3',5,5'-tetramethylbenzidine (TMB), and o-phenylenediamine (PDA) were used as acceptors. In the systems ferritin-H2O2 oxidation of amine acceptor follows Michaelis-Menten kinetics. For all aromatic amines kcat, Km, and their ratios were determined. Peroxidase efficiency of ferritin in the TMB oxidation by hydrogen peroxide (kcat/km) is characterized by a value 2.82.10(3) M-1.sec-1 comparable with ferroxidase efficiency of apoferritin in the oxidation of Fe2+ by oxygen. Reactivity of aromatic amines in the system ferritin-H2O2 is similar to the reactivity registered in their peroxides oxidation and is maximal for TMB and PDA. Bimolecular rate constants of TMB and PDA oxidation in the reaction with H2O2, TBHP, and CHP were compared in acetate buffer (pH 4.2 using 0.5 microM ferritin and 5 mM concentration of each substrate. On the oxidation of both amines activity of oxidants decreased in the following order: H2O2 > TBHP > CHP. A scheme of radical initiation in the systems ferritin-ROOH (H2O2)-amines and the influence of radical acceptors apoferritin and organic co-solvent on the rate of reactions are discussed.