[Inhibition of peroxidase oxidation of 3,3',5,5'-tetramethylbenzidine and o-phenylenediamine by 1-amino-2-naphthol-4-sulfonic acid and its polysulfide].

The kinetics of coupled peroxidation of 3,3',5,5'-tetramethylbenzidine and 1-amino-2-naphtol-4-sulfonic acid (ANSA) or its polydisulfide (poly(ADSNSA)) was studied in 0.01 M phosphate buffer (pH 6.4) at 20 degrees C. Both ANSA and poly(ADSNSA) strongly inhibited the TMB oxidation resulting in a marked delay in the product formation. Stoichiometric inhibition coefficients f, i.e., the average numbers of free-radical particles terminated by one inhibitor molecule, were estimated. The free-radical trapping effect of poly(ADSNSA) was 7.5 times greater than that of ANSA. Kinetics of coupled o-phenylenediamine (PhDA) and ANSA or poly(ADSNSA) oxidation was studied in phosphate-citrate buffers at pH 3 to 7. No lag periods in oxidation product accumulation were observed under any of the reaction conditions. A weak activation of PhDA conversion depending on pH and PhDA/ANSA ratios was observed at low ANSA concentrations, whereas increased ANSA or poly(ADSNSA) concentrations were inhibitory. The degree of PhDA inhibition was maximal in acid media, reached minimum at pH 5 to 6, and than again increased at pH above 6. Tentative mechanism of coupled aromatic amine phenol bi-substrate system peroxidation is discussed.