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Peroxidase-catalyzed oxidation of o-phenylene diamine (OPD) was competitively inhibited by trimethylhydroquinone (TMHQ), 4-tert-butylpyrocatechol (InH5), and 4,6-di-tert-butyl-3-sulfanyl-1,2-dihydroxybenzene (InH6). InH6 was the most efficient inhibitor (K i = 11 μM at 20°C in 0.015 M phosphate–citrate buffer, pH 6.0). The effects of InH5 and InH6 were not preceded by periods of induction of OPD oxidation products (contrary to TMHQ). Peroxidase-catalyzed oxidation of tetramethylbenzidine (TMB) was noncompetitively inhibited by InH6 and 3-(2-hydroxyethylthio)-4,6-di-tert-butylpyrocatechol (InH4), whereas o-aminophenol acted as a mixed-type inhibitor. The effects of all three inhibitors were preceded by an induction period, during which TMB oxidation products were formed. Again, InH6 was the most efficient inhibitor (K i = 16 μM at 20°C in 0.015 M phosphate–citrate buffer supplemented with 5% ethanol, pH 6.0). Judging by the characteristics of the inhibitors taken in aggregate, it is advisable to use the pairs OPD–InH5 and OPD–InH6 in systems for testing the total antioxidant activity of human biological fluids. |
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