Promethazine oxidation by redox mediation in peroxidase reactions

The effect of promethazine on peroxidase-catalyzed oxidation of 3,3', 5,5'-tetramethylbenzidine was investigated at pH 5.4. Promethazine dose dependently introduced a lag in the appearance of tetramethylbenzidine charge-transfer complex monitored at 652 nm. Increasing concentrations of tetramethylbenzidine however decreased the lag period proportional to the tetramethylbenzidine concentration. Addition of promethazine to preformed charge transfer complex caused rapid bleaching of the blue-colored complex. Titration of promethazine with the yellow-colored diimine gave rise to the blue charge-transfer complex and the complete reduction of the species to the colorless parent amine compound. The available evidence suggests that promethazine is oxidized via redox mediation by tetramethylbenzidine peroxidase-oxidized products.