2-Aminofluorene bioactivation by human term placental peroxidase

Earlier investigations from our laboratory demonstrated that human term placental peroxidase (HTPP) is capable of metabolism of xenobiotics and endogenous compounds. In this study, purified HTPP was found to bioactivate 2-aminofluorene (2-AF) in the presence of H2O2. 2-AF oxidation was studied spectrophotometrically while radiometry was employed to assess the bioactivation. The rate of oxidation and covalent binding to protein and DNA was dependent upon the pH of the reaction medium and the concentration of 2-AF, the enzyme, and H2O2. To observe maximal enzyme velocity of oxidation, the presence of 16.5 microM H2O2, 100 microM 2-AF, 37 micrograms of the enzyme protein/ml, and pH 7.2 was required. Under optimal assay conditions, the range of specific activity between 130 and 165 nmol of 2-AF oxidized/min/mg HTPP was observed. Using similar assay conditions, the magnitude of covalent binding of [3H]-2-AF to protein (BSA) and calf thymus DNA was found to be about 508 pmol bound/min/mg HTPP/mg BSA and 84 pmol bound/min/mg HTPP/mg DNA, respectively. Potassium cyanide and sodium azide, the known inhibitors of different peroxidases, significantly blocked both the oxidation and covalent binding of 2-AF in a dose dependent manner. These results strongly suggest that peroxidase may be one of the important pathways responsible for the bioactivation of arylamines in human term placenta.