Interaction of 2,4,4′-trichloro-2′-hydroxydiphenyl ether with microsomal cytochrome p450-dependent monooxygenases in rat liver

We studied the effects of 2,4,4′-trichloro-2′-hydroxydiphenyl ether (Irgasan DP300) on the kinetics of the cytochrome P450 (P450)-dependent monooxygenases in rat liver microsomes. The activities of 7-ethoxyresorufin 0-deethylase (EROD) and 7-pentoxyresorufin O-depentylase (PROD) in rat liver microsomes exposed to 3-methylcholanthrene (MC) and phenobarbital (PB) respectively, were substantially inhibited by Irgasan DP300. The inhibition profile of EROD was competitive, whereas that of PROD was noncompetitive; the Ki values from Hanes plots were 0.24 and 1.48 μM for EROD and PROD, respectively. Phenacetin O-deethylase (PCOD) and 4-nitrophenol hydroxylase (4NPH) activities in rats exposed to PB were also inhibited by Irgasan DP300, at Ki values lower than those for other microsomes. Irgasan DP300 slightly inhibited testosterone 6β-hydroxylase (TS6BH) activities in some microsomes. No effect of Irgasan DP300 on lauric acid ω-hydroxylase (LAOH) activity was evident in any microsomal preparations. These results indicated that Irgasan DP300 inhibits MC- and PB-inducible P450-dependent monoxygenase in vitro competitively or noncompetitively, and that the P450 enzymes of the CYP1A or CYP2B subfamily may contribute to Irgasan DP300 toxicity.