Effects of several organophosphates on hepatic cytochrome P450 activities in rats

Abstract

Four commonly used organophosphates (fenitrothion, dichlorvos, chlorpyrifos, and trichlorfon) were orally administered to male Sprague-Dawley rats for five days in order to explore their effects on the activities of liver cytochrome P450 (CYP). In addition, Michaelis-Menten kinetics of the metabolic reactions catalyzed by liver CYPs were analyzed following the addition of these compounds to the assay system to examine their potential inhibitory effects on liver CYPs activities. These reactions included ethoxyresorufin O-deethylation, midazolam 4-hydroxylation, tolbutamide hydroxylation, and bufuralol 1'-hydroxylation for CYP1A, 3A, 2C, and 2D activities, respectively. Total CYP content was also examined after oral administration of each organophosphate. Results revealed that oral giving of fenitrothion inhibited significantly CYP1A and 3A activities while elevated activity of CYP2C. Fenitrothion is a potent inhibitor for CYP1A and 2C with Ki values of 0.42 and 36.1 µM, respectively but had a weak inhibitory effect on CYP2D and 3A with Ki values of 290 and 226 µM, respectively. Chlorpyrifos is a potent inhibitor of CYP1A with Ki 0.24 µM and moderately inhibited CYP2C or 3A with Ki values of 84.8 and 77.7 µM, respectively. On the other hand, dichlorvos and trichlorfon caused extremely low or negligible inhibition of different CYP activities. From these results, it is concluded that both fenitrothion and chlorpyrifos may increase the toxicity of chemicals in environmental living organisms through their potent inhibitory effects on these CYP activities, but dichlorvos and trichlorfon may not.

Keywords: cytochrome P450; induction; inhibition; organophosphate; rat.

Fig. 1.

Effects of organophosphates on metabolic reactions catalyzed by cytochrome P450 1A, 2C, 2D, and 3A after oral administration of each organophosphate for 5 days to rats. Bars represent relative reaction activities against control; error bars indicate SD of the mean (n=5), * P ≤0.05.

Fig. 2.

Michaelis-Menten kinetics of ethoxyresorufin O-deethylation (EROD) in hepatic microsomes from rats. The solid curves represent the theoretical metabolic rates calculated using Vmax and Km values showed in Table 2. Closed and opened circles represent the observed metabolic rates (mean ± SD) in the presence or absence of organophosphates, respectively.

Fig. 3.

Michaelis-Menten kinetics of tolbutamide hydroxylation (TBH) in hepatic microsomes from rats. The solid curves represent the theoretical metabolic rates calculated using Vmax and Km values showed in Table 2. Closed and opened circles represent the observed metabolic rates (mean ± SD) in the presence or absence of organophosphates, respectively.

Fig. 4.

Michaelis-Menten kinetics of bufuralol 1’-hydroxylation (BLH) in hepatic microsomes from rats. The solid curves represent the theoretical metabolic rates calculated using Vmax and Km values showed in Table 2. Closed and opened circles represent the observed metabolic rates (mean ± SD) in the presence or absence of organophosphates, respectively.

Fig. 5.

Michaelis–Menten kinetics of midazolam 4-hydroxylation (MDZH) in hepatic microsomes from rats. The solid curves represent the theoretical metabolic rates calculated using Vmax and Km values showed in Table 2. Closed and opened circles represent the observed metabolic rates (mean ± SD) in the presence or absence of organophosphates, respectively.