Paraoxonase 1 (PON1) modulates the toxicity of mixed organophosphorus compounds

Abstract

A transgenic mouse model of the human hPON1(Q192R) polymorphism was used to address the role of paraoxonase (PON1) in modulating toxicity associated with exposure to mixtures of organophosphorus (OP) compounds. Chlorpyrifos oxon (CPO), diazoxon (DZO), and paraoxon (PO) are potent inhibitors of carboxylesterases (CaE). We hypothesized that a prior exposure to these OPs would increase sensitivity to malaoxon (MO), a CaE substrate, and the degree of the effect would vary among PON1 genotypes if the OP was a physiologically significant PON1 substrate in vivo. CPO and DZO are detoxified by PON1. For CPO hydrolysis, hPON1(R192) has a higher catalytic efficiency than hPON1(Q192). For DZO hydrolysis, the two alloforms have nearly equal catalytic efficiencies. For PO hydrolysis, the catalytic efficiency of PON1 is too low to be physiologically relevant. When wild-type mice were exposed dermally to CPO, DZO, or PO followed 4-h later by increasing doses of MO, toxicity was increased compared to mice receiving MO alone, presumably due to CaE inhibition. Potentiation of MO toxicity by CPO and DZO was greater in PON1(-/-) mice, which have greatly reduced capacity to detoxify CPO or DZO. Potentiation by CPO was more pronounced in hPON1(Q192) mice than in hPON1(R192) mice due to the decreased efficiency of hPON1(Q192) for detoxifying CPO. Potentiation by DZO was similar in hPON1(Q192) and hPON1(R192) mice, which are equally efficient at hydrolyzing DZO. Potentiation by PO was equivalent among all four genotypes. These results indicate that PON1 status can have a major influence on CaE-mediated detoxication of OP compounds.

Figure 1. CPO, DZO, and PO inhibition of plasma carboxylesterase (A), liver carboxylesterase (B), and brain AChE (C) in vitro

OPs were incubated with tissue homogenates prepared from wild-type mice. Results represent the mean ± SEM (n=3). IC₅₀ values calculated from the data are listed in Table 1.

Figure 2. PON1 levels in PON1^+/+, PON1^−/−, hPON1_Q192 and hPON1_R192 mice

Plasma PON1 levels were determined for all mice (genotypes as indicated) by measuring (A) arylesterase (AREase) activity (n=32–42 mice) or (B) low-salt diazoxonase (DZOase) activity. Note the presence of AREase activity, but not DZOase activity, in the plasma of the PON1^−/− mice. In contrast to AREase activity, DZOase activity is entirely due to PON1. Genotypes are Results represent the mean ± SEM (AREase, n=32–42; DZOase, n=16–78).

Figure 3. Time course of plasma CaE inhibition in vivo, following exposure to CPO, DZO, and PO

Time course of plasma carboxylesterase (CaE) inhibition in PON1^+/+, PON1^−/−, hPON1_Q192, and hPON1_R192 mice (genotypes as indicated) following dermal exposure to 0.75 mg/kg CPO (A), 0.5 mg/kg DZO (B), or 0.35 mg/kg PO (C). Maximal inhibition of CaE was at 4 hours. Results represent the mean ± SEM (n=5–10).

Figure 4. CPO, DZO, and PO inhibit serum CaE and liver CaE in vivo.

PON1^−/− mice were exposed dermally to CPO, DZO, or PO, and the dose response of (A) serum CaE and (B) liver CaE inhibition was measured 4-h after exposure. For PO, the highest dose given was 0.8 mg/kg. Because exposure at this dose caused lacrimation, tremors, and brain AChE inhibition of greater than 90% (not shown), higher doses would likely have been lethal. Results represent the mean ± SEM (n=4).

Figure 5. Effect of CPO exposure (0.75 mg/kg) on subsequent toxicity of malaoxon (MO)

Mice (genotypes as indicated) were exposed dermally to MO alone (A, B), or to CPO followed 4-h later by MO exposure (C, D, E, F). AChE was measured in the brain (A, C, E) and diaphragm (B, D, F) 4-h following the MO exposure. Results represent the mean ± SEM (n=4).

Figure 6. Effect of DZO exposure (0.5 mg/kg) on subsequent toxicity of malaoxon (MO)

Mice (genotypes as indicated) were exposed dermally to MO alone (A, B), or to DZO followed 4-h later by MO exposure (C, D, E, F). AChE was measured in the brain (A, C, E) and diaphragm (B, D, F) 4-h following the MO exposure. Results represent the mean ± SEM (n=4).

Figure 7. Effect of PO exposure (0.35 mg/kg) on subsequent toxicity of malaoxon (MO)

Mice (genotypes as indicated) were exposed dermally to MO alone (A, B), or to CPO followed 4-h later by MO exposure (C, D, E, F). AChE was measured in the brain (A, C, E) and diaphragm (B, D, F) 4-h following the MO exposure. Results represent the mean ± SEM (n=4).

Figure 8. Effect of TOCP exposure (10 mg/kg) on subsequent toxicity of malaoxon (MO)

PON1^+/+mice were exposed dermally to MO alone, or to TOCP followed 24-h later by MO exposure. (A) CaE inhibition in liver and serum was measured 24-h after TOCP exposure. Liver CaE was more sensitive than serum CaE to inhibition by TOCP. AChE was measured in the brain (B) and diaphragm (C) 4-h following the MO exposure. Results represent the mean ± SEM (n=4).