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Studies on the metabolism of the pneumotoxin O,S,S-trimethyl phosphorodithioate--I. Lung and liver microsomes
1Medical Research Council, Toxicology Unit, Carshalton, Surrey, U.K.
Abstract:
The metabolism of O,S,S-trimethyl phosphorodithioate (OSSMe), a pneumotoxic impurity in some organophosphorus insecticides, was investigated in rat lung and liver microsomal preparations, using OSSMe labelled with 3H or 14C on one of its thiolo-methyl (CH3S-) groups. Production of O,S-dimethyl phosphorothioate (OSMeO-) and binding of radioactivity to protein were NADPH-dependent and were shown to be, at least partly, cytochrome P-450-dependent processes in both lung and liver microsomes. Incubation with reduced glutathione prevented the binding of radioactivity without affecting OSMeO- production. The Km for the conversion of OSSMe to OSMeO- was 15-fold lower in lung (0.30 +/- 0.07 mM) than in liver (4.63 +/- 2.42 mM) microsomes. These results show that cytochrome P-450-dependent mixed-function oxidase is implicated in at least part of the metabolic activation of OSSMe, and suggest that the pulmonary isozyme(s) are more active at metabolizing OSSMe than hepatic isozymes. It is speculated, on the basis of literature data on other sulphur-containing chemicals, that the metabolic activation of OSSMe involves oxidation of a thiolo-sulphur, with subsequent formation of CH3-S-S-protein disulphides.
Insights
O,S,S-trimethyl phosphorodithioate (OSSMe) metabolism was studied in rats. Lung enzymes are more active than liver enzymes in activating this pneumotoxic compound, suggesting a role for pulmonary cytochrome P-450.
Area of Science:
- Biochemistry
- Toxicology
- Pharmacology
Background:
- O,S,S-trimethyl phosphorodithioate (OSSMe) is a pneumotoxic impurity found in organophosphorus insecticides.
- Understanding OSSMe metabolism is crucial for assessing its toxicity and developing safety measures.
Purpose of the Study:
- To investigate the metabolic pathways of OSSMe in rat lung and liver microsomes.
- To identify the enzymes involved in OSSMe activation and protein binding.
Main Methods:
- In vitro incubation of radiolabeled OSSMe with rat lung and liver microsomes.
- NADPH-dependent assays to measure metabolite production and protein binding.
- Effect of reduced glutathione on OSSMe metabolism.
Main Results:
- OSSMe metabolism and protein binding were NADPH- and cytochrome P-450-dependent in both lung and liver.
- Reduced glutathione inhibited protein binding but not metabolite production.
- The enzyme kinetics (Km) indicated significantly higher activity for OSSMe conversion in lung microsomes compared to liver microsomes.
Conclusions:
- Cytochrome P-450-dependent mixed-function oxidases play a role in the metabolic activation of OSSMe.
- Pulmonary isozymes are more efficient in metabolizing OSSMe than hepatic isozymes.
- Metabolic activation likely involves thiolo-sulfur oxidation, potentially forming protein disulfides.
