Studies on the metabolism of the pneumotoxin O,S,S-trimethyl phosphorodithioate--I. Lung and liver microsomes

The role of phosphotriesterases in the detoxication of organophosphorus compounds

The enzymes that hydrolyze organophosphorus compounds are called phosphotriesterases. The presence of phosphotriesterases has been described in a variety of tissues. The physiological role of these enzymes is not known, although a clear correlation exists between the levels of phosphotriesterases and susceptibility of the species to the toxic effects of organophosphorus compounds. Thus, mammals that possess high levels of phosphotriesterases in serum and liver are more tolerant to the toxic effects of these compounds than birds and insects - these being species considered lacking of phosphotriesterases. Because most of these enzymes are not well characterized, they are usually differentiated according to their different patterns of response to activators and/ or inhibitors. Phosphotriesterases usually depend of divalent cations and therefore EDTA usually inhibits them. A peculiar EDTA-resistant phosphotriesterase has been described in serum albumin. The biotechnological and therapeutical applications of phosphotriesterases are currently subject to study.

A direct involvement of the central nervous system in hypophagia and inhibition of respiratory rate in rats after treatment with O,O,S-trimethyl phosphorothioate

O,O,S-Trimethyl phosphorothioate (OOS-TMP) is known to induce unique symptoms, which are characterized by hypophagia, progressive weight loss, and hypothermia. To determine whether there is the possibility of a causal relationship between these toxic symptoms and a direct action of OOS-TMP on the central nervous system, we investigated the development of these symptoms in Fischer 344 female rats after oral or intracerebral treatment with OOS-TMP. Oral administration of OOS-TMP at 20 mg/kg induced marked hypophagia, progressive weight loss and hypothermia. Moreover, inhibition of respiratory rate was observed immediately after treatment. It lasts during the entire experimental period. Profound hypothermia below 34 degrees C was observed more frequently in the rats, which became hypercapnic (PaCO2 > or = 50 mmHg). In contrast, administration of OOS-TMP at 20 mg/kg (as much as the oral dose) into the cerebral lateral ventricle succeeded in inducing hypophagia, progressive weight loss and lowered respiratory rates. On the other hand, by this route of administration, OOS-TMP at 20 mg/kg failed to induce hypothermia, hypercapnia and lung injury. The present results suggest that hypophagia and inhibitions of respiratory rate are attributable to the direct action of OOS-TMP on the central nervous system, while other symptoms are associated with lung injury.

Enhanced levels of lipid peroxidation and xanthine oxidase activity in the lung of male Sprague-Dawley rats following treatment with O,O,S-trimethyl phosphorothioate

The purpose of this study was to investigate the roles of lipid peroxidation and a prototypical radical generating enzyme, xanthine oxidase, in lung injury caused by O,O,S-trimethyl phosphorothioate (OOS-TMP). Animals (five per group) were dosed with OOS-TMP at 40 mg/kg and sacrificed on the 1st, 3rd or 7th day after treatment. OOS-TMP increased lipid peroxidation (Mean +/- S.E.) to 139 +/- 9.6% of the control values in the lung and to 623 +/- 203% in the liver on the 1st day. When rats were dosed with OOS-TMP at 20, 40 and 60 mg/kg, lipid peroxidation in the lung and the liver were increased in a dose-dependent manner. In the lung, the total activity of xanthine oxidase was coincidentally increased. In contrast, the activities of superoxide dismutase and catalase were not affected. Effects of OOS-TMP on lipid peroxidation and the total activity of xanthine oxidase were completely abolished by coadministration with O,O,O-trimethyl phosphorothionate of a non-toxic dose (1 mg/kg), which antagonizes the lung injury after treatment with OOS-TMP. The present results indicate that free radical formation may be involved in lung injury after OOS-TMP treatment through activation of radical producing enzymes such as xanthine oxidase.

Modification of testicular cytochrome P-450 after fenitrothion administration

1. The effect of fenitrothion on testicular microsomal system was analyzed and compared to the modifications induced by the drug at the hepatic level. 2. Acute (165 mg/kg, 1 day) and subacute (55 mg/kg, 3 days) administration of fenitrothion caused a significant decrease on testicular cytochrome P-450 content (51 and 50% respectively) but no modification on cytochrome b5 and NADPH cytochrome c reductase. No changes were induced by fenitrothion in testicular microsomal system after chronic treatment (5.5 mg/ml, 30 days). 3. Results obtained in the liver were very similar to those observed in testis even though the percentage of cytochrome P-450 inhibition obtained after acute and subacute drug administration (45 and 43%) was smaller. 4. In addition, changes in testosterone blood concentrations were also analyzed. A significant reduction of hormone plasma levels were detected at 165 and 55 mg/kg doses of fenitrothion (98% in both cases). Fenitrothion was not able to modify the levels of testosterone in the blood after chronic administration.

Effects of pneumotoxic trialkylphosphorothioates on the pentose phosphate pathway in rat lung slices

The extent of glucose oxidation via the pentose phosphate pathway (PPP), a possible indicator of oxidative stress, was assessed in rat lung slices by measuring the production of 14CO2 on incubation with either [6-14C]glucose or [1-14C]glucose. Incubation in the presence of the pneumotoxins O,S,S-trimethyl phosphorodithioate (OSSMe, 10(-5) -10(-3) M), O,O,S-triethyl phosphorothioate (10(-4) -10(-2) M) or S,S,S-trimethyl phosphorotrithioate (10(-4) -10(-2) M) resulted in a significant, but small stimulation of PPP (max. 276%), compared to that found in the presence of the model oxidant, paraquat (1089%, 10(-5) M). Following in vivo treatment with an LD50 dose of OSSMe, PPP was unchanged (after 6 h) or decreased (after 24 h) compared to saline-treated rats. PPP was also decreased in slices which had been preincubated with OSSMe (10(-3) M) and then transferred to fresh medium. Following treatment with phorone (250 mg/kg i.p.) pulmonary levels of non-protein sulfhydryls were first reduced (20% of control at 3 h) and then increased (288% of control at 24 h), but at neither time was lung slice PPP activity affected, thus suggesting that in the rat lung PPP activity does not directly depend on pulmonary glutathione content.

Studies on the metabolism of the pneumotoxin O,S,S-trimethyl phosphorodithioate--II. Lung and liver slices

The metabolism of O,S,S-trimethyl phosphorodithioate (OSSMe), a pneumotoxic impurity in some organophosphorus insecticides, was investigated by incubating rat lung and liver slices with 1 mM OSSMe, labelled with 3H or 14C on one of its thiolo-methyl (CH3S-) groups. Protein bound radioactivity was higher in lung slices than in liver slices. In lung slices the predominant diester produced was O,S-dimethyl phosphorothioate (OSMeO-), whereas in liver slices it was S,S-dimethyl phosphorodithioate (SSMeO-). Other studies had shown binding of radioactivity and OSMeO- production to be cytochrome P-450-dependent processes in microsomes and SSMeO- production to result from the action of cytosolic glutathione-S-transferase on OSSMe. Preincubation of slices with 10(-5) M paraoxon did not influence the amount of protein-bound radioactivity, suggesting that binding of radioactivity did not simply result from protein phosphorylation. Pretreatments of the rats with O,O,O-trimethyl phosphorothioate [OOOMe(S) 0.5, 2.5 and 12.5 mg/kg p.o.], with p-xylene (1 g/kg, i.p.) or with bromophos (5.3 mg/kg, i.p.) which all protect against the lung toxicity of OSSMe probably by inhibiting pulmonary mixed-function oxidase, also led to significant decreases in both protein binding of radioactivity and OSMeO- production in lung slices, but not in liver slices. These results show that tissue slices are a convenient system for investigating xenobiotic metabolism in the lung and they suggest that the susceptibility of the lung to OSSMe probably results from a relatively high rate of activation, coupled with a relatively low rate of metabolism by non-toxic pathways and/or removal of reactive metabolites in some lung cells, possibly the alveolar type I cells.