Trialkyl phosphorothioates and glutathione S-transferases

The inhibitory effect of neuropathic organophosphate esters on neurite outgrowth in cell cultures: A basis for screening for delayed neurotoxicity

Organophosphates have previously been tested for the induction of delayed neuropathy in adult hens. An alternative in vitro test, which avoids the severe suffering caused by the test in hens, has been developed using permanent cell lines from a rat-brain glioma (C-6) or from a mouse-brain neuroblastoma (N-18). Addition of dibutyryl cAMP to these cell cultures triggers the development of neurite-like processes; the development of these processes is inhibited by the addition of various organophosphate compounds and this inhibition serves as an indicator of neurotoxicity. 26 compounds with positive results in the in vivo test in hens, and eight analogues with negative results were tested in vitro. An almost perfect correlation between the in vivo and in vitro results was found; two compounds with weak positive results in vitro (O,O,S-trimethyl phosphorothioate and O,S,S-trimethyl phosphorothioate) yielded negative results in hens but this discrepancy may be related to their bioavailability in hens. The in vitro test is recommended to avoid the pain and stress caused by testing in vivo in hens.

Effects of subacute administration of O,S,S-trimethyl phosphorodithioate on cellular and humoral immune response systems

The effects of 14-day treatment with low doses of O,S,S-trimethyl phosphorodithioate (OSS-TMP), an impurity in technical malathion, on the generation of cell-mediated and humoral immune responses were examined in female C57BL/6 mice. At a dose of 2.0 mg/kg per day OSS-TMP, the generation of antibody-secreting cells to sheep red blood cells, the generation of cytotoxic T lymphocytes (CTL) to alloantigen and the production of Interleukin-2 were elevated approximately 2-3 fold, while no changes were observed in the proliferative responses to the polyclonal activators, Concanavalin A, lipopolysaccharide, or phytohemagglutinin. In contrast, at 5.0 mg/kg per day OSS-TMP, both the CTL and specific antibody responses were suppressed, while all other immune parameters examined were unchanged. Data from cell separation and reconstitution experiments indicated that both T and B lymphocytes were affected by these treatment regimes. These data suggest that long-term exposure to low doses of OSS-TMP may enhance the ability of an animal to generate an immune response while higher doses of OSS-TMP may suppress the generation of an immune response.

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.

The interaction between phosphorothionate insecticides, pneumotoxic trialkyl phosphorothiolates and effects on lung 7-ethoxycoumarin O-deethylase activity

A number of phosphorothionate (P = S) insecticides, including bromophos and fenitrothion, prevent trialkyl phosphorothiolate (P = O)-induced lung toxicity and the resulting increase in lung weight normally observed at 3 days in the rat. Measurement of 7-ethoxycoumarin O-deethylase (7-EC) activity after both phosphorothionate and phosphorothiolate dosing revealed differing patterns of loss of enzyme activity. Depletion of 7-EC activity by phosphorothionates was maximal between 2 and 10 h after dosing, with recovery between 24 and 72 h. Phosphorothiolates, however, appear to cause two phases of loss of 7-EC activity, an initial fall of approximately 30% observed at 2 h and a secondary fall, maximal on day 3, with loss of 97% of activity, apparently associated with the pathological changes in the lung. It is suggested that oxidative metabolism of phosphorothionates known to occur at the P = S moiety, with suicidal loss of P-450, may then prevent oxidative activation of an S-methyl on the phosphorothiolates, the most likely site for production of a reactive intermediate capable of damaging the lung. Lung 7-EC in rat is sensitive to concentrations of the phosphorothionates bromophos and fenitrothion at 5-25 times less than those causing loss of liver 7-EC activity and at doses 125-600 times less than their LD50s. If repeated in man this may have implications for personnel occupationally exposed to these compounds.

Effects of O,S,S-trimethyl phosphorodithioate on immune function

The effect of acute administration of 20-80 mg/kg O,S,S-trimethyl phosphorodithioate (OSS-TMP) to C57BL/6 female mice on the murine immune system was determined. The parameters examined to evaluate overt toxicity of the compound included body weight, plasma cholinesterase levels, splenic nucleated cell number and thymic weight and nucleated cell number. Acute administration of 60 or 80 mg/kg OSS-TMP led to a 75 or 63% decrease, respectively, in plasma cholinesterase levels and a decrease in thymic size. At a dose of 80 mg/kg OSS-TMP, the animals also exhibited some lethargy and body weight loss. Below 60 mg/kg OSS-TMP, no overt toxic manifestations were observed. These studies were carried further to determine the effect of OSS-TMP on the generation of in vivo primary and in vitro secondary cellular and humoral immune responses. At nontoxic doses of the compound, i.e. 20 and 40 mg/kg OSS-TMP, the in vivo generation of a primary cytotoxic T lymphocyte (CTL) response to alloantigen was significantly elevated, but this response was unaffected following restimulation of the splenocytes by alloantigen in vitro. The generation of an in vivo primary and in vitro secondary humoral responses to sheep red blood cells (SRBC) was elevated following a single dose of 40 mg/kg OSS-TMP. Administration of toxic doses of OSS-TMP, i.e. 60 and 80 mg/kg, did not alter the ability of splenocytes to generate a primary or secondary CTL response, but suppressed the generation of humoral immune responses. These results differ significantly from those observed in a similar system following acute administration of a structural analog, O,O,S-trimethyl phosphorothioate which was previously shown to have potent immunosuppressive activity at nontoxic doses.

Enzyme-catalyzed detoxication reactions: mechanisms and stereochemistry

Enzyme catalyzed detoxication reactions are one of the primary defenses organisms have against chemical insult. This article reviews current chemical approaches to understanding the cooperative role of enzymes in the metabolism of foreign compounds. Emphasis is placed on chemical and stereochemical studies which help elucidate the mechanism of action and active-site topologies of the detoxication enzymes. The stereoselectivity of the cytochromes P-450 and flavin containing monooxygenases as well as the role of hemoglobin and lipid peroxidation in the primary metabolism of xenobiotics is discussed. Current knowledge of the mechanism and stereoselectivity of epoxide hydrolase is also presented. Three enzymes involved in secondary metabolism of xenobiotics, UDP-glucuronosyltransferase, sulfotransferase and glutathione S-transferase are discussed with particular emphasis on active site topology and cooperative participation with the enzymes of primary metabolism.