T. Roy Fukuto

Toxicological properties of phosphorothioate and related esters present as impurities in technical organophosphorus insecticides

O,O,S-Trimethyl phosphorothioate, an impurity in several technical organophosphorus insecticides, when administered orally to rats at single doses as low as 15 mg/kg caused delayed mortality, with death occurring 4-22 d after treatment. Delayed toxic signs were also observed in mice, but mice were generally less sensitive than rats. O,O,S-triethyl phosphorothioate and O,S,S-trimethyl phosphorodithioate induced the same signs of intoxication at slightly higher doses. Rats treated with O,O,S-trimethyl phosphorothioate refused food and water within 24 h after treatment and did not eat or drink until the time of death. Neither injection of nutrient solution nor atropine served to reduce or block intoxication. However, the isomeric O,O,O-trimethyl phosphorothioate was a potent antagonist of the toxicity of O,O,S-trimethyl phosphorothioate. As little as 1% of the O,O,O-trimethyl isomer protected rats from the intoxicating effects of the O,O,S-trimethyl isomer at doses as high as 200 mg/kg. Rat serum carboxylesterase and cholinesterase were inhibited for prolonged periods after a single oral dose of O,O,S-trimethyl phosphorothioate, but the duration of inhibition was significantly less when the toxicant contained 1% O,O,O-trimethyl isomer.

Studies on the chiral isomers of fonofos and fonofos oxon: I. Toxicity and antiesterase activities☆

Abstract The toxicity of the ( R ) P and ( S ) P chiral isomers and racemates of fonofos and fonofos oxon to insects and white mice were determined. ( R ) P -Fonofos and ( S ) P -fonofos oxon were 2- to 12-fold more toxic to house flies, mosquito larvae, and mice than were the corresponding enantiomers. The racemates were intermediate in toxicity. Stereoselectivity also was observed in the in vitro inhibition of house fly-head and bovine erythrocyte acetylcholinesterase, horse serum cholinesterase, chymotrypsin, trypsin, and a variety of esterases. In all cases the ( S ) P -oxon was a more potent inhibitor than the ( R ) P -oxon with k 1 ratios of (S) P (R) P ranging from 4- to 60-fold. Further, differences in levels of house fly-head, mouse brain, and blood cholinesterase obtained from house flies and mice treated with the enantiomers and racemates of fonofos and fonofos oxon were observed. Differences in toxicity of the enantiomers and racemates to house flies and mice were more closely related to in vivo than to in vitro cholinesterase inhibition.

Selective inhibition of rat pulmonary monooxygenase by O, O, S-trimethyl phosphorothioate treatment

The effects of oral administration of O,O,S-trimethyl phosphorothioate (OOS), an impurity present in widely used organophosphorus insecticides, were studied using pulmonary and hepatic microsomal enzymes of rats. The animals were treated with OOS at 10,20 and 40 mg/kg, and were killed on day 3 after treatment. Their relative lung weights increased markedly at 20 and 40 mg/kg, increasing 94% at the highest dose, whereas the weight of liver decreased. At 20 mg/kg OOS, the cytochrome P-450 content of the lung and liver decreased to 83 and 80% of the control levels respectively. Pulmonary microsomal 7-ethoxycoumarin (7-Ec) O-deethylase decreased in a dose-dependent manner; activities were less than 10% of control at the 40 mg/kg dose. The activity of pulmonary coumarin hydroxylase also decreased following OOS treatment, but the decrease was not dose-dependent since no activity was detectable at doses over 10 mg/kg. In contrast, the effect of OOS treatment on hepatic monooxygenase activity was moderate. 7-Ec deethylase activity was not affected by OOS treatment at any dose level, while p-nitroanisole (p-NA) demethylase activity was decreased only at the 40 mg/kg dose of OOS. Pulmonary malathion carboxylesterase activity was not affected by OOS treatment. In contrast, a dose-dependent decrease was observed in the liver carboxylesterase. Time course effects of OOS treatment on these parameters were examined by treating rats at 20 mg/kg. The animals were killed 0.5, 1,3 and 7 days after the treatment. The 7-Ec deethylase activity of pulmonary microsomes was decreased on days 0.5, 1 and 3 after treatment, the maximum decrease being observed on day 1. Significant decreases were not observed in hepatic microsomal activities of 7-Ec deethylase or p-nitroanisole demethylase throughout the experimental period; rather, these activities were higher on day 7. Hepatic microsomal malathion carboxylesterase was lower on days 0.5, 1 and 3 after OOS treatment.

Metabolism of carbosulfan [2,3-dihydro-2,2-dimethylbenzofuran-7-yl (di-n-butylaminothio)methylcarbamate] in the rat and house fly

Abstract The metabolism of carbosulfan, 2,3-dihydro-2,2-dimethylbenzofuran-7-yl (di- n -butylaminothio)-methylcarbamate, was studied in the rat and house fly. Carbosulfan was metabolized via at least two major pathways in the rat, by initial oxidation of the sulfur atom and by NS bond cleavage. The principal rat metabolites were the conjugated keto-phenol from the ring-labeled carbo-sulfan, carbon dioxide from the carbonyl-labeled carbosulfan, and dibutylamine from the dibutylamino-labeled carbosulfan. In house flies, carbosulfan was converted primarily into carbofuran and related oxidation products. The lower mammalian toxicity of carbosulfan compared to its insecticidal activity is explained on the basis of differences in routes and rates of metabolism of carbosulfan in mammals and insects.

Structure for the oxygenated product of peracid oxidation of Dyfonate® insecticide (O-ethyl S-phenyl ethylphosphonodithioate)

Abstract Reaction of O -ethyl S -phenyl ethylphosphonodithioate (Dyfonate®) with m -chloroperbenzoic acid gives phenyl ethoxy(ethyl) phosphinyl disulfide and not the structure proposed by McBain et al . (1). This assignment of structure is based on unambiguous synthesis as well as on interpretation of spectral and chemical evidence. Contrary to the earlier report, this product is probably not an intermediate in enzymatic oxidation of dithionophosphorus compounds in microsomal systems.

Delayed toxicity and delayed neurotoxicity of phosphorothioate and phosphonothioate esters

The delayed neurotoxicity to hens and delayed toxicity to rats of the isomeric trimethyl phosphonothioates, trimethyl phosphate, and a series of the methyl and ethyl esters of methyl-, ethyl-, and phenylphosphonate and phosphonothioates were examined. All the O,O-dialkyl phosphonothioates, phosphorothioates, and their corresponding oxons were relatively nontoxic to rats, with oral LD50 values greater than the 150-450 mg/kg tested. The O,S-dialkyl phosphorothioate esters were highly acutely toxic. The rat acute LD50 values for O,S-dimethyl methylphosphonothioate and O,S-diethyl ethylphosphonothioate were 3 and 8 mg/kg. O,S-Diethyl ethylphosphonothioate and O,O, S-trimethyl phosphorothioate were the only compounds tested that showed delayed toxicity to rats. The delayed LD50 values for these two compounds were 7 and 15-20 mg/kg, respectively, with rats dying 3-22 d after treatment The delayed toxic effects were associated with continual loss of weight, reaching 18-46% at the time of death. Of this series of compounds, only O,O-diethyl phenylphosphonothioate and its oxon showed delayed neurotoxicity to hens 45 d after treatment. The minimum effective dose for these two compounds was 25 mg/kg.d administered ip for 10 d. These findings suggest that delayed neurotoxicity in hens is not due to the same mechanism as delayed toxicity in rats.

Metabolism of 2,3-dihydro-2,2-dimethyl-7-benzofuranyl (di-n-butylaminosulfenyl)(methyl)carbamate and 2,3-dihydro-2,2-dimethyl-7-benzofuranyl (morpholinosulfenyl)(methyl)carbamate in cotton and corn plants☆

Abstract The absorption, translocation, and metabolism of two new, selectively toxic derivatives of carbofuran, 2,3-dihydro-2,2-dimethyl-7-benzofuranyl (di- n -butylaminosulfenyl)(methyl)carbamate and 2,3-dihydro-2,2-dimethyl-7-benzofuranyl (morpholinosulfenyl)(methyl)carbamate, were studied in cotton and corn plants 1, 3, 6, and 10 days following both stem injection and foliage treatment. Both carbamates were readily translocated to all plant parts following stem injection, but translocation following leaf application was restricted to within the leaf. In cotton plants, the dibutylaminosulfenyl derivative was easily hydrolyzed to form carbofuran which, in turn, was oxidized at the 3-position of the ring and the N -methyl group. These oxidized metabolites were then converted to plant conjugates. Major metabolites were carbofuran and 3-hydroxy-carbofuran followed by 3-keto-carbofuran phenol and N -hydroxymethyl-carbofuran. Five minor metabolites also were detected. In corn plants, the dibutylaminosulfenyl derivative gave the same metabolites, although the metabolism rate was significantly slower in corn relative to cotton. Overall, the results showed that there were no fundamental differences in the metabolism of the morpholinosulfenyl and dibutylaminosulfenyl derivatives. The stability of both carbamate derivatives in different solvent systems also was investigated.

Inhibition of rat liver and plasma carboxylesterases by impurities present in technical phenthoate

Phenthoate [O,O‐dimethyl S‐(α‐ethoxycarbonyl)benzyl phosphorodithioate] was rapidly hydrolyzed by rat liver and plasma carboxylesterases to the corresponding nontoxic metabolite, phenthoate acid. A partially purified enzyme isolated from rat liver microsomes was sevenfold more effective in hydrolyzing phenthoate than the microsomal fraction. O,S,S‐Trimethyl phosphorodithioate (TMPDT) and O,O,S‐trimethyl phosphorothioate (TMPT), two impurities present in technical formulations of phenthoate, were examined for their inhibiting effects on the esterase degradation of [phenyl‐14C] phenthoate in vitro. Incubation of [14C]phenthoate with rat liver and plasma carboxylesterases in the presence of these impurities greatly diminished the amount of phenthoate acid formed. TMPDT was superior in its inhibitory action against rat liver carboxylesterase to that of TMPT. TMPDT was equipotent in inhibiting crude rat liver and plasma carboxylesterases, and TMPT was more effective in inhibiting plasma carboxylesterase than r...

Penetration and metabolism of 2-isopropoxyphenyl N-methyl-N-(2-methyl-4-tert-butylphenylsulfenyl) carbamate in the house fly and honeybee

Abstract The penetration and metabolism of 2 - isopropoxyphenyl N -methyl- N -(2-methyl-4-tert - butylphenylsulfenyl)carbamate or sulfenyl-propoxur was examined in the house fly and honeybee. Reduced penetration was found to be a factor contributing to the lower toxicity of sulfenyl-propoxur to the honeybee. Honeybees and house flies metabolized sulfenyl-propoxur qualitatively in a similar manner. Quantitatively, larger amounts of propoxur were found in the house fly than in the honeybee soon after treatment with sulfenyl-propoxur. The slower rate of conversion of sulfenyl-propoxur to propoxur was considered as another factor responsible for the lower toxicity of the sulfenylated derivative to bees. The high susceptibility of bees to propoxur was related to high internal amounts of unchanged propoxur found soon after treatment.

Dysproteinuria induced in rats by O,O,S-drimethyl phosphorothioate

Administration of a single oral dose of 60 mg/kg O,O,S-trimethyl phosphorothioate (OOS-Me), a malathion impurity, resulted in a substantial increase in the amounts of amino acids along with a change in the nature of proteins excreted in the urine of treated rats. In contrast to control rats, a small increase in albumin and a small decrease in alpha 1-globulin were observed. However, alpha2-, beta- and gamma 1-globulin, which were not detected in the urine of control rats, were found in substantial amounts in the urine of OOS-Me-treated rats. These findings, coupled with observed increases in urinary glucose levels and consistent specific gravity readings of 1.01 even though treated rats were experiencing oliguria, provide evidence for OOS-Me-induced kidney tubule damage.