Junshi Miyamoto

Pyrethroids, nerve poisons: how their risks to human health should be assessed

The extensive worldwide efforts of structural modification of natural pyrethrins for better performances have resulted in successful development of a wide variety of synthetic pyrethroids with tremendously high efficacy, knock-down activity or vapor action, and/or with acceptable environmental stability and safety. Currently these pyrethroids including their preferentially manufactured stereoisomers are widely used in agriculture, and for public health as well as household insect control. The detailed toxicology and metabolism studies intended to attain human risk assessment have revealed that with voltage-dependent sodium channel as target site pyrethroids induce pronounced repetitive activity characterized grossly by tremor, hypersensitivity, choleoathetosis, and salivation. In addition, so-called cyano-pyrethroids cause transient skin paresthesia in workers. With regard to tumorigenicity, mutagenicity, teratogenicity and developmental toxicity, no significant findings have been reported. Pyrethroids are eliminated from the animals quite rapidly and completely, undergoing oxidation and ester hydrolysis followed by various conjugations, with low tissue residues. Thus, overall, sound scientific bases exist for human risk assessment under the present usage conditions.

Metabolism of tetramethrin in houseflies and rats in vitro

Abstract Degradation of radiolabeled tetramethrin or 3,4,5,6-tetrahydrophthalimidomethyl dl-trans chrysanthemumate was tested in vitro by using abdomens of SK, lab-em-7-em, R HOKOTA and P y strains of houseflies and rat liver. The effect of NADH 2 and NADPH 2 on the metabolism of tetramethrin by housefly abdomen homogenate was slight, but phosphorothioates, their oxygen analogs, carbamate insecticides, NIA 16388, p -chloromercuribenzoic acid, and mercuric chloride showed marked inhibition. The enzyme activity was localized mainly in the microsomal fraction, where the major metabolites were 3,4,5,6-tetrahydrophthalimide (TPI) (a nonenzymatic reaction from N -(hydroxymethyl) 3,4,5,6-tetrahydrophthalimide, MTI) and chrysanthemumic acid. Smaller amounts of oxidized tetramethrins and chrysanthemumic acid were also produced. The cleavage of tetramethrin into MTI and chrysanthemumic acid was inhibited by such compounds as paraoxon, carbaryl, PCMB, NIA16388, and mercuric chloride. NADPH 2 or NADPH 2 plus carbon monoxide produced little effect. Similar results were obtained with rat liver microsomal fraction. It is presumed from the above findings that the cleavage is catalyzed either by a carboxyesterase or a hydrolase, and that some pyrethroids are metabolized in insects primarily through hydrolytic pathways. Metabolites from oxidative pathways (as in mammals) are formed in minor quantities.

Comparison of inhibitory activity of various organophosphorus compounds against acetylcholinesterase and neurotoxic esterase of hens with respect to delayed neurotoxicity.

Abstract A variety of organophosphorus (OP) compounds with and without delayed neurotoxicity were examined for inhibitory power against neurotoxic esterase (NTE) and acetylcholinesterase (AchE) of hen brain in vitro and in vitro . Generally, delayed neurotoxicity induced by OP compounds correlated with high inhibition of NTE in vitro , whereas in vitro studies comparing I 50 S for both enzymes did not provide a guide to evaluation of delayed neurotoxicity. Single oral administration of delayed neurotoxic EPN, leptophos and TOCP resulted in more than 80 per cent inhibition of brain NTE at neurotoxic doses, whereas non-delayed neurotoxic methyl parathion, fenitrothion and cyanophos caused weak inhibition at near lethal doses which gave rise to severe inhibition of brain AchE. A delayed neurotoxic dose of (−)-EPN caused more severe inhibition of brain NTE as compared with the same dose of the non-delayed neurotoxic ( + )-isomer. However, a few compounds produced severe inhibition of NTE at non-delayed neurotoxic doses. Hens paralysed by repeated administration of a low level of leptophos showed significant decreases in NTE activity of the brain and spinal cord.

Purification and properties of pyrethroid carboxyesterase in rat liver microsome

Abstract Pyrethroid carboxyesterase which hydrolyzes the esters of chrysanthemumic acid was purified from rat liver microsome by cholic acid solubilization, ammonium sulfate fractionation, heat treatment, and DEAE-Sephadex A-50 column chromatography. The 45-fold purified enzyme (38% yield) is likely to consist of single protein, as evidenced by polyacrylamide gel disc electrophoresis and Sephadex G-100 column chromatography, and had a molecular weight of approximately 74,000 and a K m of 0.21 m M . It is susceptible to inhibition by organophosphates and carbamate insecticides and insensitive to p CMB, mercuric ion, and cupric ion. It is capable of hydrolyzing trans isomers of synthetic pyrethroids much more rapidly (five to ten times) than the cis counterparts. The purified pyrethroid carboxyesterase is apparently identical in nature with malathion carboxyesterase and with p -nitrophenyl acetate carboxyesterase.

Metabolic fate of resmethrin, 5-benzyl-3-furylmethyl dl-trans-chrysanthemate in the rat

Abstract Five hundred milligrams per kg of resmethrin or 5-benzyl-3-furylmethyl- dl-trans -chrysanthemate labeled with carbon-14 at 2-C of the furan ring was administered orally to Sprague-Dawley rats. Carbon-14 was rapidly absorbed from the gastrointestinal tract and distributed into the tissues. Only a little intact resmethrin was detected in the tissues. It took 3 weeks for the complete elimination of the radio-activity into urine (36%) and feces (64%). Some of the radioactive metabolites were presumed to be enterohepatically circulated. The urinary metabolites were separated and identified. The predominant one was 5-benzyl-3-furancarboxylic acid, free and bound with glucuronic acid. Other degradation products present were hydroxylation derivatives of 5-benzyl-3-furancarboxylic acid such as α-(4-carboxy-2-furyl)-benzyl alcohol, 5-benzoyl-3-furancarboxylic acid, 5-( p -hydroxybenzyl)-3-furancarboxylic acid, 5-benzyl-4-hydroxy-3-furancarboxylic acid, and sulfate and glucuronide conjugates of 5-( p -hydroxybenzyl)-3-furancarboxylic acid.

Differential metabolism of fenvalerate and granuloma formation: I. Identification of a cholesterol ester derived from a specific chiral isomer of fenvalerate

On a single po administration of the four chiral isomers of fenvalerate ([RS]-alpha-cyano-3-phenoxybenzyl [RS]-2-(4-cholorophenyl)isovalerate) to rats and mice at 2.5 mg/kg, the [2R, alpha S]-isomer showed relatively higher residues in all analyzed tissues as compared with the other three isomers. Similarly, this isomer showed higher tissue concentrations than other isomers when mice were fed a diet containing 500 ppm of the [2S, alpha S]-, and [2R, alpha R]-isomers for 2 weeks. The [2R, alpha S]-isomer produced a lipophilic metabolite in all the examined tissues on the basis of thin-layer chromatography analysis, but not for the other isomers. The amounts of lipophilic metabolite differed among tissues, being higher in adrenal, liver, and mesenteric lymph nodes following feeding to mice at 500 ppm of the [2R, alpha S]-isomer for 2 weeks. However, the amount did not increase proportionally with time and apparently reached a plateau within a rather short time. This metabolite was identified as cholesteryl [2R]-2-(4-chlorophenyl)isovalerate ([2R]-CPIA-cholesterol ester) on the basis of spectroanalysis and chromatographic behavior after purification on silica gel, Florisil, thin-layer, and high-pressure liquid chromatography. The presence of the same metabolite also was indicated in rat tissues. The CPIA-cholesterol ester was rapidly formed and found in all the analyzed tissues of mice 1 hr after a single po administration of the [2R, alpha S]-isomer.

Comparative metabolism of 3,5-Di-tert-butyl-4-hydroxytoluene (BHT) in mice and rats

In male and female DDY/Slc mice given single oral doses (20 or 500 mg/kg body weight) of 3,5-di-tert-butyl-4-hydroxytoluene (BHT) labelled with 14C at the p-methyl group, 14C was distributed mainly in the stomach, intestines, liver and kidney, and then excreted in the urine, faeces and expired air. During the 7 days after treatment, 41-65, 26-50 and 6-9% of the 14C dose was excreted in faeces, urine and expired air, respectively, and the total recovery was 96-98%. Levels of 14C in 21 male and 22 female tissues 7 days after treatment were less than 1 microgram BHT equivalents/g tissue (ppm) in mice given 20 mg/kg and less than 11 ppm in mice given 500 mg/kg. When [14C]BHT was given orally to male mice at 20 mg/kg/day for 10 days, 14C was rapidly excreted and did not exhibit any tendency to accumulate in any tissues. Thin-layer chromatography and high-performance liquid chromatography analyses showed that more than 43 metabolites were present in the urine and faeces of both species, and all of these were identified to determine metabolic pathways for BHT in mice and rats. Major metabolic reactions of [14C]BHT in mice were the oxidation of the p-methyl group attached to the benzene ring and of the tert-butyl groups. The products from the latter reaction were cyclized to some extent by reacting with the adjacent phenolic OH group to give hemiacetals or lactones. The carboxyl derivatives from the p-methyl oxidation were conjugated with glucuronic acid. When single oral doses of 20 or 500 mg [14C]BHT/kg were given to male Sprague-Dawley rats, metabolites similar to those in mice were found. However, the major biotransformation was oxidation of the p-methyl group, and oxidation of the tert-butyl groups was a minor reaction in rats.

Differential metabolism of fenvalerate and granuloma formation: II. Toxicological significance of a lipophilic conjugate from fenvalerate

Male mice of the ddY strain were fed a diet containing the [2S, alpha S]-, [2S, alpha RS]-, [2R, alpha S]-, and [2R, alpha R]-isomers of fenvalerate. Microgranulomatous changes were observed only in mice treated with the [2R, alpha S]-isomer at 125 and 1000 ppm for 1, 2, or 3 months. In contrast, the changes did not occur in mice treated with the [2R, alpha R]-isomer under the same conditions. Feeding of the [2S, alpha S]- and [2S, alpha RS]-isomers for 1 year did not cause the microgranulomatous changes at 500 or 1000 ppm. To clarify the causative agent of granuloma formation, cholesterol ester of 2-(4-chlorophenyl)isovaleric acid (CPIA), a lipophilic conjugate from the [2R, alpha S]-isomer of fenvalerate, was injected iv into ddY mice. Microgranulomatous changes were observed in the liver of mice treated with the [2R]-, [2S]-, or [2RS]-CPIA-cholesterol ester 1 week after a single treatment of 1, 10, or 100 mg/kg, as well as in liver of mice treated with a single dose of 10 or 30 mg/kg of the [2R]-CPIA-cholesterol ester and kept up to 26 weeks afterward. Histochemistry and microscopic autoradiography of the liver of mice demonstrated the presence of tritium derived from 3H-labeled[2R]-2-(4-chlorophenyl)isovalerate and cholesterol. Histochemistry also was positive for cholesterol ester in livers of mice treated with the [2R, alpha S]-isomer of fenvalerate. These results lend support for the hypothesis that CPIA-cholesterol ester is the causative agent of microgranulomatous changes induced by fenvalerate.

Comparative metabolism of fenitrothion in aquatic organisms: I. Metabolism in the euryhaline fish, Oryzias latipes and Mugil cephalus

[14C]Fenitrothion at 0.1 ppm in running water is more rapidly absorbed in the killifish (Oryzias latipes) at 25 than at 15 degrees C to a similar plateau level, and bioaccumulation ratios of fenitrothion are 235 and 339, respectively. Water of higher salinity (23%) resulted in slightly higher accumulation ratios of fenitrothion in both killifish (303) and mullet, Mugil cephalus (179), than fresh water (235 and 30, respectively), but the half-lives are independent of temperature and salinity, with values of 0.24-0.36 day. Fenitrothion was metabolized primarily through hydrolysis by the killifish, demethylation by the mullet, and conjugation of the liberated phenol with glucuronic acid by both species. Although metabolism of the compound was not affected by the different salinities and temperatures in both fish, the glucuronide conjugate was more directly excreted into water under lower temperature and higher salinity conditions. 14C-labeled compounds are distributed primarily in the gall bladder as shown by whole-body radioautography.

Metabolism of tetramethrin in houseflies in vivo

Abstract Houseflies susceptible to pyrethroids were found to metabolize in vivo radioactive tetramethrin (phthalthrin) or 3,4,5,6-tetrahydrophthalimidomethyl dl-trans chrysanthemumate mainly to N -(hydroxymethyl) 3,4,5,6-tetrahydrophthalimide and chrysanthemumic acid. Smaller amounts of oxidation products of tetramethrin or chrysanthemumic acid were formed. Pretreatment with piperonyl butoxide had little effect on the cleavage of the ester bond of tetramethrin, but another synergist NIA 16388 remarkably inhibited this reaction.