Substrate-specificity and toxicological significance of pyrethroid-hydrolyzing esterases of mouse liver microsomes

An esterase or esterases in acetone powder preparations of mouse liver microsomes hydrolyze the cyclopropanecarboxylate ester linkage of pyrethroid insecticide chemicals derived from primary alcohols. The rate of cleavage of (+)-trans-chrysanthemates with various alcohol moieties decreases in the following order: 5-propargyl-2-furylmethyl; 5-benzyl-3-furylmethyl (bioresmethrin); 3-phenoxybenzyl; tetrahydrophthalimidomethyl esters. The hydrolysis rate of benzylfurylmethyl esters with various acid moieties decreases in the order: (+)- or (?)-trans-chrysanthemate; (+)-trans-ethanochrysanthemate; tetramethylcyclopropanecarboxylate; (+)- or (?)-cis-chrysanthemate or (+)-cis-ethanochrysanthemate. The trans-isomers of chrysanthemates and ethanochrysanthemates are hydrolyzed from 2.6- to more than 50-fold more rapidly than the corresponding cis-isomers. This enzyme system does not hydrolyze secondary alcohol esters, i.e., allethronyl (+)-trans- and (+)-cis-chrysanthemates.On intraperitoneal administration to mice, the (+)-trans-chrysanthemate and -ethanochrysanthemate of benzylfurylmethanol are of very low toxicity relative to the corresponding (+)-cis-isomers and the tetramethylcyclopropanecarboxylate. S,S,S-tributyl phosphorotrithioate (DEF) pretreatment increases the toxicity of these five compounds by 2.6- to more than 188-fold, with the exception of bioresmethrin whose toxicity is not altered. When the toxicity is increased, it is probably the result of esterase inhibition since DEF strongly inhibits the esterase activity of fresh liver microsomes while the mixed-function oxidase system remains active. The oxidase system metabolizes the chrysanthemates more rapidly than the ethanochrysanthemates of benzylfuryl-methanol. Depending upon the pyrethroid involved, the esterase or the mixed-function oxidase system, or both may be responsible for limiting the toxicity of these pyrethroids to mice.     

Synthetic pyrethroids: Toxicity and synergism on dietary exposure of Tribolium castaneum (Herbst) larvae

The potency of six dietary pyrethroids, as toxicants and inhibitors of weight gain in first- and fourth-instar Tribolium castaneum (Herbst) larvae, decreased in the order of cis-cypermethrin and deltamethrin > trans-cypermethrin and cis-permethrin > fenvalerate and trans-permethrin. Dosages that reduced larval weight also delayed pupation and emergence, probably due to their antifeeding activity. Three oxidase inhibitors (piperonyl butoxide, O, O-diethyl O-phenyl phosphorothioate, and O-isobutyl O-prop-2-ynyl phenylphosphonate), at a dietary concentration of 100 mg kg^?1, had little or no effect on the toxicity of trans-permethrin, but strongly synergised the toxicity of cis-cypermethrin by about 3-, 3- and 10-fold, respectively. Piperonyl butoxide also synergised the toxicity of cis-permethrin, trans-cypermethrin and deltamethrin, but not that of fenvalerate. On the other hand, an esterase inhibitor, profenofos, did not enhance the potency of any of the α-cyano-3-phenoxybenzyl pyrethroids. Oxidases appear to be more important than esterases in pyrethroid detoxification by T. castaneum larvae.     

拟除虫菊酯类农药的免疫毒性研究进展

拟除虫菊酯类杀虫剂因其高效、对哺乳动物低毒和易降解等特性已被广泛使用,目前对其生态毒理学的研究主要集中在神经及内分泌干扰、发育和生殖毒性方面,而关于其免疫毒性的研究也越来越受到了关注。文章对拟除虫菊酯类农药的免疫毒性及其分子机制研究进展进行了总结,主要从免疫器官、免疫细胞、免疫分子和免疫功能等方面综述了此类杀虫剂对不同生物的免疫毒性及其可能的分子机制,同时总结了拟除虫菊酯类杀虫剂暴露与免疫相关疾病发生的关联,以期为拟除虫菊酯类农药免疫毒性的进一步探究提供参考。     

The toxicity of mptp (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) to Periplaneta americana l. and its effects on dopamine in the cerebral ganglia

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and MPP⁺ (1-methyl-4-phenylpyridinium ion) are potent dopaminergic neurotoxins in mammals. The mammalian toxicity of MPTP depends on its conversion, by monoamine oxidase, to MPP⁺. MPTP is toxic to cockroaches (LD₅₀ 720 μg gm^?1) and the results suggest that MPTP toxicity depends on monoamine oxidase activity at a site outside the nervous system. MPTP depletes dopamine from cockroach cerebral ganglia and MPP⁺ inhibits cockroach mitochondrial respiration. While the biochemistry of MPTP toxicity appears to be the same as in mammals it seems that insects are unable to detoxify MPTP before its action has fatal consequences outside the nervous system.     

Pyrethroids and terrestrial non-target organisms

The discovery and development of the pyrethroid insecticides represents a major advance in the techniques of crop protection and disease vector control. These compounds combine outstanding efficacy against a broad spectrum of noxious insects with low toxicity to birds and mammals. This paper considers the effects of pyrethroids on other components of the terrestrial non-target fauna, with particular reference to effects on ‘beneficial’ organisms, including natural pest-control agents, pollinators, and organisms responsible for the maintenance of soil structure and fertility. This paper uses laboratory and field data to identify which groups of organisms may be potentially at risk from the use of pyrethroids, and draws on extensive field data to consider the significance of such hazards under conditions of normal use. Emphasis will be placed throughout on the evaluation of these compounds under practical conditions, including both crop and non-crop usages. Limitations to, and opportunities for, the use of pyrethroids are discussed, and the potential for further development is considered.     

Esterase inhibitors as synergists for (+)-trans-chrysanthemate insecticide chemicals

Four synergists are used to evaluate the relative contribution of esterases and oxidases in the metabolism of four pyrethroids, the (+)-trans- and (+)-cis-isomers of resmethrin and tetramethrin, by five insect species and by mice. Three of these compounds are known pyrethroid synergists, S,S,S-tributyl phosphorotrithioate acting as an esterase inhibitor and piperonyl butoxide and O-(2-methylpropyl) O-(2-propynyl) phenylphosphonate acting as oxidase inhibitors. The fourth synergist, 1-naphthyl N-propylcarbamate, is an esterase inhibitor selected by screening 65 candidate esterase and oxidase inhibitors for maximal potency in synergizing the toxicity of trans-resmethrin to milkweed bugs. Naphthyl propylcarbamate synergizes the toxicity of trans-resmethrin and -tetramethrin to milkweed bugs, cockroaches, houseflies, cabbage loopers, and mealworms but not to mice. The persistence of trans-resmethrin in milkweed bugs treated by injection is increased by the esterase inhibitors while that of cis-resmethrin is increased by the oxidase inhibitors. The optimal synergist varies with the species and the pyrethoid, being related to both the nature of the pyrethroid alcohol moiety and the trans- or cis-configuration of the acid moiety. This probably results from species variations in the relative significance of esterases and oxidases in pyrethroid detoxification.     

Can selective peptides be combined efficiently with agrochemicals? A new approach to insect control

Sodium channels have been a major target for the development of insecticides such as synthetic pyrethroids. However, insecticides currently available induce resistance and present limited selectivity to insect pests. Molecular and biochemical studies, as well as binding experiments using radiolabelled neurotoxins, have shown that sodium channels expressed in various insect orders must be structurally and pharmacologically different. At least three groups of peptide neurotoxins derived from scorpion venom are highly active on insects and very weakly or practically inactive on mammals. It is proposed that various insecticides are examined for possible cooperative interactions with the peptide toxins highly active on insects, and pairs of ligands are identified that will increase the selectivity not only between mammals and insects but also between different pest and non-pest insects. This is feasible on the basis of the differential allosteric modulations observed between LqhαIT, an α-toxin highly active on insects, and brevetoxin on locust versus cockroach and rat brain sodium channels. Moreover, combination of LqhαIT with the pyrethroid deltamethrin increased the binding of [¹²⁵I] LqhαIT by more than 1.8-fold, and the combined presence of brevetoxin further increased the binding. Such allosteric modulation may provide a new approach to increase the selective activity of pesticides on target organisms by simultaneous application of allosterically interacting drugs, designed on the basis of the selective peptide toxins.     

Effects of pyrethroid structure on rates of hydrolysis and oxidation by mouse liver microsomal enzymes

Studies on the metabolism rates of 44 pyrethroids and 24 model compounds in mouse liver microsomal systems serve to divide the substrates into three groups based on their ease of hydrolysis and oxidation. Primary alcohol esters of trans-substituted cyclopropanecarboxylic acids are most rapidly metabolized with hydrolysis generally serving as the major component of the total metabolism rate. Although hydrolyzed slowly or not at detectable rates, the primary alcohol cis-substituted cyclopropanecarboxylates, tetramethylcyclopropanecarboxylates, and p-chlorophenyl-α-isopropylacetates are rapidly oxidized. The highly insecticidal α-cyano-3-phenoxybenzyl esters are least susceptible to metabolic attack due to both reduced esterase rates attributable to α substitution in the alcohol moiety and reduced oxidase rates for which no adequate explanation is currently available. Other structural modifications in the acid and alcohol moieties are less important in determining the metabolism rates. The substrate specificities of the microsomal esterases and oxidases are compared with in vivo pyrethroid structure-biodegradability relationships.     

昆虫对拟除虫菊酯类杀虫剂的代谢抗性机制研究进展

随着拟除虫菊酯类杀虫剂在卫生和农业害虫防治中的广泛应用,昆虫对此类杀虫剂产生抗性的报道越来越多。目前已明确昆虫对拟除虫菊酯类杀虫剂的抗性机制包括表皮穿透率下降、靶标抗性以及代谢抗性,其中代谢抗性机制较为普遍,而且其与昆虫对多种杀虫剂的交互抗性关系密切。目前,随着基因组、转录组以及蛋白质组学等新技术的发展及应用,昆虫对拟除虫菊酯类杀虫剂的代谢抗性机制研究也取得了很多新进展。昆虫体内细胞色素P450酶（P450s）、羧酸酯酶（CarE）及谷胱甘肽S-转移酶（GSTs）等重要解毒酶系的改变均与昆虫对拟除虫菊酯类杀虫剂的代谢抗性有关,其中这3类解毒酶的活性及相关基因表达量的变化是昆虫对此类杀虫剂产生代谢抗性的主要原因。明确昆虫对拟除虫菊酯类杀虫剂的代谢抗性机制,对合理使用此类杀虫剂及延缓抗药性的产生均具有重要意义。本文在总结拟除虫菊酯类杀虫剂代谢路径及相关生物酶研究概况的基础上,综述了近年来有关昆虫对此类杀虫剂代谢抗性机制研究的主要进展。     

7-Coumaryl permethrate and its cis- and trans-isomers as new fluorescent substrates for examining pyrethroid- cleaving enzymes

New esterase substrates were synthesized using cis-, trans- and cis-trans-permethrinic acid chloride and then used to measure pyrethroid-cleaving enzymes in insects. The new substrates, namely cis-, trans- and cis-trans-7-coumaryl permethrates (7-CP), show a structure very similar to permethrin insecticide and yield fluorescent products on hydrolysis. These substrates were hydrolyzed by a commercial porcine preparation that provided esterase-specific activity, and were stable at different pH values (5.2-7.8). Studies made with house fly, Musca domestica (L.), homogenates showed that these compounds are appropriate for determining pyrethroid hydrolysis activity on individual insects. The measured activity of house fly esterase was 870 relative fluorescence units (RFU) min(-1) with cis-7-CP as substrate, 1117 RFU min(-1) with trans-7-CP as substrate and 1423 RFU min(-1) with cis-trans-7-CP as substrate. The fluorescent substrates for pyrethroid-cleaving esterases described in this paper have advantages over methods already given in the literature. They are substrates with structures very similar to pyrethroids, the cleavage of which can be followed by an increase in fluorescence emission at 440 nm; it takes only about 5 min to measure the reaction, and moreover the high sensitivity of the fluorescence technique allows the quantification of esterase activity on individual insects.     

不同生境的草间钻头蛛种群对拟除虫菊酯类杀虫剂的敏感性及体内解毒酶活性的测定

采用浸渍法测定了不同生境的草间钻头蛛Hylyphantes graminicola种群对氰戊菊酯、溴氰菊酯和氯氰菊酯的敏感性,并对其体内的酯酶、多功能氧化酶-O-脱甲基和谷胱甘肽-S-转移酶的活性进行了测定。结果表明,不同生境的草间钻头蛛种群对3种拟除虫菊酯杀虫剂的敏感水平不同:草丛种群最为敏感;棉田种群的抗药性最强,其中对氰戊菊酯的抗药性最高,相对抗性指数为14.61,达到中抗水平;其他种群对3种杀虫剂较敏感或抗性较低。5种生境草间钻头蛛体内的酯酶、多功能氧化酶-O-脱甲基和谷胱甘肽-S-转移酶的活性均以草丛种群最低,棉田种群最高。酯酶和多功能氧化酶-O-脱甲基活性的增强可能是棉田草间钻头蛛种群对拟除虫菊酯类杀虫剂不敏感或产生抗性的原因。     

Actions of pyrethroid insecticides on insect axonal sodium channels

The actions of pyrethroid insecticides were tested on isolated giant axons of the cockroach Periplaneta americana, using oil-gap, single-fibre recording techniques. Current-clamp and voltage-clamp experiments were used to determine the actions of pyrethroids on axonal membrane potentials and ionic currents. Treatment with deltamethrin at micromolar concentrations caused gradual depolarisation of the axon accompanied by a reduction in amplitude of the action potential. This depolarisation was enhanced by an increase in stimulation frequency. Other synthetic pyrethroids: 3,4,5,6-tetrahydrophthalimidomethyl (1RS)-cis-3-[(RS)-2,2-dimethylcyclopropyl]-2,2-dimethylcyclopropanecarboxylate, biopermethrin and its (1S)-enantiomer, (1R)-tetramethrin, S-bioallethrin, bioresmethrin and its (1S)-enantiomer, cismethrin, and 5-benzyl-3-furylmethyl (E)-(1R)-cis-2,2-dimethyl-3-(2-oxothiolan-3-ylidenemethyl)cyclopropanecarboxylate (RU-15525, ‘Kadethrin’) were investigated. The (1S)-enantiomers were inactive, but all the other pyrethroids tested, apart from deltamethrin, induced prolonged negative (depolarising) after-potentials. All the treatments with the active pyrethroids resulted in the appearance of a voltage and time-dependent ‘maintained’ sodium conductance. The duration of this ‘slow’ conductance varied considerably depending on the pyrethroid under test. Clearly, the effectiveness of pyrethroids on whole insects is not determined only by the degree to which they directly modify the properties of sodium channels. Nevertheless, voltage-clamp experiments on isolated axons readily permit direct comparison of the actions of different pyrethroids on the sodium channels of insect neurones.     

Pyrethroid toxicology: Mouse intracerebral structure-toxicity relationships

Mouse intracerebral (ic) toxicity studies with 29 pyrethroids confirm earlier mouse intraperitoneal (ip) and rat oral and intravenous findings in three respects: α-cyano-3-phenoxybenzyl esters produce choreoathetosis, convulsions, and salivation, whereas compounds lacking the α-cyano group yield tremors and convulsions; high stereospecificity is involved in inducing both poisoning syndromes; large toxicity differences for (1R,trans) vs (1R,cis) resmethrin and permethrin do not extend to ethanomethrin and the cyanophenoxybenzyl esters. The ic investigations further establish that: profuse salivation is not unique for pyrethroids with the α-cyano group; the inactivity of (1R,trans) resmethrin in the brain is not due to detoxification; pyrethroids acting most rapidly in the brain are those with the highest knockdown activity for insects; the cyanophenoxybenzyl esters, in comparison with the non-cyano pyrethroids, have a high ic toxicity relative to their synergized ip toxicity indicating the importance of the brain in the Type II poisoning syndrome.     

Comparative properties of N-acetyl derivatives of oxime N-methylcarbamates and aryl N-methylcarbamates as insecticides and acetylcholinesterase inhibitors

N-acylation of aryl N-methylcarbamates is known to reduce their mammalian toxicity considerably without adversely affecting their insecticidal activity. It has now been found that N-acylation of several insecticidal oxime carbamates results in loss of toxicity both to insects and mammals. Kinetic data for base-catalysed solvolysis and inhibition of acetylcholinesterase, together with published work on metabolism, are combined to provide a rationale for this unexpected observation and to account for the selective toxicity of aryl N-acetyl-N-methylcarbamates.     

Influence of pyrethroid ester,oxime ether,and other central linkages on insecticidal activity,hydrolytic detoxification,and physicochemical parameters

The relative potency to target and nontarget insects of ester pyrethroids and the analogous oxime ethers, and the degree of synergism of the esters with tributyl phosphorotrithioate, provide a useful guide to the importance of esterase detoxification in species specificity. These criteria indicate that esterase detoxification is a more important component of pyrethroid tolerance in Chrysopa carnea and Cryptolaemus montrouzieri larvae than in Exochomus flavipis larvae and Musca domestica adults. Studies with 3-phenoxybenzyl 2-(4-chlorophenyl)-2-cyclopropylacetate and the corresponding 2,3,4,5,6-pentafluorobenzyl ester, their oxime ether analogs, and permethrin and its thiol ester and amide analogs provide evidence that the high insecticidal activity of some ester and E-oxime ether pyrethroids, relative to that of the corresponding thiol ester and amide, is more closely associated with the dipole moment and polarizability of the central linkage and its resistance to esterase detoxification than with bond lengths or lipophilicities conferred by a specific linkage. Pentafluorobenzyl tetramethylcyclopropanecarboxylate is very effective in the vapor state for house fly control.     

Interaction of pyrethroids with mammalian spinal neurons

The effects of intravenous administration of non-cyano (cis-permethrin) and cyano-substituted (deltamethrin) pyrethroids were studied on spontaneous and evoked ventral root activity in rat spinal cord and on spontaneous firing of ventral horn interneurons in the cat. Both pyrethroids had dramatic facilitatory effects on spontaneous firing rates of ventral roots and spinal interneurons and increased the amplitude of mono- and polysynaptically mediated ventral root responses to dorsal root stimulation. Spontaneous and evoked afferent sensory activity was slightly enhanced by cis-permethrin, but not by deltamethrin. In the cat diazepam (0.5 mg/kg, iv) was equally effective in antagonizing the facilitation of interneuronal firing resulting from either deltamethrin or cis-permethrin. These effects of pyrethroids on spinal neurons may underly the production of tremor and choreoathetosis-salivation toxicity symptoms in mammals.     

害虫对新烟碱类杀虫剂的抗药性及其治理策略

烟碱和新烟碱类杀虫剂都是作为后突触烟碱乙酰胆碱受体(nAChRs)的激动剂作用于昆虫中枢神经系统,但这两类杀虫剂存在明显不同的选择毒性:烟碱类对哺乳动物毒性较高,而杀虫活性低;新烟碱类具有高杀虫活性,而对哺乳动物低毒。由于新烟碱类杀虫剂的作用方式独特,对以前使用的如拟除虫菊酯类、氯化烃类、有机磷类和氨基甲酸酯类等杀虫剂很少或无交互抗性,该类杀虫剂为防治一些世界性重大害虫(包括对以前使用的杀虫剂具有长期抗性的害虫)作出了重要贡献。但现已发现不少害虫对新烟碱类杀虫剂产生了抗性。文章就害虫对新烟碱类杀虫剂的抗性概况、抗性机理和抗性治理策略进行了综述。     

Incidence of pyrethroid‐resistant oilseed rape pests in Germany

BACKGROUND: Failures in pollen beetle control using pyrethroids since 2005 indicated pyrethroid resistance in Germany. Therefore, resistance monitoring using bioassays was established in Germany for oilseed rape pest insects. RESULTS: The spread and intensity of pyrethroid resistance of Meligethes aeneus increased from 2005 onwards, with no sensitive samples left in any region of Germany in 2011. Sensitivity also declined for the newly introduced actives bifenthrin, etofenprox (both class‐I pyrethroids) and tau‐fluvalinate; all three claimed to be less affected by resistance, although there was no clear cross‐resistance to lambda‐cyhalothrin (class‐II pyrethroid). In the German region with the longest tradition and high intensity of oilseed rape production, pyrethroid resistance of Psylliodes chrysocephala and Ceutorhynchus obstrictus, with resistance factors of up to 81 and 140 respectively, was detected. CONCLUSION: The intensive use of only one mode of action for many years is risky, because even pest insects with a low intrinsic resistance risk may develop resistance. Therefore, resistance strategies need to include several control options for pest insects needing regular treatments. Copyright © 2012 Society of Chemical Industry     

Is Apis mellifera more sensitive to insecticides than other insects?

BACKGROUND: Honey bees (Apis mellifera L.) are among the most important pollinators in natural and agricultural settings. They commonly encounter insecticides, and the effects of insecticides on honey bees have been frequently noted. It has been suggested that honey bees may be (as a species) uniquely sensitive to insecticides, although no comparative toxicology study has been undertaken to examine this claim. An extensive literature review was conducted, using data in which adult insects were topically treated with insecticides. The goal of this review was to summarize insecticide toxicity data between A. mellifera and other insects to determine the relative sensitivity of honey bees to insecticides. RESULTS: It was found that, in general, honey bees were no more sensitive than other insect species across the 62 insecticides examined. In addition, honey bees were not more sensitive to any of the six classes of insecticides (carbamates, nicotinoids, organochlorines, organophosphates, pyrethroids and miscellaneous) examined. CONCLUSIONS: While honey bees can be sensitive to individual insecticides, they are not a highly sensitive species to insecticides overall, or even to specific classes of insecticides. However, all pesticides should be used in a way that minimizes honey bee exposure, so as to minimize possible declines in the number of bees and/or honey contamination. Copyright © 2010 Society of Chemical Industry     

Pyrethroid resistance and cross‐resistance in the German cockroach,Blattella germanica (L)

A German cockroach (Blatella germanica (L)) strain, Apyr‐R, was collected from Opelika, Alabama after control failures with pyrethroid insecticides. Levels of resistance to permethrin and deltamethrin in Apyr‐R (97‐ and 480‐fold, respectively, compared with a susceptible strain, ACY) were partially or mostly suppressed by piperonyl butoxide (PBO) and S,S,S,‐tributylphosphorotrithioate (DEF), suggesting that P450 monooxygenases and hydrolases are involved in resistance to these two pyrethroids in Apyr‐R. However, incomplete suppression of pyrethroid resistance with PBO and DEF implies that one or more additional mechanisms are involved in resistance. Injection, compared with topical application, resulted in 43‐ and 48‐fold increases in toxicity of permethrin in ACY and Apyr‐R, respectively. Similarly, injection increased the toxicity of deltamethrin 27‐fold in ACY and 28‐fold in Apyr‐R. These data indicate that cuticular penetration is one of the obstacles for the effectiveness of pyrethroids against German cockroaches. However, injection did not change the levels of resistance to either permethrin or deltamethrin, suggesting that a decrease in the rate of cuticular penetration may not play an important role in pyrethroid resistance in Apyr‐R. Apyr‐R showed cross‐resistance to imidacloprid, with a resistance ratio of 10. PBO treatment resulted in no significant change in the toxicity of imidacloprid, implying that P450 monooxygenase‐mediated detoxication is not the mechanism responsible for cross‐resistance. Apyr‐R showed no cross‐resistance to spinosad, although spinosad had relatively low toxicity to German cockroaches compared with other insecticides tested in this study. This result further confirmed that the mode of action of spinosad to insects is unique. Fipronil, a relatively new insecticide, was highly toxic to German cockroaches, and the multi‐resistance mechanisms in Apyr‐R did not confer significant cross‐resistance to this compound. Thus, we propose that fipronil could be a valuable tool in integrated resistance management of German cockroaches. © 2001 Society of Chemical Industry