Studies on organophosphorus impurities in technical malathion: Inhibition of carboxylesterases and the stability of isomalathion

Four organophosphorus esters found as impurities in technical malathion were synthesized, and their abilities to inhibit monomeric and oligomeric carboxylesterases from rabbit liver, as well as type I and type II esterases from porcine liver, were studied. The equilibrium dissociation constant (K_d), phosphorylation constant (k₂), and bimolecular rate constant (k_i) were determined in the presence of substrate. Inhibition, as judged by the k_i, was in the following order: isomalathion > O,S,S-trimethyl phosphorodithioate > O,O,S-trimethyl phosphorothioate > O,O,S-trimethyl phosphorodithioate. All of the k₂ values were relatively small, indicating that the k_d values contribute most to the overall inhibitory power. Rabbit liver carboxylesterases are more sensitive to inhibition by isomalathion than porcine liver esterases, reflecting a species difference. Isomalathion undergoes nonenzymatic degradation in the presence of fluoride ions, producing diethyl thiomalate. A kinetic investigation of the nonenzymatic degradation of isomalathion was conducted, and reaction rate constants were determined in phosphate buffer at various molarities, temperatures, and pH values.     

Effect of a mixture of iprobenfos and malathion on the development of malathion resistance in the mosquito Culex pipiens pallens Coq

A malathion-resistant (RM) strain of Culex pipiens pallens Coq was obtained by successively selecting a field population with malathion in the laboratory. The synergistic effect of iprobenfos on malathion toxicity and alpha-naphthyl acetate (alpha-NA) esterase assay revealed that malathion resistance in the RM strain was associated with increased alpha-NA esterase activity and the synergism was mainly due to the inhibition by iprobenfos of this activity. There was no difference in alpha-NA esterase activity between the larvae and female adults in the susceptible (S) strain, but the activity in the adults was 13-fold higher than in the larvae of the RM strain. To understand the effect of the application of a mixture of iprobenfos and malathion on the evolution of malathion resistance, an artificial strain (Syn) was generated by mixing the RM and S strains with 0.1 frequency of the malathion-resistant individuals. The offspring of the Syn strain were divided into two sub-strains, Rm and Rm+ibp, which were successively treated with, respectively, malathion alone and malathion + iprobenfos (1:2) at LC70. In the mixture, the fungicide iprobenfos acted as a synergist of malathion. After treatment for 10 generations, the resistance level to malathion was 317.4-fold for the Rm sub-strain, whereas for the Rm+ibp sub-strain it was only 38.9-fold, compared with the Syn strain. Similar results were obtained by measurement of alpha-NA esterase activity from both larvae and female adults. The alpha-NA esterase activities in larvae and female adults at F10 generation were 2.6- and 10.9-fold from the Rm+ibp sub-strain and 5.7- and 98.5-fold from the Rm sub-strain, respectively, compared with the Syn strain. The above results suggested that iprobenfos, although it cannot completely stop or prevent the onset of malathion resistance, could dramatically delay its evolution.     

The biochemical basis of malathion resistance in the sheep blowfly,Lucilia cuprina

The in vivo and in vitro metabolism of [¹⁴C]malathion was studied in susceptible (LS) and malathion resistant (RM) strains of the sheep blowfly, Lucilia cuprina (Wiedemann). No difference was found between strains in the penetration, excretion, storage, or inhibitory potency of the insecticide. However, RM degraded malathion to its α- and β-monocarboxylic acid metabolites more rapidly than LS, both in vivo and in vitro. This enhanced degradation of [¹⁴C]malathion occurred in vitro in both mitochondrial and microsomal fractions of resistant flies. Kinetic analysis revealed that these fractions degraded malathion by discrete mechanisms. The enzymes from the mitochondria of both strains had the same K_m, whereas the microsomal enzyme from the RM strain had a fivefold higher K_m than that from the LS strain. Studies of esterase activities and the effect of enzyme inhibitors showed that both the mitochondrial and microsomal resistance mechanisms were the result of enhanced carboxylesterase activity. It was concluded that increased carboxylesterase detoxification of malathion adequately explained the high level of malathion resistance in RM if rate-limiting factors such as cuticular penetration were taken into account.     

Influence of malathion on the catalytic potential of glutamate dehydrogenase in sheep brain

Malathion stimulated sheep brain glutamate dehydrogenase. The maximal velocity (V_max) and Michaelis-Menten constant (K_m) showed alterations characteristic of nonessential activation. The activation energy required by the enzyme subjected to pesticidal stress was found to decrease, suggesting a decrement in energy barrier for the catalysis of the enzyme. The possible implications of malathion-stimulated glutamate dehydrogenase activity are discussed.     

东亚飞蝗对马拉硫磷抗性研究初报

马拉硫磷是当前防治东亚飞蝗的主要农药。通过东亚飞蝗对马拉硫磷的抗药性测定,结果表明,用点滴法测定东亚飞蝗抗性为2.9倍,用浸虫法测定东亚飞蝗抗性为4.5倍。     

Effect of short-time malathion administration on glucose homeostasis in Wistar rat

The main objective of this study is to investigate the 24 h kinetic effect of acute administration of malathion, an organophosphorus insecticide, on glucose homeostasis in adult rats. A single dose of malathion (400 mg/kg, 1/5 of LD₅₀) was administered orally to rats, blood glucose was measured, liver and pancreas were removed to determine the level of hepatic glycogen, the activity of pancreatic acetylcholinesterase and butyrylcholinesterase, as well as the level of TBARs. Blood glucose increased in rats treated with a single dose of malathion. This effect was observed in the first hours of treatment, reached a 2.2 fold peak after 2 h, and then decreased after 4 h, followed by a decrease in the level of hepatic glycogen. The storage of glycogen starts from 6 to 12 h of administration. A decrease in cholinesterase activities was noted. The level of TBARs increased considerably in liver and pancreas. Results of this study can be explain by a stimulation of glycogenolysis and gluconeogenesis by liver, with a temporarily loss of endocrine functions of pancreas leading to hyperglycemia.     

Accumulation of malathion residues in grain stored on malathion-treated wood surfaces

Laboratory studies were conducted to determine whether insecticides applied to storage structures are able to move past a single layer of grain kernels to give adequate control of stored-grain insects. The extent of uptake of insecticides was assessed on three layers of wheat in contact with wood surfaces treated with malathion 836 g litre^?1 e.c. at 1–0 g a.i. m^?2 using chemical and biological methods at predetermined time intervals. There was a progressive uptake of malathion beyond the layer directly in contact with the treated surfaces and the uptake depended on the duration of storage. After 8 weeks a sufficient amount of malathion accumulated on the top layer of grain kernels to cause 100% mortality of adult Tribolium castaneum (Herbst).     

The biochemical nature of malathion resistance in Anopheles stephensi from Pakistan

Malathion resistance in Anopheles stephensi from Pakistan was synergized by triphenyl phosphate, primarily a carboxylesterase inhibitor. There was a slight degree of antagonism with piperonyl butoxide. The major metabolite of malathion in larvae of both the resistant and susceptible strains was malathion monocarboxylic acid. Resistant larvae produced about twice as much of this product as the susceptible larvae. This suggests that a qualitative or a quantitative change in a carboxylesterase enzyme may be the basis of malathion resistance in this strain. Analysis of general esterase levels to α- and β-naphthyl acetate showed that there was no quantitative change in the amount of carboxylesterase enzyme present in the resistant strain as compared to the susceptible.     

Determination of malathion on tomatoes and brinjals by multiband chromatoplates

A one step clean-up for malathion in extracts of tomatoes and brinjals using a multiband chromatoplate and its quantitative estimation by measuring spot areas are described.     

Impact of malathion stress on lipid metabolism in Limnonectus limnocharis

Despite mounting concerns about amphibian population declines, information on impact of pesticides on physiological changes is meager. The present study deals the influence of an organophosphate pesticide—malathion on the lipid metabolism of Limnonectus limnocharis under laboratory conditions. Changes in the lipid metabolism were analyzed in the liver, muscle, ovary, and testis of frogs exposed to lethal (10.67 mg L⁻¹ for 1, 2, 3, and 4 days) and sub-lethal (2.13 mg L⁻¹ for 1, 5, 10, 15, 20, and 25 days) concentrations of malathion. Upon lethal concentration treatment, against the increase of fatty acids, glycerol, and lipase activities in all tested tissues, there was decrease in the total lipids content over different durations. On the other hand, exposure to sub-lethal concentration, the amount of total lipids content, free fatty acids, glycerol and lipase activity increased. Changes in the lipid metabolism due to lethal concentration of malathion exposure could depict the negative impact on the reproductive success, which would result in decline of amphibian population.     

Iodometric analysis and shelf life of malathion in formulations

Using an iodometric method for the determination of malathion in emulsifiable concentrates, values of active ingredient in four commercial formulations and one primary reference standard were within ±0.7% weight/volume (w/v) of the stated concentrations. A standard deviation of 0.42% w/v and 95% confidence limits of ±0.9% w/v for a single determination were calculated for the method. Added moisture and elevated temperature were found to accelerate malathion breakdown in the commercial formulations. The greatest breakdown rate of 5.2% w/v per 100 days occurred with a formulation stored at 27 °C containing 0.5% v/v added moisture.     

四种乳酸菌对马拉硫磷的手性选择性降解研究

乳酸菌广泛存在于自然界,为明确其对手性农药马拉硫磷的选择性降解情况,采用CHIRALCEL OD-H手性色谱柱及高效液相色谱分析方法,研究了马拉硫磷对映异构体的拆分以及在植物乳杆菌Lactobacillus plantarum subsp.plantarum、干酪乳杆菌Lactobacillus casei、保加利亚杆...     

Studies on hydrolases in various house fly strains and their role in malathion resistance

Aliesterase, carboxylesterase, and phosphorotriester hydrolase activities in six house fly strains were studied in relation to malathion resistance. Selection of two susceptible strains with malathion for three generations resulted in an increase in both carboxylesterase activity and LD₅₀ of malathion, indicating that the increased detoxication by the enzyme was the major mechanism selected for malathion resistance. With the highly resistant strains, however, the carboxylesterase activity alone was not sufficient to explain the resistance level, and the involvement of additional mechanisms, including phosphorotriester hydrolase activity, was suggested. The E₁ strain, which had high phosphorotriester hydrolase activity but normal or low carboxylesterase activity, showed a moderate level, i.e., sevenfold resistance. Upon DEAE-cellulose chromatography, two or three esterase peaks were resolved from susceptible, moderately resistant, and highly resistant strains. The substrate specificity, the sensitivity to paraoxon inhibition, and the $\text{α}\text{β}$ ratio of malathion hydrolysis were studied for each esterase peak from the different strains. The results suggested the existence of multiple forms of esterases with overlapping substrate specificity in the house fly.     

The detection and characterization of malathion resistance in field populations of Anopheles culicifacies B in Sri Lanka

Malathion resistance was first detected in Sri Lankan Anopheles culicifacies in limited regions of the island in 1982. The frequency of resistance has been increasing slowly since then, but is not yet high enough to be considered an operational problem. Malathion toxicity is synergised in the resistant population by triphenyl phosphate, and metabolism studies suggest the involvement of a carboxylesterase enzyme. The spread of general esterase activity in individuals in an area of the island where resistance is present is wider than that in a totally malathion-susceptible area. However, the frequency of individuals with high esterase activity does not correlate well with resistance in the two field populations studied in detail. This suggests that a qualitative rather than a quantitative change in esterase activity may be involved in this resistance. Extrapolation from similar qualitatively changed carboxylesterases in other anophelines leads us to predict that the resistance in A. culicifacies will be malathion specific and inherited as a single semidominant characteristic.     

Characterization of malathion resistance in a Mexican population of Rhizopertha dominica

Malathion resistance of a field-collected population of Rhizopertha dominica (Coleoptera: Bostrichidae) from Mexico was evaluated and the resistance mechanisms were characterized both in vivo and in vitro. The Mexican population showed a resistance level of 50-fold at LC₅₀ as compared with that of a susceptible laboratory population. Malathion bioassays with the synergists triphenyl phosphate, piperonyl butoxide and diethyl maleate suggested that esterases were likely to contribute to the resistance whereas cytochrome P450 monooxygenases and glutathione S-transferases were not. In-vitro assays of esterases indicated that the general esterase activity was 1·3-fold higher in the Mexican population than in the susceptible population. However, the phosphotriesterase activity in the resistant population was 3·7-fold higher than in the susceptible population. Significantly higher phosphotriesterase activity in the resistant population was further indicated by 3·4-fold increase of V_max in enzyme kinetics and higher frequency of individuals with high phosphotriesterase activity in this population. All these findings suggested that phosphotriesterases play a role in malathion resistance in the Mexican population of lesser grain borer. © 1998 SCI     

Comparison of the effect of subacute organophosphate exposure on the cortical and peripheral evoked activity in rats

Organophosphates are irreversible blockers of acetylcholinesterase and are widely used as insecticide agents. Their action is not limited to the target organisms so that occupational or food-borne exposure of humans usually leads to neurotoxicity in which several other mechanism, apart from cholinesterase inhibition, may play a role. In the present study, rats were treated with three different organophosphates (chlorfenvinphos, diisopropyl fluorophosphate, and dimethoate) for 4 weeks, and alterations in two forms of stimulus evoked activity—somatosensory and visual cortical sensory evoked potentials and peripheral nerve action potential—were compared. In the treated rats, there was significant increase in the somatosensory evoked response latency and non-significant increase in its duration. In the visual evoked potential, only duration was altered. The conduction velocity of the peripheral nerve was decreased. Comparison of the changes in the cortical- and peripheral evoked activity showed that the slowed peripheral impulse conduction only partly explains the increase in the cortical response latency. Hence, possible mechanisms of direct cortical action of the organophosphates are also discussed.     

Role of a microsomal carboxylesterase in reducing the action of malathion in eggs of Triatoma infestans

A microsomal malathion carboxylesterase present in Triatoma infestans eggs was active from the first day of embryonic development. This microsomal egg malathionase (MEM) showed a unique band in polyacrilamide gel electrophoresis (PAGE) when malathion was used as substrate. In vivo metabolism of [¹⁴C]malathion during all embryonic development showed a 10% degradation due to carboxylesterases. The in vitro evaluation of the same metabolic pathway catalyzed by the microsomal fraction of T. infestans eggs showed partial inhibition by paraoxon. α- and β-malathion monoacids were identified as the main metabolites of the in vivo and in vitro metabolic pathways. The carboxylesterase band that appeared in PAGE (MEM) from the first day of embryonic development could be the main cause of malathion tolerance in T. infestans eggs.     

Interaction of phosphodiesterase 5 inhibitor with malathion on rat brain mitochondrial-bound hexokinase activity

The main objective of this study was to investigate the possible protective effect of pentoxifylline as a phosphodiesterase 5 inhibitor used as a cardiovascular medication on malathion-induced changes on rat mitochondrial-bound hexokinase activity.Animals in four various groups received moderate toxic dose of malathion (200 mg/kg/day), effective dose of pentoxifylline (50 mg/kg/day) alone and in combination, and the control group that received only vehicle. All administrations were done intraperitoneally for one week. At the end of the experiment, the brain was removed and the mitochondria were isolated. Hexokinase (HK) activity, cellular lipid peroxidation (LPO) and total antioxidant capacity (TAC) were analyzed in brain mitochondria.Malathion noticeably decreased TAC and increased HK activity and LPO in the mitochondria whereas pentoxifylline significantly restored malathion-induced changes in LPO, HK, and TAC.The results of the present study indicate that phosphodiesterase 5 inhibition remarkably protects brain mitochondria from malathion-induced changes on HK activity and oxidative stress.     

Interaction of phosphodiesterase 5 inhibitor with malathion on rat brain mitochondrial-bound hexokinase activity

The main objective of this study was to investigate the possible protective effect of pentoxifylline as a phosphodiesterase 5 inhibitor used as a cardiovascular medication on malathion-induced changes on rat mitochondrial-bound hexokinase activity.Animals in four various groups received moderate toxic dose of malathion (200 mg/kg/day), effective dose of pentoxifylline (50 mg/kg/day) alone and in combination, and the control group that received only vehicle. All administrations were done intraperitoneally for one week. At the end of the experiment, the brain was removed and the mitochondria were isolated. Hexokinase (HK) activity, cellular lipid peroxidation (LPO) and total antioxidant capacity (TAC) were analyzed in brain mitochondria.Malathion noticeably decreased TAC and increased HK activity and LPO in the mitochondria whereas pentoxifylline significantly restored malathion-induced changes in LPO, HK, and TAC.The results of the present study indicate that phosphodiesterase 5 inhibition remarkably protects brain mitochondria from malathion-induced changes on HK activity and oxidative stress.     

应用溴敌隆膨化颗粒毒饵防治田鼠的效果试验

应用0.005％溴敌隆膨化颗粒毒饵防治花生地害鼠试验结果表明：0.005％溴 敌隆膨化颗粒毒饵对花生地的墨 线姬鼠，黄毛鼠及社鼠适口性很好，投饵后进行3d取食率在37.8％-57.4％；平均为45.2％。投毒饵后15d采用夜夹法测定灭鼠效果，0.005％溴敌隆膨化颗粒毒饵灭鼠效果达93.54％。