| Carboxylesterases (CarE) play important roles in detoxifying a phenomenal number of endogenous and exogenous compounds, especially involved in metabolism of organophosphates and carbamate insecticides. In this study, the toxicity of nine insecticides belonging to organophosphates, carbamate and pyrethroids against Liposcelis bostrychophila Badonnel, a dominant stored-product insect, was assessed, and the synergism effects of three synergists (TPP, DEM and DEF) on these insecticides were also evaluated, respectively. Furthermore, the biochemical and toxicological characterizations of CarE and the expression profiles of two CarE genes encoding of CarE in L. bostrychophila were determined. The main results were as follows:The contact toxicity of nine insecticides on the psocid was evaluated by using the pesticide-membrane method. The results showed that the five tested organophosphates insecticides had excellent toxicity on the psocid, the order was Paraoxon-methyl> Chlorpyrifos> Malathion> Paraoxon-ethyl> Omethoate. The toxicity of paraoxon-methyl was 3 times higher than paraoxon-ethyl. In the two tested carbamate insecticides, aldicarb exhibited significantly higher toxicity than propoxur and the other insecticides. The toxicity of two pyrethroids (deltamethrin and fenpropathrin) was significantly lower than the other insecticides. However, deltamethrin showed better knockdown effects against L. bostrychophila in comparison with the others in the experiment. It is concluded that organophosphorus insecticides generally had better effect on psocids control. Therefore, it was suggested that organophosphorus insecticides could be applied as perfect grain protectant against the psocids in stored products.In the synergism experiments, DEF exhibited strong synergism with 7.50-,5.90- and 3.86- fold of the synergistic ratio (SR) on chlorpyrifos, malathion and aldicarb, respectively. DEF exhibited some synergism on omethoate (SR= 2.19), paraoxon-methyl (SR= 2.15), paraoxon-ethyl (SR= 1.64), and fenpropathr (SR= 1.43), while DEF did not show any synergism effect on deltamethrin and propoxur. The results suggested that esterases were probably involved in the metabolism of seven insecticides except deltamethrin and propoxur. Furthermore, except paraoxon-methyl, TPP exhibited varying synergism effiects on eight insecticides. Especially, TPP had a high SR (6.75) value on aldicarb. It implied that CarE were related to the metabolism of insecticides except paraoxon-methyl and played a major role in the metabolism of aldicarb. DEM showed no obvious synergism on omethoate, chlorpyrifos, propoxur and deltamethrin, in which DEM even had antagonistic effect on omethoate with an SR value of 0.78. However, DEM exhibited the strongest synergism on paraoxon-ethyl with 2.97-fold of SR value. This indicated that GSTs possibly played roles in the metabolism of paraoxon-ethyl, while had no relationship to other insecticides. Thus, the rational use of pesticides and corresponding synergists should be taken into account to control psocid pests effectively.The biochemical and toxicological characteristics of CarE (α-NA andβ-NA as substrates, respectively) from L. bostrychophila were analyzed via the microplate reader method. Usingα-NA as subsrtate, the optimum temperature and pH for the enzyme activity were determined as 24.9℃and pH 6.5, respectively, while 25.2℃and pH 6.2 forβ-NA as subsrtate. In vitro inhibitions on CarE by nine insecticides were analysed. Eitherα-NA orβ-NA was used for subsrtates, both paraoxon-methyl and paraoxon-ethyl showed efficient inhibition with lower I50S. Furthermore, the inhibitory effect was highter whenα-NA was used as subsrtate thanβ-NA. However, both of deltamethrin and fenpropathrin had no effect, whenα-NA orβ-NA was used as substrates. Usingβ-NA as substrate, the inhibition of five remaining insecticides was ranked as Malathion> Omethoate> Propoxur> Aldicarb> Chlorpyrifos. Meanwhile, the ranking of the five insecticides was Malathion> Chlorpyrifos> Omethoate> Propoxur> Aldicarb, whenβ-NA was used as substrates. These results are in accordance with bioassay. Additionally, kenetic study of CarE revealed that the seven insecticides tested were capable to increase the affinity of CarE to substrates, while reduce the catalytic activity.Based on the bioassay, the time dynamics of CarE activity of L. bostrychophila treated by nine insecticides with LC50S were evaluated. The results showed that there was no difference of CarE activity betweenα-NA andβ-NA as subsrtate. After psocids were treated by malathion, omethoate, paraoxon-methyl and aldicarb, CarE activity decreased immediately, then increased gradually, and reduced to control level subsequently. The enzyme activity showed sustained inhibition after the psocids were treated by paraoxon-ethyl, chlorpyrifos, propoxur and fenpropathr, where chlorpyrifos exhibited the best efficacy of inhibition. However, deltamethrin had no obvious inhibitory effect on CarE. In addition, the in vivo effects of different insecticides on the kinetic parameters of CarE in L. bostrychophila were also analyzed. The results indicated that the in vitro inhibition on CarE was stronger than in vivo inhibition. Furthermore, usingα-NA as substrate, only malathion and chlorpyrifos had strong impact on Km and Vmax values. Meanwhile, whenβ-NA was used as substrate, only paraoxon-ethyl and deltamethrin had little effect on Km and Vmax values among those tested insecticides.The esterase isozyme patterns of L. bostrychophila were analyzed by the polyacrylamide gel electrophoresis (PAGE). The results showed that esterase isozyme was composed of eight bands, the majority of which were of higher molecular weight. Compared to the control group, Paraoxon-methyl and Paraoxon-ethyl treatment inhibited the eaterase isozymes effectively and resulted in no bands being detected, which suggested that the esterase isozymes were more sensitive to paraoxon-methyl and paraoxon-ethyl than the others. This is consistent with previous in vitro inhibition study.In addition, we also investigated the time course effects (6,12,24,36 and 48 h) of insecticides (Deltamethrin, Paraoxon-methyl, and Aldicarb) on the expression patterns of two CarE genes(Lb est1, EU854151; Lb est2; EU854152) of L. bostrychophila. After the treatment of paraoxon-methyl, aldicarb and deltamethrin, the transcripts of Lb estl decreased immediately, then increased gradually, and recovered to control level subsequently. The highest quantities of Lb est1 mRNA were detected 24 h after treatment. Treated by deltamethrin, the transcripts of Lb est2 showed the similar trend to Lb est1, while the highest expression level was at 36 h after the treatment. The highest expression level of Lb est2 was determined at 24 h after treated by paraoxon-methyl then returned to the control level. However, the expression level of Lb est2 was suppressed after the aldicarb treatment.
|
PDF Full Text Request