Developing antibodies from cholinesterase derived from prokaryotic expression and testing their feasibility for detecting immunogen content in Daphnia magna

To yield cholinesterase（ChE） from prokaryotic expression, the ChE gene that belongs to Daphnia magna was amplified by reverse transcription-polymerase chain reaction（RT-PCR） using forward primer 5＇-CCCYGGNGCSAT GATGTG-3＇ and reverse primer 5＇-GYAAGTTRGCCCAATATCT-3＇. To express the gene, one sequence of the amplified DNA, which was able to encode a putative protein containing two conserved carboxylesterase domains, was connected to the prokaryotic expression vector PET-29a（＋）. The recombinant vector was transformed into Escherichia coil BL21（DE3）. Protein expression was induced by isopropy-D-thiogalactoside. The expressed ChE was used as an immunogen to immunize BALB/c mice. The obtained antibodies were tested for their specificity towards crude enzymes from species such as Alona milleri, Macrobrachium nipponense, Bombyx mori, Chironomus kiiensis, Apis mellifera, Eisenia foetida, Brachydanio rerio, and Xenopus laevis. Results indicated that the antibodies had specificity suitable for detecting ChE in Daphnia magna. A type of indirect and non-competitive enzyme-linked immunosorbent assay（IN-ELISA） was used to test the immunoreactive content of ChE（ChE-IR） in Daphina magna. The detection limit of the IN-ELISA was found to be 14.5 ng/ml at an antiserum dilution of 1：22 000. Results from tests on Daphnia magna exposed to sublethal concentrations of triazophos indicated a maximal induction of 57.2% in terms of ChE-IR on the second day after the animals were exposed to a concentration of 2.10 μg/L triazophos. Testing on animals acclimatized to a temperature of 16 °C indicated that ChE-IR was induced by 16.9% compared with the ChE-IR content detected at 21 °C, and the rate of induction was 25.6% at 10 °C. The IN-ELISA was also used to test the stability of ChE-IR in collected samples. Repeated freezing and thawing had no influence on the outcome of the test. All these results suggest that the polyclonal antibodies developed against the recombinant ChE are as efficient as those developed against the native ChE in detecting ChE content in Daphnia magna.

Purification and studies on characteristics of cholinesterases from Daphnia magna

Due to their significant value in both economy and ecology,Daphnia had long been employed to investigate in vivo response of cholinesterase(ChE) in anticholinesterase exposures,whereas the type constitution and property of the enzyme remained unclear.A type of ChE was purified from Daphnia magna using a three-step procedure,i.e.,Triton X-100 extraction,ammonium sulfate precipitation,and diethylaminoethyl(DEAE)-Sepharose Fast-Flow chromatography.According to sodium dodecyl sulfate polyacrylamide gel electrophoresis(SDS-PAGE),molecular mass of the purified ChE was estimated to be 84 kDa.Based on substrate studies,the purified enzyme preferred butyrylthiocholine iodide(BTCh) [with maximum velocity(Vmax)/Michaelis constant(Km)=8.428 L/(min·mg protein)] to acetylthiocholine iodide(ATCh) [with V max /Km =5.346 L/(min·mg protein)] as its substrate.Activity of the purified enzyme was suppressed by high concentrations of either ATCh or BTCh.Inhibitor studies showed that the purified enzyme was more sensitive towards inhibition by tetraisopropylpyrophosphoramide(iso-OMPA) than by 1,5-bis(4-allyldimethylammoniumphenyl) pentan-3-one dibromide(BW284C51).Result of the study suggested that the purified ChE was more like a type of pseudocholinesterase,and it also suggested that Daphnia magna contained multiple types of ChE in their bodies.