PURIFICATION AND CHARACTERIZATION OF CARBOXYLESTERASES OF A RICE BROWN PLANTHOPPER, NILAPARVATA-LUGENS STAL

More than 10 molecular forms of carboxylesterases were observed with alpha-naphthyl acetate as substrate in a rice brown planthopper (BPH), Nilaparvata lugens Stal, using isoelectric focusing. The three most active ones, E(1), E(2) and E(3), purified with gel permeation/chromatofocusing chromatography were characterized. Their subunit molecular mass varied between 62 and 64 kDa and the pI ranged from c. 4.7 to 4.9. They were immunologically related and showed no difference in sensitivity toward the inhibition of paraoxon, methyl paraoxon, and malaoxon. E(1) consistently exhibited much higher activity than the other two isozymes toward some model substrates, i.e. alpha-naphthyl acetate and butyrate, beta-naphthyl acetate, and 4-nitrophenyl acetate as well as some insecticides. While malathion and trans-permethrin were readily hydrolyzed by these isozymes, very limited or no degradation of cypermethrin and cis-permethrin was detected. The carboxylesterases of BPH, being unable to hydrolyze parathion, could bind strongly the potent anticholinesterase paraoxon and oxons of several organophosphorus insecticides, rendering them nontoxic. Resistant BPH had higher activity and quantity of carboxylesterases (notably E(1)) than susceptible BPH. Protein subunits immunologically related to E(1) of BPH were detected in two other rice planthoppers (Laodelphax striatellus and Sogatella furcifera) and two aphids (Myzus persicae and Aphis gossypii), but not in the green rice leafhopper (Nephotettix cincticeps) and southern house mosquito (Culex quinquefasciatus).