Kinetic and molecular differences in the amplified and non-amplified esterases from insecticide-resistant and susceptible Culex quinquefasciatus mosquitoes

Two non-amplified esterases were purified from the insecticide- susceptible Pel SS strain of Culex quinquefasciatus. These were the two major esterase activity peaks in this strain. The two corresponding amplified carboxylesterases, Estα2 and Estβ2, involved in organophosphate sequestration were purified from two resistant C. quinquefasciatus strains. The Pel SS esterases were significantly less reactive with the organophosphates than those from the resistant strains. One of the Pel SS esterases was electrophoretically identical to amplified Culex Estβ1. However, it differed kinetically, and in its nucleotide and predicted amino acid sequences from the two characterized amplified Estβ1s, it is classified as Estβ13. Restriction fragment analysis suggested Pel SS has only one Estα and one Estβ gene, while the resistant Pel RR has both amplified and non-amplified forms of Estα and Estβ. The EcoRI fragments for both Pel SS esterases were distinct from those of the amplified Estα21, Estβ21, or Estβ1(1 and 2). An esterase with the same size EcoRI fragment as Estβ13 was also present in Pel RR. This and restriction enzyme fragment analysis of C. quinquefasciatus field populations suggest that variability of the susceptible alleles may be lower than previously suggested. A non-amplified Estα with a unique EcoRI band was present in Pel RR. The previous esterase purification procedures may not have separated these amplified and non- amplified alleles. Hence, the small differences between the purified esterases from resistant strains may reflect mixtures of identical amplified alleles with different non-amplified alleles, which have significantly different k(a) values.