Hydrolysis of paraoxon and malaoxon in three strains of Myzus persicae with different degrees of parathion resistance

Homogenates of three strains of Myzus persicae, A, R, and E, with an LD₅₀ for topically applied parathion of 9, 93, and 263 ng per aphid, showed an in vitro hydrolytic degradation of paraoxon of 2.3, 4.7, and 8.6 pmol/mg aphid/h, respectively. These values represent V_max; K_m was <10^?7M. The three strains showed a malaoxon degradation of 2.4, 11.9, and 18.8 pmol/mg/h at 10^?6M substrate concentration. V_max for R and E was 21 and 27 pmol, respectively and K_m 7 and 4 × 10^?7M. Activity in strain A was too low to estimate these entities. The breakdown product of paraoxon was mainly diethyl phosphoric acid, that of malaoxon mainly dimethyl phosphoric acid. No hydrolysis of the carboxylester groups of malaoxon was found. Hydrolysis of paraoxon and malaoxon was inhibited by isopropyl and n-propyl paraoxon and by the salioxon-analog K₂. The two latter compounds were shown to act as synergists with parathion when added in amounts that caused little mortality when given alone. The hydrolytic enzyme is soluble and retains its activity during incubations of several hours. It is likely that it is responsible for at least part of the resistance. Resistance was maintained without selection over a period of three years. There was no correlation between degree of resistance and carboxylesterase activity of the strains.