Published online by Cambridge University Press: 27 March 2009
Effects of temperature, hydrogen ion and substrate concentrations on conjugation of l-chloro-2,4-dinitrochlorobenzene by glutathione S-transferase from susceptible and dichlorvos-resistant strains of cotton aphid (Aphis gossypii Glover (Homoptera: Aphididae)) were evaluated. Enzymes from both strains had common optimum temperature and substrate concentration values of 30 °C and 10 mM respectively. Also, while enzyme activity of the susceptible strain peaked at pH 7·2, that of the resistant strain showed complete linear dependency up to pH 8·0. Of four subcellular fractions, the 100 000 g supernatant (soluble fraction) gave the highest enzyme activity in both phosphate and Tris/HCl buffers. There was no linear relationship between insecticide application frequency and production of enzyme activity in the susceptible strain but there was a very high positive correlation between these two parameters in the resistant strain.