Induction and catalytic properties of grasshopper (Zonocerus variegatus) glutathione transferase fed on different food plants.

In order to establish the role of diet on the induction and catalytic properties of glutathione transferase (GST) in insects, variegated grasshopper (Zonocerus variegatus) was exposed to different food plants separately for 30 days and the properties of the induced enzyme were then investigated. Insects fed on cassava (M. esculenta) leaves had the highest GST induction followed by insects fed on bitter leaf (V. amygdalina). Z. variegatus that fed in the wild on different food plants had the least suggesting that allelochemicals in the food plants have a compensatory toxicity-alleviating actions on one another. 1-Chloro-2,4-dinitrobenzene (CDNB) was the best substrate for all the induced GST however, the mode of binding of the substrate to the induced enzyme was not the same. GST from M. esculenta-fed insect showed ping-pong kinetic mechanism whereas GSTs from V. amygdalina and T. procumbens-fed insects showed random sequential mode of substrate binding. Catalytic efficiency (kcat/Km) of GST from M. esculenta-fed insects was 3-8-fold higher than other induced enzymes. Commercial insecticides- cypermethrin and lindane had an inhibition constant, Ki, of 0.13±0.004 mM and 0.68±0.09 mM, respectively, suggesting that the concentration as used in the field (0.03 mM for cypermethrin and 0.3 mM for lindane) would have little effect on the insect's GST. The study concluded that higher GST activity are induced in insects that fed on monotonous diets than those that fed on various food plants. Hindgut appears to be the primary organ of detoxication. The catalytic properties of the induced enzymes are different from one another.