Carboxylamidases inSpodoptera exigua: properties and distribution in the larval body

Resistance to many insecticides demonstrated by the beet armyworm,Spodoptera exigua (Hübner), can be caused by the action of carboxylamidases. A colorimetric method, based on the hydrolysis of 4-nitroacetanilide to 4-nitroaniline by carboxylamidases, was used for evaluating biochemical properties of these detoxifying enzymes in beet armyworm. The optimum pH and temperature were 7.5 and 38°C, respectively. K_m (Michaelis constant) and V_max (maximal velocity) at 28°C were 2.3 X 10^-4 M and 2.06 nmol min^-1 mg protein^-1, respectively. The enzyme activity was evaluated in several body parts and located mostly (66.2%) in the midgut. The soluble fraction (supernatant of 105,000g) contained the highest enzyme activity relative to the total (69.3%), and exhibited the highest specific activity. Carboxylamidase activity was totally inhibitedin vitro at a concentration of 10^-6 M methomyl. The analysis of thein vitro inhibition kinetics indicated the ability of methomyl and diflubenzuron to inhibit carboxylamidases noncompetitively. Over 95% inhibitionin vivo was obtained when the larvae were fed with castor bean leaves dipped in 250 mgl ^-1 of methomyl. Thein vivo enzyme activity could be reduced to half with a pretreatment of 15 mgl ^-1 diflubenzuron.