Effect of insecticide injection on pyruvate kinase of the insect Tenebrio molitor (Coleopteran)

In the present study, we report on the changes observed in the kinetic properties of pyruvate kinase (PK) from whole body homogenate in the three developmental stages of the insect Tenebrio molitor after insecticide or acetone injection, and after injury and 24 h starvation. The apparent K_m value of the enzyme towards phosphoenolpyruvate was higher in the pupae stage than in the rest developmental stages although the same enzyme type appears to be present throughout the life span of the insect as judged by its V_max and sensitivity towards ATP and Alanine. The highest specific activity, twice as high as that of larvae and pupae, was observed in adults. Starvation resulted to activation of the enzyme by increasing the K_m value in the two feeding stages, namely larvae and adult, while injury had the opposite effect. Acetone injection did not have any significant effect at any stage. Insecticide injection had varying effects, depending upon the developmental stage, the dose and the type of the insecticide. In larvae, low doses of methyl paraoxon and methyl parathion activated the enzyme but as the dose increased the K_m value returned to normal levels. Low dose of decamethrin activated the enzyme also but at high dose it caused a severe inactivation as judged by a fivefold increase of the observed K_m. In pupae, decamethrin had no effect, methyl parathion had similar effects to those observed in larvae, and methyl paraoxon caused activation of the enzyme at any dose given. In the adult stage none of the injected insecticides had any effect on the K_m value although V_max and specific activity were significantly altered.