The effect of phosphine treatment on superoxide dismutase, catalase, and peroxidase in the granary weevil, Sitophilus granarius

Abstract

Previous studies have shown that the fumigant insecticide phosphine (PH 3) inhibits cytochrome c oxidase and that a direct relationship exists between oxygen concentration during fumigation and insect mortality. Recently, it was shown that PH 3 stimulated the release of hydrogen peroxide (H 2O 2) from isolated insect mitochondria in vitro and it was hypothesized that treatment with PH 3 in vivo could result in the generation of superoxide radicals (O 2−) by the inhibited electron transport chain. The cell contains a complex oxygen defense system to protect itself against oxygen-derived free radicals, including three enzymes; superoxide dismutase (SOD), which removes O 2−, the catalase (CAT), and peroxidase (PER), which remove H 2O 2. The effect of PH 3 treatment on this antioxidant enzyme system was investigated using PH 3-susceptible ( S) and -resistant ( R) granary weevils. No glutathione peroxidase activity was found in this species. However, it did contain peroxidase activity that was observed using p-phenylenediamine as an indicator. Peroxidase activity was the same in S- and R-insects and was reduced by 65% in S- and 45% in R-insects 3 days after treatment (LD 30). Catalase activity was significantly higher (62%) in S-insects than R. This activity was inhibited by 34% in S-insects 3 days after treatment (LD 30), but was unaffected in R-insects. A pyrogallol assay was used to measure superoxide dismutase. Two isozymes were present, a cyanide (CN)-insensitive form in the mitochondria and a CN-sensitive form in the cytosol. Activity of the latter enzyme increased twofold after in vivo PH 3 treatment (LD 30) in S-insects, while no change was observed in R-insects. This study demonstrates that PH 3 treatment has a significant effect on the enzymes involved in oxygen defense. Elevated SOD activity probably occurred in response to an increase in O 2− generation and this coupled with a reduction in both CAT and PER activity could result in an accumulation of H 2O 2 and the consequent production of the hydroxyl radical (HO •), a powerful oxidizing agent. These results indicate that insect mortality could be attributed to accumulation of oxygen-derived free radicals which eventually destroy the cell integrity.