BIOCHEMICAL DEFENSE OF PROOXIDANT PLANT ALLELOCHEMICALS BY HERBIVOROUS INSECTS

A new aspect of interactions among insect herbivores and defensive chemistry of plants in the regulation of oxygen toxicity exerted by pro-oxidant allelochemicals is described. Endogenous oxygen toxicity results from activation of the ground state of molecular oxygen to the superoxide anion radical (O2.-), hydrogen peroxide (H2O2), hydroxyl radical (.OH), lipid hydroperoxides (LOOHs), and peroxyl radicals (LO2. or RO2.). The strongly lipid-peroxidizing singlet oxygen (1-DELTA-gO2) is also produced during light activation of photosensitizers. Ingestion of pro-oxidants exacerbates oxygen toxicity by increasing the production of these deleterious forms of oxygen. The role of ascorbate, alpha-tocopherol, glutathione, carotenoids and urate as antioxidants in insects is apparent, but needs more work for the elucidation of their roles. The major defence mechanism includes a group of antioxidant enzymes represented by superoxide dismutese (SOD), catalase (CAT), glutathione-S-transferase's peroxidative activity (GSTPX), glutathione reductase (GR), and DT-diaphorase. SOD converts O2.- radicals to H2O2 and O2, CAT decomposes H2O2 to H2O and O2, GSTPX reduces LOOHs to LOHs with GSH as reductant and GSSG formed from GSH during the GSTPX reaction is reduced to GSH by GR. DT-diaphorass is an important antioxidant in that it reduces quinones by a two-electron reduction to stable products, thereby preventing the one-electron reduction to semiquinone radicals which generate O2.- radicals. Therefore, these enzymes are crucial for insect herbivores for preventing the free-radical cascade of oxygen, and terminating the toxic lipid peroxidation chain reaction, in response to the endogenous and potential exogenous oxidant-induced injury.