ANTIOXIDANT DEFENSE OF THE CABBAGE LOOPER, Trichoplusia ni: ENZYMATIC RESPONSES TO THE SUPEROXIDE‐GENERATING FLAVONOID, QUERCETIN, and PHOTODYNAMIC FURANOCOUMARIN, XANTHOTOXIN

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Abstract
Abstract— Superoxide dismutase (SOD) activity was induced by ca 2‐fold (to5–6 U) when Trichoplusia ni midfifth‐instar larvae were exposed to two toxic oxygen species generating plant pro‐oxidants, quercetin (a flavonoid) and xanthotoxin (8‐methoxypsoralen; a photoactive furanocoumarin). Very high catalase (CAT) activity (ca 300 U) of this insect was not affected by 8‐methoxypsoralen, but was slightly decreased by quercetin. No Se‐dependent glutathione peroxidase (GPOX) activity was observed, but high glutathione transferase (GST) peroxidase activity (over 50 U) in this insect was slightly induced by 8‐methoxypsoralen (8‐MOP), and was partially inhibited by quercetin, 8‐Methoxypsoralen induced the activity of glutathione reductase (GR), but quercetin partially inhibited the activity of this enzyme. An increase in SOD activity appears to be the main response of this insect to dietary exposure to pro‐oxidant compounds. High CAT activity guarantees the destruction of large cellular increases in H2O2, a product of rapid dismutation of superoxide from induced activity of SOD. Moreover, GST with its peroxidase activity apparently substitutes for GPOX, forming a GST/GR enzyme pair as a primary line of defense against deleterious organic hydroperoxides. These studies clearly point out the key role for an insect's antioxidant enzymatic countermeasures against defensive pro‐oxidant compounds produced by plants.