Activities of enzymes that detoxify superoxide anion and related toxic oxyradicals in Trichoplusia ni

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Abstract
In third‐, fourth‐, and fifth‐instar larvae of the cabbage looper moth, Trichoplusia ni, the activities of the antioxidant enzymes, superoxide dismutase (SOD*), catalase (CAT), glutathione peroxidase (GPOX), and glutathione reductase (GR) were examined using 850 g supernatants of whole‐body homogenates. The enzyme activities, expressed as units mg−1 protein min−1 at 25°C ranged as follows: SOD, 0.67‐2.13 units; CAT, 180.5‐307.5 units; GPOX, none detectable; and GR, 0.40‐1.19 units. There was a similar pattern of changes for SOD and CAT activities with larval ontogeny, but not for GR. The cabbage looper apparently uses SOD and CAT to form a “defensive team” effective against endogenously produced superoxide anion (O2⪸). Glutathione may serve as an antioxidant for the destruction of any organic/lipid peroxides formed, and GSH oxidized to glutathione disulfide would be recycled by GR. Bioassays against pro‐oxidant compounds exogenous sources of (O2⪸) show high sensitivity of mid‐fifth instars to the linear furanocoumarin, 8‐methoxypsoralen (xanthotoxin) primarily from photoactivation (320‐380 nm), and auto‐oxidation of the flavonoid, quercetin. The LC50s are 0.0004 and 0.0045% (w/w) concentration of xanthotoxin and quercetin, respectively. Both pro‐oxidants have multiple target sites for lethal action and, in this context, the role of antioxidant enzymes is discussed.