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

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.