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Effects of two biosynthetically distinct plant phototoxins-xanthototoxin, a furanocoumarin, and harmine, a P-carboline alkaloid, which are known to produce toxic oxygen species-on the food utilization efficiencies and enzymatic detoxification systems of the polyphagous cabbage looper. fiichoplusia ni (Lepidoptera: Noctuidae), were studied. Newly molted fifth-instar larvae were allowed 36 h to ingest diets containing these two phototoxins at 0.15% wet weight in the presence of near ultraviolet (UVA). The growth and development of the larvae, as well as the corresponding activities of the antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPOX), and glutathione reductase (GR) and the detoxification enzyme cytochrome P450, were measured. Xanthotoxin reduced rates of relative growth and consumption and efficiencies of conversion of ingested and digested food to biomass. harmine reduced rates of growth and consumption without affecting efficiencies of conversion. Specific activities of SOD, CAT, GPOX, and GR of whole-body homogenates in the absence of compounds were0.88 units, 153pmol H202 decomposed.mg protein -'.min -I, 38.3 nmol NADPH 0 x i d i z e d . q protein-l-min-l, and 0.56 nmol NADPH oxidized-mg protein-l-min-l, respectively. SOD activity was induced 2.9-fold and 3.8-fold by dietary xanthotoxin and harmine, respectively. CAT and GPOX activities were induced 1.2-fold by harmine only, and GR activity was not changed by either chemical. The P-450 activity toward xanthotoxin in the microsomal fraction of midguts was low (0.15 nmol xanthotoxin metabo1ized.q protein -'.min-') and was not induced by xanthotoxin ingestion. These studies indicate that P-450 and antioxidant enzyme systems may be independent but consequential, the induction of antioxidant enzymes by phototoxins occurring when low P-450 activity toward the phototoxin permits the accumulation of oxidative stress from unmetabolized phototoxin, which in turn induces antioxidant enzymes.

The parsnip webworm,Depressaria pastinacella (Lepidoptera: Oecophoridae), feeds on plants rich in furanocoumarins, phototoxic allomones. Final-instar larvae possess high levels of activities of antioxidant enzymes (Superoxide dismutase, catalase, glutathione reductase), which detoxify oxygen radicals generated from the furanocoumarins of their host plants. When added to an artificial diet, three linear furanocoumarins (xanthotoxin, bergapten, imperatorin) do not increase levels of the antioxidant enzymes. However, on diets containing both xanthotoxin and piperonyl butoxide, a cytochrome P-450 inhibitor, food utilization indices of the insect are reduced and superoxide dismutase activity is enhanced. These data suggest that cytochrome P-450s act as a primary detoxification system of ingested furanocoumarin, and antioxidant enzymes as a backup system to detoxify oxygen radicals generated by unmetabolized furanocoumarins.

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Phototoxic compounds are widely distributed among plant families; due to their ability to bind covalently to DNA or to react with oxygen and generate toxic oxyradicals, these compounds are toxic to a variety of herbivorous organisms.Black swallowtail (Papilio poiyxenes) larvae feed exclusively on phototoxic host plants i n the Apiaceae and Rutaceae. In this study, we examined the toxicity of four photoroxins-three furdnocoumarins and one p-carboline alkaloid-to P.polyxenes, as well as the inducibility of antioxidant enzyme defenses in response to these phototoxins. Neither the furanocoumarins nor the p-cdrboline alkaloid demonstrated any toxic effect on digestive efficiencies of P. polyxenes in the presence of light; harrnine, the alkaloid, did significantly reduce growth and consumption rates. None of the compounds had a significant effect on antioxidant enzyme levels. These findings contrast with those reported in earlier studies forTrichoplusia ni, a generalist noctuid sensitive to both furanocoumarins and P-carboline alkaloids. Greater detoxicative metabolic capabilities, coupled with substantially higher constitutive levels of antioxidant enzyme activity, likely explain at least in part rhe ahsence of induced antioxidant enzyme responses in the specialist feeder on phototoxic plants. Q lY93 Wiley-Liss, Inc.