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In the mid-fifth instar larvae of the cabbage looper moth, Trichoplusia ni, the subcellular distribution of total superoxide dismutase was as follows: 3.05 units (70.0%), 0.97 units (22.3%), and 0.33 units (7.6%) mg-1 protein in the mitochondrial, cytosolic and nuclear fractions, respectively. No superoxide dismutase activity was detected in the microsomal fraction. Catalase activity was unusually high and as follows: 283.4 units (47.3%), 150.1 units (25.1%), 142.3 units (23.8%), and 22.9 units (3.8%) mg-1 protein in the mitochondrial, cytosolic, microsomal (containing peroxisomes), and nuclear fractions. No glutathione peroxidase activity was found, but appreciable glutathione reductase activity was detected with broad subcellular distribution as follows: 3.86 units (36.1%), 3.68 units (34.0%), 2.46 units (23.0%), and 0.70 units (6.5%) mg-1 protein in the nuclear, mitochondrial, and cytosolic fractions, respectively. The unusually wide intracellular distribution of catalase in this phytophagous insect is apparently an evolutionary adaptation to the absence of glutathione peroxidase; hence, lack of a glutathione peroxidase-glutathione reductase role in alleviating stress from lipid peroxidation. Catalase working sequentially to superoxide dismutase, may nearly completely prevent the formation of the lipid peroxidizing .OH radical from all intracellular compartments by the destruction of H2O2 which together with O2- is a precursor of .OH.

The pea aphid Acyrthosiphon pisum (Harris) incorporated [l-14C]acetate into a phospholipid dienoic fatty acid in a time-dependent manner. In 2-h incubations, the incorporation of radioactivity into the 18:2 fraction was minimal, whereas after 45 h 18:2 was the major fatty acid labeled. Ozonolysis of the isolated dienoic fatty acid methyl ester followed by radio-gas-liquid chromatography showed that radioactivity was associated with fragments containing carbons 1-9 and 13-18. These data established the location of the double bonds in the 9,12 positions and indicated that the entire molecule was labeled from [l-14C]acetate. Tetracycline-treated aphids synthesized linoleic acid in the same proportions as untreated controls. Scanning electron microscopy showed that over 50% of the treated insects had greatly reduced numbers of intracellular symbiotes or lacked them or most of the existing symbiotes had an abnormal appearance. Therefore, we conclude that intracellular symbiotes are not involved in the biosynthesis of linoleic acid in the pea aphid.

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In mid-fifth-instar larvae of the southern armyworm, Spodoptera eridania, the subcellular distribution of four antioxidant enzymes-superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPOX), and glutathione reductase (GR)-were examined. Two-thirds (4.26 units -mg protein-') of the SOD activity was found in the cytosol, and one-third (2.13 units -mg protein-') in the mitochondria. CAT activity was unusually high and not restricted to the microsomal fraction where peroxisomes are usually isolated. The activity was distributed as follows: cytosol (163 units) mitochondria (125 units) and microsomes (119 units). Similar to CAT, the subcellular compartmentalization of both GPOX and GR was unusual. N o activity was detected in the cytosol, but in mitochondria and microsornes, GR levels were 5.49 and 3.09 units. Although GPOX activity exhibited 14-16-fold enrichment in mitochondria and microsornes, respectively, over the 850g crude homogenate, the level was negligible (mitochondria = 1.4 x IOW3 units; microsomes = 1.6 x lop3 units), indicating that this enzyme is absent. The,unusual distribution of CAT has apparently evolved as an evolutionary answer to the absence of GR from the cytosol, and the lack of GPOX activity.Key words: antioxidant enzymes, catalase, detoxification of oxyradicals, glutathione peroxidase, glutathione reductase, superoxide dismutase INTRODUCTIONIn all aerobic organisms the superoxide anion free radical is generated through one-electron reduction of dioxygen from numerous biological sources Received October 5,1987; accepted January 6,1988. Address reprint requests to Dr. Sami Ahmad, Department of Biochemistry, University of Nevada, Reno, NV 89557-0014. which include small to large autoxidizable molecules (e.g., catecholamines, hemoproteins), oxidoreductases (e.g ., xanthine oxidase), and subcellular organelles such as nuclei, mitochondria, microsomes, and chloroplasts [l] .Damages from 02-* generating systems have been recently reviewed [2].Further, 0,-in its protonated form, -0OH radical, and the conversion of 0,-via the metal-catalyzed Haber-Weiss reaction (Fenton reaction) produces in vivo H202 and -OH [3]. These products are more reactive than their precursor, and in particular, -OH is very deleterious, as it causes lipid peroxidation [4]. The potential is especially high for harmful consequences of lipid peroxidation in insects in that lipids not only are essential components of cellular membranes but also have unique physiological functions in insects [5].Phytophagous insects are additionally subject to exogenous plant prooxidant sources of toxic oxygen compounds and free radicals. Plant pro-oxidants consist of photodynamically active compounds such as furanocoumarins, e.g., xanthotoxin [6], which mainly generates ' 0 2 and only secondarily some 0,: [q. Phenolic compounds such as flavonoids, e.g., quercetin, generate 0,-and H202 apparently nonphotoactively upon enzymatic bioactivation [8] and these products further generate -OH 191.Among well-known defenses are antioxidant ...