Cabbage looper antioxidant enzymes: Tissue specificity

Abstract

A unique pattern of antioxidant enzymes exists in phytophagous insects for defense against endogenous and exogenous sources of toxic forms of oxygen, and data presented herein describe a profile of these enzymes in many tissues of larvae of the cabbage looper moth, Trichplusia ni. The specific activities of the antioxidant enzymes were high in tissues of high metabolic activities, i.e. Malpighian tubules, hindgut, muscles and gonads. A unique finding was the high constitutive activity of a superoxide dismutase (SOD) in hemocytes, probably consisting predominantly of the CuZn-SOD, which is analogous to the exclusive presence of this form of SOD in vertebrate erythrocytes and leukocytes. In all other tissues, the activity of Mn-SOD was higher than that of the CuZn-SOD which is converse to the pattern in vertebrate tissues. The glutathione peroxidase (GPOX) activity, present in all tissues and with highest levels in the gonads, did not seem to be the selenoprotein typical of the mammalian GPOX. $\text{Glutathione}\text{-S-}\text{transferase}$ (GST) activity paralleled that of its glutathione peroxidase activity (GSTPX). The high GSTPX activity suggests that GSTPX and not GPOX, forms a sequential team with glutathione reductase (GR) to reduce deleterious lipid hydroperoxides and to reduce the oxidized glutathione, GSSG, back to GSH. Catalase (CAT) which decomposes H₂O₂ has very high activity apparently correlated with the low GPOX activity. Finally, the integumental epithelium, and the gut (combined sections) possessed higher amounts of antioxidant enzymes than other tissues. Thus, a physiological relationship may occur between the antioxidant enzyme levels in tissues of T. ni with particularly high metabolic activity and associated endogenous oxidative stress. In addition, another physiological role of these enzymes may be to protect from exogenous oxidative stress exerted by dietary redox-active pro-oxidants in the gut, and to the potential of photodynamically mediated oxygen toxicity in peripheral organs such as the integument.