Antioxidant defenses and lipid peroxidation during anoxia stress and aerobic recovery in the marine gastropod, Littorina littorea
The effect of anoxia exposure (6 d under N2 gas at 5°C) and aerobic recovery (at 5°C) on the antioxidant defenses of the marine periwinkle, Littorina littorea L., were assessed in hepatopancreas and foot muscle. In hepatopancreas, the maximal activities of antioxidant enzymes clearly responded to changes in oxygen availability. Activities of five enzymes were suppressed (to 44-70 % of controls) in hepatopancreas during anoxia exposure: superoxide dismutase (SOD), catalase (CAT), total glutathione peroxidase (GPox), glutathione reductase (GR) and glutathione-S-transferase. When returned to aerobic conditions, activities of the glutathione-related enzymes in hepatopancreas all rose again in concert reestablishing control levels within 12 h whereas SOD and CAT activities remained suppressed even after 24 h recovery. In foot muscle only SOD activity decreased during anoxia (to 56 % of control) whereas during aerobic recovery GPox activity decreased by about 85 % and SOD, CAT and GR activities rose. Anoxia exposure stimulated an increase in the amount of the low molecular weight antioxidant, glutathione, in both organs; total glutathione (GSH + 2 GSSG) was 2.8- and 1.6-fold higher than control levels in hepatopancreas and foot, respectively. Elevated GSH may be needed when oxygen is reintroduced or synthesis of the tripeptide may be favored under the reducing conditions of anoxia. Total glutathione content of both tissues continued to rise during aerobic recovery but only after 24 h recovery did a significant increase in the GSH/GSSG ratio occur. The changes in enzymatic and metabolite antioxidant defenses during anoxia and recovery suggest that these are naturally adaptable in response to changes (or anticipated changes) in the generation of oxygen free radicals within tissues, a feature that could serve the natural lifestyle of this species which experiences cyclic periods of oxygen availability/deprivation with the changing tides. To determine whether antioxidant defenses were responding to free radical damage to tissue macromolecules, peroxidative damage to lipids was also measured by three methods that quantify damage at different stages of lipid degradation: initial (conjugated dienes), middle (lipid hydroperoxides), and terminal (thiobarbituric acid reactive substances [TBARS] measure breakdown products such as malondialdehyde). Hepatopancreas showed no change in either initial or terminal products of radical attack over anoxia/recovery (6 d anoxia followed by 0.5, 1, 5 or 12 h aerobic recovery) whereas the level of lipid hydroperoxides was strongly suppressed during anoxia and remained low throughout recovery. Thus, the antioxidant defenses of hepatopancreas appear to be fully capable of handling an increase in oxygen free radical generation associated with the reintroduction of oxygen after anoxia. Foot muscle showed a different response with increased damage detected at both initial and middle stages during anoxia exposure. However, lipid hydroperoxide levels were reduced again within 30 min of recovery whereas levels of conjugated dienes returned to control values after 5 h. TBARS were largely unaffected in foot suggesting that peroxidative damage can be repaired in the tissue so that terminal breakdown products do not accumulate.