Reduced enzymatic antioxidative defense in deep-sea fish

Oxygen, while being an obligate fuel for aerobic life, has been shown to be toxic through its deleterious reactive species, which can cause oxidative stress and lead ultimately to cell and organism death. In marine organisms, reactive oxygen species (ROS), such as the superoxide anion and hydrogen p eroxide, are generated within respiring cells and tissues and also by photo chemical processes in sea water. Considering both the reduced metabolic rat e of nektonic organisms thriving in the deep sea and the physico-chemical c onditions of this dark, poorly oxygenated environment, the meso- and bathyp elagic waters of the oceans might be considered as refuges against oxidativ e dangers. This hypothesis prompted us to investigate the activities of the three essential enzymes (superoxide dismutase, SOD; catalase, CAT; glutath ione peroxidase, GPX) constitutive of the antioxidative arsenal of cells in the tissues of 16 species of meso- and bathypelagic fishes occurring betwe en the surface and a depth of 1300 m, While enzymatic activities mere detec ted in all tissues from all species, the levels of SOD and GPX decreased in parallel with the exponential reduction in the metabolic activity as estim ated by citrate synthase activity, In contrast, CAT mas affected neither by the metabolic activity nor by the depth of occurrence of the fishes. High levels of metabolic and antioxidative enzymes were detected in the light or gans of bioluminescent species. The adjustment of the activity of SOD and G PX to the decreased metabolic activity associated with deep-sea living sugg ests that these antioxidative defense mechanisms are used primarily against metabolically produced ROS, whereas the maintenance of CAT activity throug hout all depths could be indicative of another role. The possible reasons f or the occurrence of such a reduced antioxidative arsenal in deep-sea speci es are discussed.