The paradigm that all oxygen-respiring eukaryotes have cytosolic CuZn-superoxide dismutase and that Mn-superoxide dismutase is localized to the mitochondria does not apply to a large group of marine arthropods.

The enzyme superoxide dismutase (SOD), which catalyzes the dismutation of the superoxide radical, is present in the cytosol and mitochondria of all oxygen-respiring eukaryotes. The cytosolic form contains copper and zinc (CuZnSOD), whereas the mitochondrial form contains manganese (MnSOD). The latter protein is synthesized in the cytosol as a MnSOD precursor, containing an N-terminal mitochondrial-targeting sequence. CuZnSOD is sensitive toward cyanide (CN) and hydrogen peroxide (H2O2), but MnSOD is not. Assays for SOD activity in cytosol from the hepatopancreas of the blue crab, Callinectes sapidus, showed the presence of a CN/H2O2-insensitive form of SOD. No CN/H2O2-sensitive CuZnSOD was found. This unexpected phenomenon was shown to occur in all decapod crustacea (crabs, lobsters, shrimp) examined. The cytosolic and mitochondrial SODs of C. sapidus were purified by means of ion-exchange, size-exclusion, and reverse-phase HPLC. The cytosolic SOD is a homodimeric protein, which exists in a monomer-dimer equilibrium (24 kDa left and right arrow 48 kDa). The protein contains approximately 1 Mn per subunit. No copper or zinc is present. Amino acid sequence analysis identified the novel cytosolic SOD as a MnSOD precursor with an abnormal mitochondrial-targeting sequence. The mitochondrial SOD of C. sapidus is similar to the MnSOD found in other eukaryotes. N-Terminal amino sequences of mitochondrial and cytosolic blue crab MnSOD differ in several positions. The MnSODs are thus encoded for by two different genes. The paradigm that all eukaryotes contain intracellular CuZnSOD and that MnSOD occurs exclusively in the mitochondria appears not to apply to a large group of marine arthropods.