MnSOD

Superoxide dismutases (SODs) are a family of cytosolic metalloenzymes that specifically remove (reviewed by Omar etal., 1992). SOD distribution within the body is ubiquitous, being found in erythrocytes as well as most organs and cell types. Three distinct mammalian SOD forms exist CuZnSOD, MnSOD and extracellular SOD (EC-SOD). Their amino-acid sequences differ as well as the transition metals at their active sites. Rheumatoid synovial fluid contains low levels of SOD activity and hence little protection from ROM generated by infiltrating PMNs (Blake etcU., 1981). Furthermore, leucocytes from patients with RA are deficient in MnSOD, which might promote the extracellular leakage of O2 (Pasquier et al., 1984). 

The mitochondrial dysfunctionality seen in manganese neurotoxicity might be related to the accumulation of reactive oxygen species (Verity, 1999). Mitochondrial Mn superoxide dismutase (MnSOD) is found to be low or absent in tumour cells and may act as a tumour suppressor. It is induced by inflammatory cytokines like TNF, presumably to protect host cells. In a rat model, iron-rich diets were found to decrease MnSOD activity, although a recent study reported that in rat epithelial cell cultures iron supplementation increased MnSOD protein levels and activity, but did not compromise the ability of inflammatory mediators like TNF to further increase the enzyme activity (Kuratko, 1999). 

Gardner et al. [165] have shown that the redox-cycling agent phenazine methosulfate (PMS), mitochondrial ubiquinol-cytochrome c oxidoreductase, or hypoxia inactivated aco-nitase in mammalian cells. It has been proposed that the inactivation of aconitase is mediated by superoxide produced by prooxidants because the overproduction of mitochondrial MnSOD protected aconitase from inactivation by the prooxidants mentioned above except hyperoxia. Later on, the reaction of superoxide with aconitases began to be considered as one of the most important ways to NTBI generation in vivo. 

In earlier studies [5,6] superoxide detection in mitochondria was equated to hydrogen peroxide formation. However, while it is quite possible that superoxide is a stoichiometric precursor of mitochondrial hydrogen peroxide, it is understandable that the level of hydrogen peroxide may be decreased due to the reactions with various mitochondrial oxidants. Moreover, superoxide level can be underestimated due to the reaction with mitochondrial MnSOD. Several authors [7,8] assumed that mitochondrial superoxide production may be estimated through cyanide-resistant respiration, which supposedly characterizes univalent dioxygen reduction. This method was applied for the measurement of superoxide production under in vitro normoxic and hyperoxic conditions, in spite of the finding [7] that cyanide-resistant respiration reflects also the oxidation of various substrates (lipids, amino acids, and nucleotides). Earlier,... 

It is important that mitochondrial oxygen radical production depends on the type of mitochondria. Recently, Michelakis et al. [78] demonstrated that hypoxia and the proximal inhibitors of electron transport chain (rotenone and antimycin) decreased mitochondrial oxygen radical production by pulmonary arteries and enhanced it in renal arteries. This difference is probably explained by a lower expression of the proximal components of electron transport chain and a greater expression of mitochondrial MnSOD in pulmonary arteries compared to renal arteries. 

The mechanisms of superoxide-dismuting activity of SODs are well established. Dismutation of superoxide occurs at copper, manganese, or iron centers of SOD isoenzymes CuZnSOD, MnSOD, or FeSOD. These isoenzymes were isolated from a variety of sources, including humans, animals, microbes, etc. In the case of CuZnSOD, dismutation process consists of two stages the one-electron transfer oxidation of superoxide by cupric form (Reaction (1)) and the one-electron reduction of superoxide by cuprous form (Reaction (2)). 

Similar reactions are catalyzed by Mn and Fe centers of MnSOD and FeSOD. It is obvious that before participation in Reaction (2), superoxide must be protonized to form hydroper-oxyl radical HOO by an outer-sphere or an intra-sphere mechanisms. All stages of dismuting mechanism, including the measurement of elementary rate constants, have been thoroughly studied earlier (see, for example, Ref. [2]). 

Numerous excellent reviews on the possible role of oxygen radicals in cancer and carcinogenesis have been published 10-20 years ago [147-153], Earliest studies have been much concerned with the role of SOD in tumor cells. Despite some contradictory results, it is general conclusion that tumor cells are usually characterized by lowered CuZnSOD activity and always by lowered MnSOD activity [147]. The origin of SOD declining in cancer cells is unknown. It has been suggested that MnSOD is not induced in cancer immortalized cells in response to oxidative stress, but the reason of this is uncertain [154],... 

Thus, oxygen radical production by leukocytes can be responsible for cancer development. However, the levels of leukocyte oxygen radical generation depend on the type of cancer. For example, PMNs and monocytes from peripheral blood of patients with lung cancer produced a diminished amount of superoxide [169], Timoshenko et al. [170] observed the reduction of superoxide production in bronchial carcinoma patients after the incubation of neutrophils with concanavalin A or human lectin, while neutrophils from breast cancer patients exhibited no change in their activity. Chemotherapy of lung and colorectal carcinoma patients also reduced neutrophil superoxide production. Human ALL and AML cells produced, as a rule, the diminished amounts of superoxide in response to PMA or FMLP [171], On the other hand total SOD activity was enhanced in AML cells but diminished in ALL cells, while MnSOD in AML cells was very low. It has been proposed that decreased superoxide production may be responsible for susceptibility to infections in cancer patients. 

The importance of superoxide-mediated damage to cancer cells was also demonstrated in the experiments with overexpressed mitochondrial MnSOD. Hirose et al. [186] showed that the overexpression of mitochondrial MnSOD enhanced the survival of human melanoma cells exposed to cytokines IL-1 and TNF-a, anticancer antibiotics doxorubicin and mitomycin C, and y-irradiation. Similarly, Motoori et al. [187] found that overexpression of MnSOD reduced the levels of reactive oxygen species in mitochondria, the intracellular production of 4-hydroxy-2-nonenal, and prevented radiation-induced cell death in human hepatocellular... 

The enzyme superoxide dismutase (SOD) occurs in three forms in mammalian systems (1) CuZnSOD (SOD1) found in the cytosol, (2) MnSOD (SOD2) found in mitochondria, and (3) CuZnSOD found in extracellular space (SOD3). Additionally, many bacterial SOD enzymes contain iron. SOD 1 has been discussed in detail... 

Among many different complexes that have been synthesized in attempt to mimic the structure and/or functionality of SODs (16-22), the most active SOD mimetics known to date are seven-coordinate Mn(II) complexes with macrocyclic ligands derived from C-substituted pentaazacyclopentadecane [15]aneNs and its pyridine derivative (Scheme 4) (12d,16a,23-25). Some of them possess SOD activity that exceeds the one of native mitochondrial MnSOD, and are the first SOD mimetics which entered clinical trials (12d,16a,23,26-28). A few Fe(III) complexes with the same type of ligands have also been studied and they are one of the best iron-based SOD catalysts (18). It should be stressed that the decomposition of superoxide catalyzed by these complexes has been quantified by direct stopped-fiow method, in the presence of a substantial superoxide excess over catalyst, as a reliable method for determining true SOD activity (29). 

The intriguing question is how the seven-coordinate geometry around the metal center favors its remarkable catalytic activity, knowing that in the native MnSOD and FeSOD enzymes the active metal center has a five-coordinate geometry (3a,14f30). All SOD... 

Protons are in general indispensable for the dismutation of superoxide (Eq. (4)). Also in the case of its dismutation catalyzed by a metal center, two protons are needed for the dissociation of the product (H2O2) from the metal center (Scheme 9). Therefore, a complex which can accept two protons upon reduction and release them upon oxidation is an excellent candidate for SOD activity. The studies on proton-coupled electron transfer in Fe- and Mn-SODs 48), demonstrated that the active site of MnSOD consists of more than one proton acceptor (Scheme 10). Since the assignment of species involved in proton transfer is extremely difficult in the case of enzymatic systems, relevant investigations on adequate model complexes could be of vast importance. H2dapsox coordinates to Fe(II) in its neutral form, whereas in the case of Fe(III) it coordinates in the dapsox form. Thus, oxidation and reduction of its iron complex is a proton-coupled electron transfer process 46), which as an energetically favorable... 

Bacterial SODs typically contain either nonheme iron (FeSODs) or manganese (MnSODs) at their active sites, although bacterial copper/zinc and nickel SODs are also known (Imlay and Imlay 1996 Chung et al. 1999). Catalases are usually heme-containing enzymes that catalyze disproportionation of hydrogen peroxide to water and molecular oxygen (Eq. 10.2) (Zamocky and Koller 1999 Loewen et al. 2000). 

It is believed that MnSOD plays a pivotal role in many diseases. There have been many reviews of the biochemistry of MnSOD " and focusing on the structural aspects of the enzyme. Four different types of SOD are known, a Cu/Zn-containing SOD, a FeSOD, a NiSOD, and MnSOD. MnSODs, which are structurally related to the FeSODs, have a of 23,000 ( 200 amino acids) and function as a dimer or as a tetramer. MnSOD catalyzes the dismutation reaction by cycling between the - -2 and +3 oxidation states. One proton is taken up by the system in each step (Equation (2)) ... 

Figure 24 Active site of the MnSOD molecule. Active site residues are numbered in accordance with the...

Crystal structure determinations of MnSODs from organisms ranging from E. coli to humans have been reported. Structural determinations of note include those by Jameson et al. on the E. coli enzyme and mutant forms of this enzyme with atomic resolution,a cambialistic superoxide dismutase from Porphyromonas gingivalis, and mutant forms of the human enzyme the Q143N, and Q143A mutants.The coordination sphere of the... 

---