Aqueous solutions superoxide dismutase

CL Eisher, J-L Chen, J Li, D Bashford, L Noodleman. Density-functional and electrostatic calculations for a model of a manganese superoxide dismutase active site in aqueous solution. J Phys Chem 100 13498-13505, 1996. 

FIGURE 10.6 Comparison of solid-state and liquid-state spectra from a copper protein. The figure illustrates shifts in apparent gz and Az-values of the S = 1/2 and I =3/2 spectrum from Cu11 in bovine superoxide dismutase as a function of the surrounding medium. Top trace frozen aqueous solution at 60 K middle trace frozen water/glycerol (90/10) solution at 60 K bottom trace aqueous solution at room temperature. (Modified from Hagen 1981.)... 

H.J. Forman and I. Fridovich, Electrolytic univalent reduction of oxygen in aqueous solution demonstrated with superoxide dismutase. Science. 175, 339 (1972). 

The demonstration that PMNs formed O2- in the respiratory burst necessitated the consideration of all the species which result when dioxygen is reduced one electron at a time (Fig. 1). Superoxide, the result of the reduction of dioxygen by one electron, appears to act mainly as a mild reductant in aqueous solutions. But when it coexists with H2O2, its spontaneous dismutation product, O can initiate a number of potentially injurious events [reviewed by Fridovich The primary means by which cells deal with superoxide anions appears to be through the catalysis of their dismutation by a family of metalloenzymes collectively designated superoxide dismutases. 

Effect of Pressure on Proton-Coupled Electron Transfer Reactions of Seven-Coordinate Iron Complexes in Aqueous Solution It has been shown that seven-coordinate Fe(III) diaqua complexes consisting of a pentaaza macrocyclic ligand possess superoxide dismutase (SOD) activity, and therefore could serve an imitative SOD function.360 Choosing appropriate chemical composition of a chelate system yielded suitable pKa values for the two coordinated water molecules so that the Fe(III) complexes of 2,6-diacetylpyridine-bis(semicarbazone) (dapsox) and 2,6-diacetylpyridine-bis(semioxamazide) (dapsc) (see Scheme 7.12) would be present principally in the highly active aqua-hydroxo form in solution at physiological pH.361... 

M17. McClune, G. J., and Fee, J. A., Stopped flow spectrophotometric observation of superoxide dismutase in aqueous solution. FEBS Lett. 67, 294-298 (1976). 

Or From Tetrabutyl-Ammonium Superoxide. Tetrabutyl-ammonium superoxide was found convenient for the superoxide dismutase assay since it dissolved readily without decomposition in N,N-dimethyl form-amide (70). Infusions of this solution into a cuvette containing aqueous oxidized cytochrome c reduced the available cytochrome c. In the presence of different erythrocuprein concentrations the reduction rate was progressively diminished. According to McCord and Fridovich (70) an enzyme unit was defined as 50% inhibition of the rate of reduction of cytochrome c. Alternatively tetranitromethane was found appropriate for monitoring Or (70, 136—148) where the stable nitroform anion C(N02)3 is being formed (Equ. (a)) ... 

Proof of the generation of Or during electrolysis of aqueous solutions was obtained in the laboratory of Fridovich (206). The OI catalysed oxidation of adrenaline served as a monitor which could be inhibited by superoxide dismutase. Ultrasonication of buffered aerated solutions gave rise to the formation of Or which could be detected using the cytochrome-c reductase assay (207). This sonication induced cyto-chrome-c reduction was also inhibited by native erythrocuprein. Another sensitive superoxide dismutase assay using the reduction of nitro blue tetrazolium by Or was developed by Beauchamp and Fridovich (208). This assay allowed the detection of erythrocuprein in the ng/ml region. During the metalloenzyme conference in Oxford, 1972, Fridovich summarized the basic facts on superoxide dismutase (erythrocuprein) (209). 

The term auto-oxidation is often used to refer to the oxidation of l-ascorbic acid by molecular oxygen in aqueous solution in the absence of any catalysts. The many studies on this topic still have not produced an entirely satisfactory mechanism for the electron transfer from ascorbate to the dioxygen molecule. E.s.r. studies have shown that when ascorbate reacts with dioxygen in aqueous solution in the pH range 6.6-9.6, a steady-state concentration of ascorbate radicals is produced. There is, however, little direct evidence for the formation of H2O2 in this reaction. Data from the inhibition of the reaction by certain enzymes, notably superoxide dismutase, suggest that H2O2 is produced in the process. 

This method has proven to be a powerful tool for routine determinations of superoxide dismutases in nonpurified samples. The dropping mercury electrode acts in an Oj-saturated aqueous solution as a source of superoxide and a detector of its dismutation as well The electroreduction of molecular oxygen in aqueous solutions... 

Concerning the catalytic superoxide dismutase activity of low molecular mass copper complexes, some general comments are neccessary. Firstly, it should be emphasized, that apart from the inactive Cu-penicillamine, all complexes described above do not survive high concentrations of biological chelators in aqueous solutions. For example, bovine serum albumine is able to remove most of the copper from these complexes (Fig. 14). 

The amount of copper in the body is 80-100 mg. Copper is a component of a number of oxido-reductase enzymes (cytochrome oxidase, superoxide dismutase, tyrosinase, uricase, amine oxidase). In blood plasma, it is bound to ceruloplasmin, which catalyzes the oxidation of Fe + to Fe +. This reaction is of great significance since it is only the Fe + form in blood which is transported by the transferrin protein to the iron pool in the liver. The daily copper requirement is 1-1.5 mg and it is supplied in a normal diet. Copper is even less desirable than iron during food processing and storage since it catalyzes many unwanted reactions. Cu +-Ions are taste bearing. The threshold value 2.4-3.8 mg/1 was determined with aqueous solutions of CuSOa or CuCl2. 

The solution produced in the cathodic con artment by electrolyzing a dilute solution of NaCl exhibits low dissolved oxygen and high dissolved hydrogen (i). These properties of the cathodic solution suggest that it is potentially usefiil for preventing the oxidation of polyunsaturated lipids and their related compounds in an aqueous systems. Furthermore, this solution is reported to have superoxide dismutase-like and catalase-Uke activities (7). However, this data has not yet been confirmed. 

The efficiency of the electron transfer from a-terthienyl to oxygen was estimated to be lower than 1% 209). The efficiency is somewhat higher, 6%, with 2,2 -bithienyl 103). The generation of superoxide was revealed experimentally by the photoreduction of cytochrome c or of nitro blue tetrazolium sensitized by a-terthienyl in aqueous solution under aerobic conditions, a reaction which is suppressed in the presence of the very specific enzyme superoxide dismutase. Electron transfer reactions can also occur directly from electronically excited a-terthienyl to cytochrome c, but they are not favored in the presence of oxygen 150). 

Combined ESR and optical titrations with azide point to the presence of two binding sites on iron (III), with only the first being occupied at room temperature and the second azide binding as a result of freezing the aqueous solution (Slykhouse and Fee, 1976). The results suggest that each of the two iron (III) atoms of the super oxide dismutase has two coordination positions available for the interaction with solute species, but only one is necessary for catalysis of the superoxide dismutation reaction. 

---