Superoxide Forms

Not all oxidants formed biolc cally have the potential to promote lipid peroxidation. The free radicals superoxide and nitric oxide [or endothelium-derived relaxing aor (EDRF)] are known to be formed in ww but are not able to initiate the peroxidation of lipids (Moncada et tU., 1991). The protonated form of the superoxide radical, the hydroperoxy radical, is capable of initiating lipid peroxidation but its low pili of 4.5 effectively precludes a major contribution under most physiological conditions, although this has been suggested (Aikens and Dix, 1991). Interestingly, the reaction product between nitric oxide and superoxide forms the powerful oxidant peroxynitrite (Equation 2.6) at a rate that is essentially difiiision controlled (Beckman eta/., 1990 Huie and Padmaja, 1993). 

Residues of potassium-sodium alloy in metal containers were covered with oil prior to later disposal. When a lid was removed later, a violent explosion occurred. This was attributed to frictional initiation of the mixture of potassium superoxide (formed on long standing of the alloy) and oil. 

Knoller, S., Shpungin, S., and Pick, E. (1991) The membrane-associated component of the amphiphile-activated, cytosol-dependent superoxide-forming NADPH oxidase of macrophages is identical to cytochrome b559./. Biol. Chem. 266, 2795-2804. 

Babior, B. M., Kipnes, R. S. (1977). Superoxide-forming enzyme from human neutrophils Evidence for a flavin requirement. Blood, 50, 517-24. 

Work by Harbour, Chow and Bolton (1974) on the spin adducts of superoxide (or HOO )13 with nitrones paved the way for a number of investigations of superoxide and hydroperoxyl radical chemistry. Harbour and Bolton (1975) used DMPO to trap superoxide formed by spinach chloroplasts in the presence of 02. The signal strength was greatly enhanced when methylviologen was present, consistent with the hypothesis that this bis-pyridinium dication accepts an electron from the primary acceptor of photoprotein I, and then transfers it to molecular oxygen. 

Oxidation of Potassium Peroxide. Determination of Potassium Superoxide. Potassium peroxide was prepared by the addition of a tert-butyl alcohol solution of 90% hydrogen peroxide to potassium tert-butoxide in DMSO or tert-butyl alcohol. Oxygen absorption was followed in the standard manner (20). Analysis of solid precipitates for potassium superoxide followed exactly the method of Seyb and Kleinberg (23). Potassium superoxide formed in the oxidation of benzhydrol was determined in a 15-ml. aliquot of the oxidation solution. To this aliquot 10 ml. of diethyl phthlate was added to prevent freezing of the solution. The mixture was cooled to 0°C., and 10 ml. of acetic acid-diethyl phthlate (4 to 1) added over a period of 30 minutes with stirring. The volume of the evolved oxygen was measured. 

The extended x-ray absorption fine structure (EXAFS) spectrum of the MnnMnn catalase from L. plantarum [82] revealed a nearest neighbor at 2.19 A and no evidence of a short Mn -0 shell [82], There is no evidence of a Mn—Mn vector. These EXAFS data support the coordination of one to two imidazoles per Mn in the superoxidized form and two to four in the reduced form, suggesting... 

Hydroxyl radical may hydroxylate tyrosine to 3,4-dihydroxyphenylalanine (DOPA). DOPAs are the main residues corresponding to protein-bound reducing moieties able to reduce cytochrome c, metal ions, nitro tetrazolium, blue and other substrates (S32). Reduction of metal ions and metalloproteins by protein-bound DOPA may propagate radical reactions by redox cycling of iron and copper ions which may participate in the Fenton reaction (G9). Abstraction of electron (by OH or peroxyl or alkoxyl radicals) leads to the formation of the tyrosyl radical, which is relatively stable due to the resonance effect (interconversion among several equivalent resonant structures). Reaction between two protein-bound tyrosyl radicals may lead to formation of a bityrosine residue which can cross-link proteins. The tyrosyl radical may also react with superoxide, forming tyrosine peroxide (W13) (see sect. 2.6). 

CAT may react with superoxide forming Compound III which is a resonance hybrid between the forms CAT-Fe -Oj (predominating) and CAT-Fe- -O, [209]. 

D2. Dewald, B., Baggiolini, M., Curnutte, J. T, and Babior, B. M., Subcellular localization of the superoxide-forming enzyme in human neutrophils. J. Clin. Invest. 63,... 

Figure 7.12. Correlation between the OSC at 400°C of Ce,Zr(i.,)02 samples and the amount of superoxides formed upon oxygen adsorption (O.Smbar) at room temperature.

T. Mild, L.S. Yoshida, and K. Kakinuma, Reconstitution of superoxide-forming NADPH oxidase activity with cytochrome b55g purified from porcine neutrophils. Requirement of a membrane-bound flavin enzyme for reconstitution of activity,/. Biol Chem. 267 18695 (1992). 

It is known that superoxide forms complexes with Ca ", thereby permitting formation of calcium peroxide. Since the majority of biological membranes translocate Ca or other alkali metal ions, the opportunity for interaction with superoxide within the membrane may be considerable (Ca " and calmodulin are required for superoxide production by leukocytes ). Under pathological circumstances calcium peroxide could be formed and lipid peroxidation initiated. The breakdown of membrane capacitance as a consequence of this may be deleterious in a number of ways. For example, the loss of a critical electric field may remove the driving force for energy-dependent processes associated with charge transfer. On the other hand, circumstances may be established which bring about unrestrained and unmodulated metabolic activity, as in cancer. 

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