The mechanism of the enzymic reaction of an iron-containing superoxide dismutase purified from the marine bacterium Photobacterium leiognathi was studied by using pulse radiolysis. Measurements of activity were done with two different preparations of enzyme containing either 1.6 or 1.15 g-atom of iron/mol. In both cases, identical values of the second-order rate constant for reaction between superoxide dismutase and the superoxide ion in the pH range 6.2-9.0 (k=5.5 X 10(8) M-1-S-1 at pH 8.0) were found. As with the bovine erythrocuprein, there was no evidence for substrate saturation. The effects of reducing agents (H2O2, sodium ascorbate or CO2 radicals) on the visible and the electron-paramagnetic-resonance spectra of the superoxide dismutase containing 1.6 g-atom of ferric iron/mol indicate that this enzyme contains two different types of iron. Turnover experiments demonstrate that only that fraction of the ferric iron that is reduced by H2O2 is involved in the catalysis, being alternately oxidized and reduced by O2; both the oxidation and the reduction steps have a rate constant equal to that measured under turnover conditions. These results are interpreted by assuming that the superoxide dismutase isolated from the organism contains 1 g-atom of catalytic iron/mol and a variable amount of non-catalytic iron. This interpretation is discused in relation to the stoicheiometry reported for iron-containing superoxide dismutases prepared from several other organisms.
Superoxide radical ions (O2-) produced by the radiolytic reduction of oxygenated formate solutions and by the xanthine oxidase-catalysed oxidation of xanthine were shown to oxidize the haem groups in oxyhaemoglobin and reduce those in methaemoglobin as in reactions (1) and (2): (see articles) Reaction (1) is suppressed by reaction (8) when [O2-]exceeds 10 muM, but consumes all the O2- generated in oxyhaemoglobin solutions when [oxyhaemoglobin] greater than 160 muM and [O2-]less than 1 nM at pH 7. The yield of reaction (2) is also maximal in methaemoglobin solutions under similar conditions, but less than one haem group is reduced per O2- radical. From studies of (a) the yield of reactions (1) and (2) at variable [haemoglobin] and rates of production of O2-, (b) their suppression by superoxide dismutase, and (c) equilibria observed with mixtures of oxyhaemoglobin and methaemoglobin, it is shown that k1/k2=0.7 +/- 0.2 and k1 = (4 +/- 1) × 10(3) M-1-S-1 At pH7, and k1 and k2 decrease with increasing pH. Concentrations and rate constants are expressed in terms of haem-group concentrations. Concentrations of superoxide dismutase observed in normal erythrocytes are sufficient to suppress reactions (1) and (2), and hence prevent the formation of excessive methaemoglobin.