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PubMed · 179587

The one-electron transfer redox potentials of free radicals. I. The oxygen/superoxide system.

Y A Ilan·G Czapski·D Meisel
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Abstract

The method of determination of Redox potentials of radicals, using the pulse radiolysis technique, is outlined. The method is based on the determination of equilibrium constants of electron transfer reactions between the radicals and appropriate acceptors. The limitations of this technique are discussed. The redox potentials of several quinones-semiquinones are calculated, as well as the standard redox potential of the peroxy radical. EO2/O2=-0.33 V and the redox oxidation properties of the peroxy radical in various systems and pH are discussed. The value determined for the redox potentials of O2/O2 is higher by more than 0.2 V than earlier estimates, which has important implications on the possible role of O2 in biological processes of O2 fixation.

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Electron TransportFree RadicalsKineticsMathematicsModels, BiologicalOxidation-ReductionOxygenPotentiometryQuinonesSuperoxidesJournal ArticleResearch Support, U.S. Gov't, Non-P.H.S.

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Y A Ilan, G Czapski, D Meisel. 1976-05-14. The one-electron transfer redox potentials of free radicals. I. The oxygen/superoxide system.. https://doi.org/10.1016/0005-2728(76)90080-3

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Kinetic and mechanistic studies on the reduction of melilotate hydroxylase by reduced pyridine nucleotides.

The reduction of the melilotate hydroxylase . 2-OH-phenyl propionate complex by NADH and reduced 3-acetyl pyridine adenine dinucleotide (AcPyNADH) has been investigated using steady state kinetic and rapid reaction techniques. Reduction by NADH appeared to involve only one charge-transfer-type intermediate (between reduced enzyme and NAD) as previously described (Strickland, S., and Massey, V. (1973) J. Biol. Chem. 248, 2953-2962). Reduction by AcPyNADH was shown to involve two charge-transfer-type intermediates. The first was between oxidized enzyme and AcPyNADH and the second was between reduced enzyme and AcPyNAD. Reaction of AcPyNADH with oxidized enzyme . 2-OH-phenyl propionate complex to form the first charge-transfer complex reached equilibrium within the mixing time of the stopped flow apparatus (5 ms). Subsequent steps in the reaction appeared to be first order and were independent of the AcPyNADH concentration. An 8-fold deuterium isotope effect on the step involving flavin reduction was found when reduced 3-acetyl[4A-2H]pyridine adenine dinucleotide (AcPyNADD) was used as the reductant. Analysis of the rapid reaction results for the reaction of oxidized pyridine nucleotide with reduced enzyme . 2-OH-phenyl propionate complex indicated the presence of two forms of reduced enzyme (in equilibrium) of which only one form was capable of reacting with the oxidized pyridine nucleotide. Based on the rapid reaction data, a mechanism for the reduction half-reaction is proposed. The turnover number calculated from this mechanism is in good agreement with that determined from the steady state data.

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