The Theory of Reversible Two-step Oxidation Involving Free Radicals.

Michaelis, Leonor; Schubert, Moritz

doi:10.1021/cr60073a003

Cited by 145 publications

(33 citation statements)

References 11 publications

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“…Synthetic reactions, requiring oxidative phosphorylation, would be covered by this type of mechanism Perhaps a few words will not be out of order about the possible nature of the active intermediate that is generated in the course of oxidation. The work of Michaelis (1935Michaelis ( , 1938 strongly suggests that all reversible oxidations, involving the removal of two electrons, actually occur in single steps, thereby giving rise to free-radical type of intermediates. Compounds of this type, having an unshared electron, are, under usual conditions, extremely unstable, owing to their great reactivity.…”

Section: On Mechanisms Of Reactionmentioning

confidence: 99%

The Cyclophorase Complex of Enzymes

Green

1951

Biological Reviews

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SUMMARY The cyclophorase complex of enzymes which implement the citric acid cycle is contained within the mitochondrial bodies. Mitochondria behave as fibre‐like macromolecules which can be comminuted first to submicroscopic particles (microsomes) and then eventually to soluble proteins. The cyclophorase system of enzymes can be prepared by differential centrifugation of mitochondria at low temperatures. Even at oo the cyclophorase activity declines sharply in a matter of hours. The freshly prepared enzyme gel contains all the necessary co‐factors and enzymes for the reactions of the citric acid cycle and of fatty acid oxidation. With time, a requirement for adenosine‐5‐phosphate and magnesium ions emerges. Various amino‐acids and fatty acids (even‐numbered) are burned to carbon dioxide and water in the cyclophorase system by virtue of their ability to give rise to members of the citric acid cycle, either directly or indirectly. Although intermediates do not accumulate during the normal activity of the cyclophorase system, experimental devices can be employed which permit the study of one‐step oxidations or conversions. Fatty acids are not oxidized unless ‘sparked’ by the co‐oxidation of some member of the citric acid cycle. The sparking effect is eliminated by either 2:4‐dinitrophenol or gramicidin. The oxidation of fatty acids can be shown to proceed by way of β‐oxidation. The β‐keto acids interact with oxalacetate in a transacetylation reaction leading to formation of citric acid and a fatty acid with two carbon atoms less than the parent acid. Under conditions for active oxidation acetoacetic acid arises in liver cyclophorase principally if not exclusively as the terminal 4‐carbon residue of the even‐numbered fatty acids. Odd‐numbered fatty acids from C3 to C7 do not give rise to acetoacetic acid in significant amount, but C9 and C11 acids do. Propionic acid which arises as the terminal residue of the odd‐numbered fatty acids is inert in kidney, but is oxidizable in liver to carbon dioxide and water by way of pyruvic acid as intermediate. In presence of malonate or under conditions where the citric acid cycle is inhibited, both odd‐ and even‐numbered fatty acids can be converted quantitatively to acetoacetic acid in liver cyclophorase. The cyclophorase system catalyses a group of synthetic condensations which are all distinguished by (1) the necessity for sparking by some members of the citric acid cycle, and (2) inhibition by 2:4‐dinitrophenol and gramicidin. The oxidases which implement the oxido‐reductions of glycolysis are present at least in part in the cyclophorase complex of rabbit kidney and liver. None of the other enzymes of the glycolytic system occur in association with mitochondria. The pyridinoprotein enzymes of the cyclophorase complex appear to be firmly linked with their prosthetic groups. With the transition from particulate to soluble pyridinoproteins, there is a loss of the capacity to bind the prosthetic group. The classical, soluble pyridinoprotein enzymes, with one exception, are full...

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Section: On Mechanisms Of Reactionmentioning

confidence: 99%

The Cyclophorase Complex of Enzymes

Green

1951

Biological Reviews

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“…Assuming that for A, K, = 0, the only complication in the reversible redox sequence is the reaction substitutions from [8], [9], and [I I] into [7] and rearrangement gives Equation 12 is similar to but simpler than equations for the calculation of Kc developed by Michaelis (6,9). It allows calculation of Kc from any point on the potentiometric titration curve.…”

Section: Introductionmentioning

confidence: 99%

The Redox Behavior of 9,10-Anthraquinone-2-sulfonate in Acidic Aqueous Solution

Broadbent¹,

Melanson²

1975

Can. J. Chem.

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A green intermediate, observed during reduction of 9,10-anthraquinone-2-sulfonate (A) to the corresponding anthrahydroquinone (AH2) in acidic aqueous solution, has been identified as a quinhydrone-like charge-transfer complex of A and AH2. Spectrophotometric and potentiometric measurements have shown that changes in the slope of the potentiometric titration curves with varying initial [A] are caused only by this complex [Formula: see text]and not by any semiquinone radical (AH·) or radical anion (A·−) intermediate. The very low semiquinone formation constant in acidic solution indicates that the pKa of AH· < 7.0. Electrochemically determined pKa values for similar radicals in the range 8.5–10.5 are invalid because of interference by charge-transfer complexes of this type.

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“…The possibility might be considered that ubiquinone is reduced to a free radical or semiquinone (Michaelis & Schubert, 1938) Fig. 7.…”

Section: Discussionmentioning

confidence: 99%