Evidences for Substrate Activation of Copper Catalyzed Intradiol Cleavage in Catechols
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Catechol
Dioxygenase
Semiquinone
Cleavage (geology)
Reactivity
Semiquinone
Moiety
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Catechol
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The metabolism of quinone-containing antitumor agents involves enzymatic reduction of the quinone by one or two electrons. This reduction results in the formation of the semiquinone or the hydroquinone of the anticancer drug. The consequence of these enzymatic reductions is that the semiquinone yields its extra electron to oxygen with the formation of superoxide radical anion and the original quinone. This reduction by a reductase followed by oxidation by molecular oxygen (dioxygen) is known as redox-cycling and continues until the system becomes anaerobic. In the case of a two electron reduction, the hydroquinone could become stable, and as such, excreted by the organism in a detoxification pathway. In some cases such as aziridine quinones, the hydroquinone can be oxidized by one electron at a time resulting in the production of superoxide, the semiquinone and the parental quinone. Quinone anticancer agents upon reduction can also set up an equilibrium between the hydroquinone, the parental quinone and the semiquinone which results in a long-lived semiquinone. Depending on the compound, aziridine quinones, for example, this equilibrium is long lasting thus allowing for the detection of the semiquinone under aerobic conditions. This phenomenon is known as comproportionation-disporportionation equilibrium. The series of reviews in this Special Issue address the consequences of bioreduction of quinone alkylators used in the treatment of cancer. In this particular review we are interested in describing the phenomenon of redox-cycling, how it is measured, and the biological consequences of the presence of the semiquinone and the oxygen radicals generated.
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Hydroquinone
Aziridine
Benzoquinone
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Catechol oxidase
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Catechol oxidase
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Catechol oxidase
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The electrochemical properties of 17 furanquinones, 5 pyridoquinones, and the iminoquinone diplamine in aprotic solvent systems were investigated. For the furanquinone and pyridoquinone derivatives, the quinone/semiquinone and semiquinone/dianion redox couples were observed as two successive one‐electron transfer steps during cyclic voltammetry. For the pyridoquinones, two additional voltammetric waves attributed to nitro group reduction were observed at more negative potentials. The influence of molecular structure on quinone reduction potential is addressed. Reduction of diplamine was analogous to reduction of the quinones, occurring in two successive one‐electron processes. In the presence of a proton donor, pyridoquinone reduction occurred via an ECEC mechanism. For the furanquinones, a general relationship is observed between reduction potential and reported inhibitory activity against various cancer cell lines.
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Dioxygenase
Semiquinone
Cleavage (geology)
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Abstract Electrochemical characterization of topa quinone (6-hydroxydopa quinone), the organic cofactor of copper-containing amine oxidases, has been performed with the aid of spectroscopy and ab initio energy minimization technique. Topa quinone exhibits a totally reversible cyclic voltammogram at a mercury electrode, which is ascribed to a two-step one-electron conversion between topa quinone and topa via topa semiquinone intermediate. Digital simulation of the reversible wave has afforded the separated estimation of each one-electron redox potential. The acid-dissociation constants of the phenolic hydroxyl groups of topa quinone, topa semiquinone and topa have been evaluated electrochemically and supported by electronic and electron spin resonance spectra. At pH 7.0, topa quinone is acid-dissociated and has two-electron redox potential of 0.079 V vs. NHE coupled with a three-proton transfer. Redox catalytic activity of topa quinone for the oxidation of amines and NADH was not observed over conventional voltammetric time periods. Energy minimization calculation of acid-dissociated topa quinone anion indicates an intermediate electronic structure between the p - and o -quinone types with three almost equivalent carbonyl groups. The lack of the redox catalytic activity of free topa quinone appears to be attributable to the partial contribution of the p -quinone-type structure.
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