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A free radical resulting from the removal of one hydrogen atom with its electron during the process of dehydrogenation of a hydroquinone to quinone or similar compound (for example, flavin mononucleotide).
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A second commonly studied antioxidant gene activated by Nrf2 is the quinone oxidoreductase gene (NQO1), which produces an enzyme that prevents semiquinone redox cycling and consequent oxidative stress.
Cigarette smoke contains substantive amounts of toxic oxidants and free radicals including reactive aldehyde, quinone, hydroquinone, semiquinone and superoxide (Cantin, 2010; Yao and Rahman, 2011), which trigger off multiple series of pro-inflammatory responses.
One electron addition to quinine moiety in the tetracyclic ring of anthracyclines results in the formation of semiquinone and free oxygen radicals such as superoxide anion ([O.
B], the secondary electron acceptor), and the semiquinone [Q.
This semiquinone has been shown to bind to the reductase domain of endothelial nitric oxide synthase (eNOS), resulting in the diversion of electron flow from the oxygenase domain of the enzyme, thus leading to an increase in superoxide generation and reduction in NO production (6).
The data showed a signature typical of free radicals and similar to that of semiquinone, a free radical found in cigarette smoke.
Flavin mononucleotide (FMN), which may be photoreduced under illumination of some tetrapyrrolic compounds to the corresponding semiquinone radical [45], increased slightly the quantum yield of HPD photobleaching (Table 7).
117-121) Eumelanin is characterized by the presence of paramagnetic centers that are solely of the semiquinone type, while pheomelanin contains a hyperfine structure with an unpaired electron near the nucleus of [sup.
24-26) Second, superoxides react directly with catecholamines to produce semiquinone radicals and hydrogen peroxide; the former feeds into many other oxidant chain reactions, while the latter can mediate tissue injury by alkylative adduct formation or by redox cycling to produce other toxic oxidizing species.
Although a link between free radicals in these samples and health impacts was suspected, their potential health impacts were not recognized because they were thought to be "inaccessible to cells and too stable to play any part in carcinogenesis" until the publication of a series of papers by Pryor and colleagues demonstrating the viability of catalytic cycles involving semiquinone radicals (Pryor and Squadrito 1995; Pryor et al.
2], semiquinone as QH* and quinone as Q, the antioxidant behaviour comes from the reaction Q[H.