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. 2022 Oct 4;23(19):11746. doi: 10.3390/ijms231911746

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© 2022 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

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Diagrammatic pathway related to the effects of QC as both antioxidant, and pro-antioxidant in the presence of low and high levels of reduced GSH, both effects are strongly dependent upon the reduced GSH enzyme. During oxidative stress, in the presence of peroxidase, QC reacts with H2O2 to form a semiquinone radical that is rapidly oxidized to QQ, has a pro-oxidant effect, high reactivity towards protein thiols, and DNA leads to cell damage, and cytotoxicity. QQ preferentially reacts with GSH to stable protein oxidized QC adducts namely 6-GSQ, and 8-GSQ. In the presence of high GSH concentration, QQ reacts with GSH to form GSQ, and QQ cannot exert its cytotoxic effects, by contrast QQ reacts with protein thiols, and leading to cellular damage in the depletion of the GSH enzyme level.