![]() 2,8,9,12,17–20 The redox regulation typically involves controlled production of reactive oxygen and nitrogen species. 9 Mounting evidence suggests that ROS actually may have a beneficial physiological role acting as messengers in cellular signaling, a new paradigm in the rich and diverse chemistry of ROS which has attracted increased attention in the last decade (see also Figure 1.1). This hypothesis is, however, currently under revision. 11–14 According to the long-held “free radical theory of aging” 15,16 advanced by Denham Harman in 1956, the noxious effects of ROS, generated during cellular respiration at the mitochondrial level are directly involved in aging processes. 10 Oxidative stress caused by the imbalance between excessive formation of ROS and limited antioxidant defences is connected to many pathologies including age-related disorders, cancer, cardiovascular, inflammatory, and neurodegenerative diseases such as Parkinson’s and Alzheimer’s diseases. 9 ROS overproduction is associated with numerous disorders. Reactive oxygen species, in particular hydroxyl and peroxyl radicals, hydrogen peroxide and superoxide radical anion, have long been implicated in oxidative damage inflicted on fatty acids, DNA and proteins as well as other cellular components. 7,8 RNS that bear oxygen atoms include nitric oxide radical (NO or NO˙), nitrogen dioxide radical (NO 2˙), nitrite (NO 2 −), and peroxynitrite (ONOO −). ![]() 6,7 ROS may also be classified as free radicals and nonradical species. ![]() Incorporation of peroxyl (ROO˙), alkoxyl (RO˙), semiquinone (SQ˙ −) and carobonate (CO 3˙ −) radicals and organic hydroperoxides (ROOH) is also frequently encountered within the definition of ROS. 5 A somewhat more encompassing definition also includes within ROS compounds such as hypochlorous (HOCl), hypobromous (HOBr), and hypoiodous acids (HOI). According to some definitions the term ROI describes the chemical species formed upon incomplete reduction of molecular oxygen, namely superoxide radical anion (O 2˙ −), hydrogen peroxide (H 2O 2), and hydroxyl radicals (OH˙), while ROS includes both ROI and ozone (O 3) and singlet oxygen ( 1O 2). 3,4 Over the years, the terms ROS (reactive oxygen species), ROI (reactive oxygen intermediates) and RNS (reactive nitrogen species) have been coined to define an emerging class of endogenous, highly reactive, oxygen- (and also nitrogen-) bearing molecules. proposed that oxygen-containing free radicals were responsible for toxic effects in aerobic organisms. Certain derivatives of oxygen are, however, highly toxic to cells. 2 For all currently living aerobic species, molecular oxygen is a central molecule in cellular respiration. The increasing concentration of molecular oxygen (O 2) in the atmosphere roughly 2.5 billion years ago, 1,2 due to oxygenic photosynthesis by cyanobacteria, allowed for the evolution of aerobic respiration, leading to the development of complex eukaryotic organisms.
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