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Molecules 2020, 25, 3191 2 of 24 provide a brief overview on antioxidant activity of silver nanoparticles, its measurement, and future perspectives in relationship between AgNPs and antioxidant activity. 2. Antioxidant Activity 2.1. Oxidative Stress Oxidative stress is a phenomenon that can be defined as a state when the equilibrium between the antioxidative defense of cell and oxidants is disrupted by the effect of excess of the oxidants, for example reactive oxygen or nitrogen species (ROS or RNS, respectively) and organic compounds containing sulphur producing alkyl sulfanyl radicals (RS•). For example, transition metal ions at their lower oxidation states are not oxidant species by themselves, but may provoke the formation of ROS or RNS by reacting with hydrogen peroxide or molecular oxygen, thereby serve as prooxidants. Of course, also the presence of the oxidants leads to oxidative modifications of biological system on molecular level (unsaturated bonds of lipids, proteins, DNA, etc.,) causing damage and finally, cellular death is accelerated [12]. This phenomenon occurs when oxidative substances are excessively formed or accumulated and defense mechanisms have failed. Reactive oxygen species represent the most important group of oxidants containing radicals (hydroxyl •OH, superoxide ion O•−), as well as 2 non-radicals (hydrogen peroxide, organic peroxides) [13]. Superoxide and hydroxyl radical are products of oxygen reduction by electrons (Figure 1) [14,15]. Figure 1. Some reactive oxygen and nitrogen compounds and relationships between them, redrawn from Greguška [15]. From biological point of view, superoxide anion radical O•− is generated by the mitochondrial 2 respiratory chain and phagocytic NADPH oxidase (nicotinamide adenine dinucleotide phosphate oxidase; NOX), so it is produced by respiration and component part of defense system [16]. It is known that human body produces around 5 g of ROS per day. These are with dynamic balance with the production of any oxygen forms essential for living and protection against the toxic influence of ROS [17]. In addition, an excessive amount of reactive nitrogen species is formed during the oxidative stress. The most common representative of RNS is nitroxide (NO•) produced by nitrogen oxidation catalyzed by NO-synthase [18]. NO• is able to rapidly react with reactive oxygen species (mainly with superoxide anion radical) and produce further reactive nitrogen compounds, for example peroxynitrite or peroxynitrous acid which can be further transformed to NO2• or •OH (Figure 1). These products may initiate new radical reactions leading to damage of biomolecules (nitrosylation of DNA or proteins) [19]. The action of ROS and RNS leads to an oxidation of double bond of polyunsaturated fatty acids in lipids (lipoproteins, membrane structures) and formation of aldehydes or peroxides and a result of these processes is changing the membrane permeability [20,21]. The effect of oxidative stress on proteins leads to changes of ion transport (mainly Ca2+ ions homeostasis), protein inactivation, and enzymatic activity modification [22]. DNA is damaged by deoxyribose ring cleavage, base modification or chainPDF Image | Antioxidant Activity Determination of Silver Nanoparticles
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