**1.1 Reactive oxygen species**

Reactive Oxygen Species (ROS) are by-products of chemical reactions that involve a one-electron reduction of oxygen, leading to the production of a diatomic oxygen radical known as superoxide. Superoxide serves as a precursor for multiple ROS generation. The oxygen anion can be produced mainly by enzymatical or nonenzymatical means. Enzymes such as Phagocytic Nicotinamide Adenine Dinucleotide Phosphate Oxidase (NADPH Oxidase), cytochrome P450–dependent oxygenase,

and the proteolytic conversion of cytosolic Xanthine dehydrogenase can produce singlet-oxygen molecules [1, 2]. The enzymatic generation of superoxide is the main generator of superoxide, especially NADPH oxidase. However, non-enzymatic generation can also cause a significant increase in reactive species. This occurs mostly in the mitochondria during ATP synthesis. The electron transport chain (ETC) has been reported to consist of centers that can leak electrons to oxygen, thereby causing a reduction of the oxygen [1]. Additionally, reduced coenzymes and prosthetic groups such as flavins and metal ions can directly transfer an electron to oxygen. The produced singlet-oxygen undergoes dismutation, catalyzed by the enzyme superoxide dismutase, to produce hydrogen peroxide, a more stable and less reactive molecule. The increased availability of ROS can lead to an imbalance between oxidants and antioxidants, resulting in a state that is known as oxidative stress [2]. During this state, the various deleterious effects of ROS such as disruption of cellular homeostasis, structures and function are manifested in the cell. Therefore, ROS overproduction has a pathological role in the development of various conditions and diseases, including inflammation and autoimmune disease.
