5.2 ROS
When ischemic tissue undergoes reperfusion, blood brings oxygen and nutrients to the tissue. At the same time, due to the low concentration of antioxidants in cells, the production of reactive oxygen species (ROS) increases. In the I /R process of biology, ROS will be produced by many ways, including mitochondrial electron transfer chain (ETC), xanthine oxidase system (XOD), NADPH oxidase system and nitric oxide synthase (NOS), etc[85]. The first three are related to oxidative stress in multiple organs, such as the heart, brain, lungs, liver, pancreas, kidneys, and gastrointestinal tract[86]. NOS mainly acts as an oxidative stress factor in vascular endothelial cells[87]. During the metabolism of normal mitochondria, the respiratory chain complex on the inner mitochondrial membrane can produce a small amount of ROS[88]. As mentioned earlier, when I/R occurs, due to hypoxia, changes in ATP, pH, and calcium overload occur in cells, which can lead to mitochondrial damage and produce more ROS. However, ROS further exacerbates oxidative stress, leading to a vicious cycle of cells[82, 88].
The xanthine oxidase (XOD) system is an important pathway for ROS production. Under ischemia, ATP synthesis is reduced and xanthine dehydrogenase (XDH) is converted into xanthine oxidase (XOD). At the same time, ATP degradation products (ADP, AMP, hypoxanthine) accumulate. When reperfusion is resumed, a large amount of oxygen molecules enter the tissue with the blood, and XOD catalyzes the conversion of hypoxanthine into xanthine and uric acid, producing a large amount of ROS[89]. The oxygen free radicals generated by this pathway have chemotactic effects, attracting and activating a large number of white blood cells to aggregate. When the tissue resumes oxygen supply, the activated white blood cells’ oxygen consumption increases sharply, further producing a large amount of oxygen free radicals, causing cell damage.
The NOx/Deox family of NADPH oxidase mainly includes 7 subtypes, such as Nox-1, Nox-2, Nox-3, Nox-4, Nox-5, Duox-1 and Duox-2, these enzymes have the ability to produce ROS[90]. Under hypoxic conditions, hypoxia inhibitory factor-1α (HIF-1α). Promote the activation of NOX enzyme, and after reperfusion, cells will release some chemical factors to further activate NADPH oxidase, such as phospholipase A2 (PLA2), TNF-α, IL-1β, IFN-γ and angiotensin II(Ang II), etc. Overexpression of NADPH oxidase after activation enhances ROS production[91, 92].
In addition to the aforementioned pathways, NOS is also an important pathway for generating ROS. Tetrahydrobiopterin (BH4) is a cofactor of NOS enzyme. In I/R, oxidative stress oxidizes BH4 to BH2, leading to a decrease in BH4 cell level and uncoupling of NOS, thereby promoting ROS production[93].