Table 1 Overview of nitric oxide synthases, reactive oxygen species, and antioxidant enzymes

Nitric oxide synthases

Inducible NOS (iNOS)

Produced by many cell types

Essential in immune function and has major rolls in inflammatory pathology and septic shock

[165, 166]

Endothelial NOS (eNOS)

Produced in endothelial tissue

Essential for normal cardiovascular system function, blood pressure control, anti- atherosclerotic properties

[165, 166]

Neuronal NOS (nNOS)

Produced mainly in central and peripheral neurons of the central nervous system (CNS)

Essential for synaptic plasticity, central regulation blood pressure, vasodilation

[165, 166]

ROS

Superoxide (O₂^•−)

Mitochondrial damage, NADPH oxidase, xanthine oxidoreductase, uncoupled NOS, NOX, cytochrome P450-dependent oxygenases, non-enzymatically, when a single electron is directly transferred to O₂

Oxidation of proteins, lipids, and DNA; mitochondrial damage and cell death signaling; innate immune response; generation of ONOO⁻; coordination with MPO to facilitate respiratory burst to enhance chloramine and hypochlorite through H₂O₂ production

[39, 40, 167,168,169]

Peroxynitrite (ONOO⁻)

Near diffusion rate reaction of O₂^•− with NO

Protein nitration; DNA damage and biomolecule modification including amino acids, proteins, enzymes, and cofactors; tyrosine nitration

[44, 167]

Hydroxyl radical (^•OH)

Fenton reaction (H₂O₂ with transition metals), MPO compound II with xenobiotics substrates for cytochrome p450

Oxidative modification of amino acids, purine and pyrimidine bases of DNA, and lipids

[167, 170]

Hypochlorous acid (HOCl)

Mammalian peroxidases reaction with chloride ion and H₂O₂

Hemoprotein heme destruction; innate immune response with anti-microbial, anti-fungal, and anti-viral properties

[167, 170]

Hydrogen peroxide (H₂O₂)

Monoamine, monoacid oxidase, glucose/glucose oxidase, Superoxide dismutase, NOX4

Induction of cellular damage and arrest during cell cycle progression; facilitation of cell death; promoter for cell cycle progression

[39, 167, 170]

Antioxidant enzymes

Superoxide Dismutase (SOD)

Cytosolic copper/zinc-SOD, mitochondrial manganese-SOD, and extracellular SOD

Catalyzes the dismutation of the superoxide anion to O₂ and to the less reactive species H₂O₂

[39, 169, 171]

Catalase (CAT)

Mammalian catalase is present in peroxisomes

Reacts with H₂O₂ to form water and molecular oxygen and reacts with H donors (methanol, ethanol, formic acid, or phenols) with peroxidase activity; protects cells from self-generated H₂O₂

[39, 171]

Glutathione peroxidase (GP)

Cytosolic and mitochondrial glutathione peroxidase (or GPX1) is found in most tissues and in erythrocytes, kidney, and liver. The phospholipid hydroperoxide glutathione peroxidase (GPX4) are found in most tissues and is highly expressed in renal epithelial cells and testes. Cytosolic GPX2 and extracellular GPX3 are found in the gastrointestinal tract and kidney, respectively.

Catalyzes the reduction of fatty acid hydroperoxides and H₂O₂ using glutathione (GSH)

[171]