Table 1.

Mechanisms of action of selected enzymatic and non-enzymatic antioxidants [12,13,17,19].

Enzymatic Antioxidants
Superoxide dismutase (SOD)	- requires a metal as a cofactor (is a metalloenzyme) - catalyzes the dismutation of O2•− into O2 and H2O2
Catalase (CAT)	- uses iron or manganese as a cofactor - catalyzes H2O2 into O2 and H2O
Glutathione peroxidase (GPx)	- is an important intracellular enzyme - its activity depends on selenium - breaks down H2O2 into water and lipid peroxides
Non-enzymatic Antioxidants
Glutathione (GSH)	- serves as a scavenger of O2•− and •OH - essential cofactor for antioxidant enzymes - regenerates other oxidized antioxidants (vit C, vit E)
Uric acid	- protects against oxidative damage by scavenging O2, •OH - strong electron donor and a selective scavenger of ONOO−
Phenolic compounds	- classified as primary antioxidants (capable of HAT (e.g., gallic acid, caffeic acid, epicatechin) and SET (e.g., kaempferol, resveratrol) - function as secondary oxidants due to the ability to bind to potentially pro-oxidative metal ions
Carotenoids	- react as antioxidant agents through three mechanisms: SET, the formation of one adduct, and HAT - excellent peroxyl radical scavengers
Vitamin C	- can produce reactions with oxidizing agents through HAT, SET or a concerted transfer of electron/protons (SET/HAT) - reacts with O2•− and •OH in the cytoplasm
Vitamin E	- prevents lipid peroxidation chain reactions and quenches O2 in cellular lipid compartments - reduces LOO• by transferring the phenolic hydrogen atom of the chroman ring

O₂^•−, superoxide anion radical; H₂O₂, hydrogen peroxide; ^•OH, hydroxyl radical; HAT, hydrogen-atom transfer; SET, single-electron transfer; LOO^•, lipid peroxyl radicals; ONOO⁻, peroxynitrite anion; O₂, molecular oxygen.