Several lines of antioxidant defense against oxidative stress: antioxidant enzymes, nanomaterials with multiple enzyme-mimicking activities, and low-molecular-weight antioxidants

Abstract

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are well recognized for playing a dual role, since they can be either deleterious or beneficial to biological systems. An imbalance between ROS production and elimination is termed oxidative stress, a critical factor and common denominator of many chronic diseases such as cancer, cardiovascular diseases, metabolic diseases, neurological disorders (Alzheimer's and Parkinson's diseases), and other disorders. To counteract the harmful effects of ROS, organisms have evolved a complex, three-line antioxidant defense system. The first-line defense mechanism is the most efficient and involves antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). This line of defense plays an irreplaceable role in the dismutation of superoxide radicals (O₂^•-) and hydrogen peroxide (H₂O₂). The removal of superoxide radicals by SOD prevents the formation of the much more damaging peroxynitrite ONOO^- (O₂^•- + NO^• → ONOO^-) and maintains the physiologically relevant level of nitric oxide (NO^•), an important molecule in neurotransmission, inflammation, and vasodilation. The second-line antioxidant defense pathway involves exogenous diet-derived small-molecule antioxidants. The third-line antioxidant defense is ensured by the repair or removal of oxidized proteins and other biomolecules by a variety of enzyme systems. This review briefly discusses the endogenous (mitochondria, NADPH, xanthine oxidase (XO), Fenton reaction) and exogenous (e.g., smoking, radiation, drugs, pollution) sources of ROS (superoxide radical, hydrogen peroxide, hydroxyl radical, peroxyl radical, hypochlorous acid, peroxynitrite). Attention has been given to the first-line antioxidant defense system provided by SOD, CAT, and GPx. The chemical and molecular mechanisms of antioxidant enzymes, enzyme-related diseases (cancer, cardiovascular, lung, metabolic, and neurological diseases), and the role of enzymes (e.g., GPx4) in cellular processes such as ferroptosis are discussed. Potential therapeutic applications of enzyme mimics and recent progress in metal-based (copper, iron, cobalt, molybdenum, cerium) and nonmetal (carbon)-based nanomaterials with enzyme-like activities (nanozymes) are also discussed. Moreover, attention has been given to the mechanisms of action of low-molecular-weight antioxidants (vitamin C (ascorbate), vitamin E (alpha-tocopherol), carotenoids (e.g., β-carotene, lycopene, lutein), flavonoids (e.g., quercetin, anthocyanins, epicatechin), and glutathione (GSH)), the activation of transcription factors such as Nrf2, and the protection against chronic diseases. Given that there is a discrepancy between preclinical and clinical studies, approaches that may result in greater pharmacological and clinical success of low-molecular-weight antioxidant therapies are also subject to discussion.

Keywords: Antioxidant enzymes; Chronic disease; Enzyme mimics; Low-molecular antioxidants; Oxidative stress; ROS.

Fig. 1

DNA damage by carbonate radicals (CO₃^·−). CO₃^·− can be generated from the interaction of superoxide radical (O₂^·−) and nitric oxide (NO^·), forming peroxynitrite (ONOO⁻), which in turn reacts with CO₂ to finally form CO₃.^·−. Adapted from Fleming and Burrows (2020)

Fig. 2

The mechanism of superoxide radical dismutation catalyzed by Cu,Zn-SOD showing inner- and outer-sphere electron transfers. Adapted from Quist et al. (2017)

Fig. 3

Catalase deficiency and human diseases

Fig. 4

Examples of catalase-like mimics: metalloporphyrins (AEOL 11207) and metallo-salens (EUK 134)

Fig. 5

Concerted action of antioxidant enzymes superoxide dismutase, catalase, and glutathione peroxidase. SOD converts superoxide radical anions to hydrogen peroxide. The physiological level of hydrogen peroxide is maintained by catalase. Under pathological conditions, redox metals (Cu, Fe) can catalyze the formation of hydrogen radicals by the decomposition of hydrogen peroxide (Fenton reaction). Glutathione peroxidase (GPx) converts hydrogen peroxide (H₂O₂) to 2H₂O using reduced glutathione (GSH) as a substrate, which is converted to its oxidized form, glutathione disulfide (GSSG). GSSG is converted back to its reduced form, GSH, using the enzyme glutathione reductase (GR). The glutathione reductase reaction contributes significantly to the cellular maintenance of reduced glutathione, which is important for redox defense

Fig. 6

Structures of selenocysteine and cysteine

Fig. 7

Selected structures of GPx mimics

Fig. 8

Nanozymes with both SOD-like increasing and catalase-like suppressing activities resulted in enhanced levels of H₂O₂, supporting the efficacy of chemodynamic therapy in cancer cells. The enhanced damage to cancer cells is caused by ^·OH formed via the Fenton reaction

Fig. 9

pH-dependent transformation of ascorbic acid and its reactions with ROS and redox-active metals

Fig. 10

Dehydroascorbate (DHA) can be catalytically reduced by a two-electron reduction step to an ascorbate monoanion (AscH⁻) by dehydroascorbate reductase (DHAR), a key enzyme involved in ascorbate recycling. This reduction process is accompanied by the oxidation of GSH to GSSG. GSH is restored by glutathione reductase (GR) in the presence of NADPH

Fig. 11

EPR spectra of ascorbyl radical (Asc^·−) observed from rat brain homogenates in dimethyl sulfoxide. A) Reference–normal rat brain (the concentration of Asc^·− was estimated to be 0.92 ± 0.1 nM/1 mg brain tissue); B) Ischemic rat brain 5 months after three-vessel occlusion (the concentration of the Asc^·− radical was estimated to be 1.95 ± 0.1 nM/1 mg brain tissue). EPR spectra were measured at 20 °C. EPR spectral parameters: microwave frequency 9.512 GHz, g-factor = 2.0064 (calibrated to DPPH standard, g-factor 2.00362; hyperfine splitting constant A(H) = 2.03 gauss

Fig. 12

Co-anticancer therapy using vitamin C. Intravenously administered vitamin C (ascorbate) oxidizes the hydrogen peroxide-converting enzyme peroxiredoxin-2. The level of the H₂O₂-converting enzyme catalase is usually decreased in cancer cells; thus, an elevated level of hydrogen peroxide is a potential source of hydroxyl radicals (via the Fenton reaction), which can cause damage to cancer cells

Fig. 13

Structures of α-tocopherol and γ-tocopherol

Fig. 14

Immunomodulatory and antioxidant functions of vitamin E

Fig. 15

Structures of selected carotenoids

Fig. 16

A simplified scheme of the interaction between peroxyl radicals (ROO^·), carotenoids (CAR), and a reduced form of vitamin C (AscH⁻) and vitamin E (α-T-OH). Asc^·− is the ascorbyl radical of vitamin C, and α-T-O.^· is the tocopheryl radical of vitamin E. Adapted from Krinsky and Yeum (2003)

Fig. 17

Subclasses of flavonoids. M denotes metals coordinated to the metal-binding sites of quercetin

Fig. 18

Flavonoid radical reactions