Antioxidant Metabolites and Enzymatic Antioxidants: Their Roles in Redox Balance

Tehseen Gull

Contribution ID: RN-C-9B6282A464C0

Submitted by Tehseen Gull · TIMES University, Multan, Pakistan · ResearcherNest ID RN-RQC7H329KYGP

Article summary

Abstract

Reactive oxygen species (ROS) are produced during normal metabolism in animals and plants. Although excessive ROS can damage lipids, proteins, and DNA, controlled amounts support cell signaling, growth, and responses to stress. Antioxidants therefore maintain a balance between ROS production and removal rather than eliminating ROS completely. Antioxidant metabolites and enzymes work together across cellular compartments. Ascorbate helps remove hydrogen peroxide, particularly through ascorbate peroxidase in plants. Glutathione supports antioxidant recycling, detoxification, and redox regulation. Tocopherols help protect lipid-rich membranes, while plant melatonin contributes to stress responses through interactions with ROS signaling. Among antioxidant enzymes, superoxide dismutase converts superoxide to hydrogen peroxide; catalase, peroxiredoxins, and peroxidases then help control peroxide levels. Glutathione reductase replenishes reduced glutathione so these defenses can continue functioning. These mechanisms are relevant to plant adaptation and human health, but laboratory antioxidant activity does not establish a clinical benefit. Evidence from randomized trials has not shown that commonly studied antioxidant vitamin supplements prevent cardiovascular disease or cancer, and beta-carotene supplements have shown harm in some populations. Future research should examine specific pathways, doses, and outcomes in their biological context. It should also distinguish protection against damaging oxidation from the preservation of useful ROS signals in living tissues.

Full article

Introduction

Reactive oxygen species (ROS) are produced during normal metabolism in animals and plants. They include superoxide, hydrogen peroxide, and the highly reactive hydroxyl radical. For many years, ROS were discussed mainly as causes of cellular damage. Current research presents a more nuanced picture: controlled ROS production contributes to signaling, development, and responses to stress, whereas excessive or poorly regulated ROS can damage lipids, proteins, and DNA. Antioxidant defenses must therefore control ROS without removing the signals that cells need (Averill-Bates, 2024; Wang et al., 2024).

This balance depends on a network of small antioxidant molecules and enzymes. Their activities vary across cellular compartments and are closely connected: an enzyme may convert one ROS into another, while a metabolite used in that reaction must subsequently be regenerated. Examining a single antioxidant in isolation can consequently give an incomplete account of cellular protection.

Antioxidant metabolites

Antioxidant metabolites differ in their chemical properties and location. Water-soluble compounds, including ascorbate and glutathione, participate in reactions within aqueous cellular compartments. Lipid-soluble tocopherols are associated with membranes, where oxidation of lipids is a particular concern. The contribution of each compound depends on its availability, its reaction partners, and the oxidative challenge faced by the cell.

Ascorbate (vitamin C) is an electron donor in several antioxidant reactions. Its role in plants is particularly well defined: ascorbate peroxidase uses it to reduce hydrogen peroxide to water. Yoshimura and Ishikawa (2024) reported that ascorbate peroxidase isoforms occur in the chloroplasts, cytosol, mitochondria, and peroxisomes. This distribution is a useful finding because hydrogen peroxide must be regulated near its sites of production; one enzyme confined to a single location could not provide the same coverage. Ascorbate is restored through reactions linked to glutathione, forming part of the ascorbate–glutathione cycle.

Glutathione is another central metabolite. Its reduced form participates in antioxidant reactions, and glutathione reductase helps restore it after oxidation. A recent review by Noctor et al. (2024) found that glutathione’s functions in plants extend beyond peroxide control. It also contributes to detoxification, responses to pathogens, and changes in protein function and gene expression. This broader evidence suggests that a measured change in glutathione concentration should not automatically be interpreted as a change in antioxidant protection alone.

Tocopherols, members of the vitamin E family, help protect lipid-rich membranes from oxidation. Plant phenolic compounds and carotenoids can also contribute to antioxidant defenses, although their effects depend on their structure, concentration, and cellular location. These metabolites act within an interacting system; a high concentration of one does not guarantee that other parts of the system can process its reaction products or restore its active form (Wang et al., 2024).

Melatonin has attracted interest in plant stress research. Ahammed et al. (2024) described a reciprocal relationship between melatonin and ROS: melatonin can attenuate excessive ROS accumulation, while ROS signaling helps mediate melatonin-associated responses to environmental stress. The authors identified respiratory burst oxidase homolog proteins, which generate ROS outside the plasma membrane, as participants in this signaling process. The finding helps explain why an effective stress response may require carefully controlled ROS production alongside ROS removal.

Enzymatic antioxidant defenses

Antioxidant enzymes work through linked reactions. Superoxide dismutase (SOD) converts superoxide into hydrogen peroxide and oxygen. It does not complete ROS removal on its own: the hydrogen peroxide produced must be handled by other enzymes. Wang et al. (2024) described SOD as a major defense in aerobic organisms and reported that, in plants, its isoforms occur across several compartments. Their review also noted that SOD-catalyzed conversion of superoxide is far faster than the spontaneous reaction, illustrating the importance of enzymatic control when ROS production increases.

Catalase converts hydrogen peroxide into water and oxygen. Peroxiredoxins, glutathione peroxidases, and plant ascorbate peroxidase provide additional routes for processing peroxides. These enzymes differ in their substrates, locations, and sources of reducing power. Plant glutathione peroxidases also differ from many animal counterparts: Wang et al. (2024) explained that plant forms generally contain cysteine at their active site and commonly rely on thioredoxin for recycling. Thus, enzymes with similar names should not be assumed to function identically across organisms.

Enzymes that replenish antioxidant metabolites are equally important. Glutathione reductase regenerates reduced glutathione using cellular reducing power, helping sustain subsequent antioxidant reactions. The ascorbate–glutathione cycle links this recycling to hydrogen peroxide control. Together, these pathways show why antioxidant defense is better assessed as a coordinated process than by measuring the activity of SOD or catalase alone (Wang et al., 2024; Yoshimura &amp;amp;amp;amp; Ishikawa, 2024).

ROS signaling and plant stress

The benefits and harms of ROS depend on context. Averill-Bates (2024) described how hydrogen peroxide can contribute to signaling through reversible changes in protein cysteine residues. The review also noted an unresolved mechanistic question: hydrogen peroxide reacts relatively slowly with many protein thiols, whereas peroxiredoxins react with it rapidly. Peroxiredoxins may therefore help relay some redox signals rather than acting solely as peroxide-removing enzymes.

In plants, ROS signaling contributes to responses to drought, salinity, and other environmental challenges. Excess accumulation can impair cellular structures, while eliminating ROS signals could disrupt acclimation. This explains why stress-tolerant plants cannot be identified simply by looking for the lowest ROS measurement. More informative assessments examine ROS alongside antioxidant enzyme activity, metabolite status, plant growth, and performance under stress (Wang et al., 2024).

Health claims and measurement

The biological importance of antioxidants does not establish that taking antioxidant supplements prevents disease. In a systematic review prepared for the US Preventive Services Task Force, O’Connor et al. (2022) found no clear evidence that commonly studied vitamin supplements prevent cardiovascular disease or cancer. Their pooled analysis of four randomized trials involving 94,830 participants associated beta-carotene supplementation with increased lung cancer risk (odds ratio = 1.20; 95% confidence interval [1.01, 1.42]). This finding directly challenges the assumption that increasing antioxidant intake is always harmless.

Laboratory antioxidant assays provide valuable chemical information, but their results require careful interpretation. Different assays measure different reactions, and results can vary with the method used. As Kotha et al. (2022) emphasized, no single in vitro assay adequately represents every antioxidant mechanism or predicts a health effect after a food or extract is consumed. Food preservation, cellular protection, and clinical benefit are distinct outcomes that require appropriate evidence.

Conclusion

Recent findings show that antioxidants operate through connected pathways that both limit oxidative damage and preserve useful ROS signals. Evidence about enzyme location, metabolite recycling, and differences between plant and animal systems has refined understanding of these defenses. Clinical findings also show why promising biochemical activity cannot be treated as proof of benefit from supplementation. Future studies should assess specific pathways and measurable outcomes in the organisms and settings where a benefit is proposed.

References

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Kotha, R. R., Tareq, F. S., Yildiz, E., &amp;amp;amp;amp; Luthria, D. L. (2022). Oxidative stress and antioxidants—A critical review on in vitro antioxidant assays. Antioxidants, 11(12), 2388. https://doi.org/10.3390/antiox11122388

Noctor, G., Cohen, M., Trémulot, L., Châtel-Innocenti, G., Van Breusegem, F., &amp;amp;amp;amp; Mhamdi, A. (2024). Glutathione: A key modulator of plant defence and metabolism through multiple mechanisms. Journal of Experimental Botany, 75(15), 4549–4572. https://doi.org/10.1093/jxb/erae194

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Wang, P., Liu, W.-C., Han, C., Wang, S., Bai, M.-Y., &amp;amp;amp;amp; Song, C.-P. (2024). Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development. Journal of Integrative Plant Biology, 66(3), 330–367. https://doi.org/10.1111/jipb.13601

Yoshimura, K., &amp;amp;amp;amp; Ishikawa, T. (2024). Physiological function and regulation of ascorbate peroxidase isoforms. Journal of Experimental Botany, 75(9), 2700–2715. https://doi.org/10.1093/jxb/erae061

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