Pro-oxidants are chemicals that induce oxidative stress, either by generating reactive oxygen species or by inhibiting antioxidant systems. The oxidative stress produced by these chemicals can damage cells and tissues, for example an overdose of the analgesic paracetamol (acetaminophen) can fatally damage the liver, partly through its production of reactive oxygen species.
Some substances can serve as either antioxidants or pro-oxidants, depending on conditions. Some of the conditions that are important include the concentration of the chemical and if oxygen or transition metals are present. While thermodynamically very favored, reduction of molecular oxygen or peroxide to superoxide or hydroxyl radical respectively is spin forbidden. This greatly reduces the rates of these reactions, thus allowing aerobic life to exist. As a result, the reduction of oxygen typically involves either the initial formation of singlet oxygen, or spin-orbit coupling through a reduction of a transition-series metal such as manganese, iron, or copper. This reduced metal then transfers the single electron to molecular oxygen or peroxide.
Transition metals can serve as pro-oxidants. For example, chronic manganism is a classic "pro-oxidant" disease. Another disease associated with the chronic presence of a pro-oxidant transition-series metal is hemochromatosis, associated with elevated iron levels. Similarly, Wilson's disease is associated with elevated tissue levels of copper. Such syndromes tend to be associated with a common symptomology. Thus, all are occasional symptoms of (e.g) hemochromatosis, another name for which is "bronze diabetes". The pro-oxidant herbicide paraquat, Wilson's disease, and striatal iron have similarly been linked to human Parkinsonism. Paraquat also produces parkinsonian-like symptoms in rodents.
Fibrosis or scar formation is another pro-oxidant-related symptom. E.g., interocular copper or vitreous chalcosis is associated with severe vitreous fibrosis, as is interocular iron. Liver cirrhosis is also a major symptom of Wilson's disease. The pulmonary fibrosis produced by paraquat and the antitumor agent bleomycin is also thought to be induced by the pro-oxidant properties of these agents. It may be that oxidative stress produced by such agents mimics a normal physiological signal for fibroblast conversion to myofibroblasts.
Vitamins that are reducing agents can be pro-oxidants. Vitamin C has antioxidant activity when it reduces oxidizing substances such as hydrogen peroxide, however, it can also reduce metal ions which leads to the generation of free radicals through the Fenton reaction.
- 2 Fe2+ + 2 H2O2 → 2 Fe3+ + 2 OH· + 2 OH−
- 2 Fe3+ + Ascorbate → 2 Fe2+ + Dehydroascorbate
The relative importance of the antioxidant and pro-oxidant activities of antioxidant vitamins are an area of current research, but vitamin C, for example, appears to have a mostly antioxidant action in the body. However, less data is available for other dietary antioxidants, such as polyphenol antioxidants, zinc, and vitamin E.
Use in medicine
Several important anticancer agents both bind to DNA and generate reactive oxygen species. These include adriamycin and other anthracyclines, bleomycin, and cisplatin. These agents may show specific toxicity towards cancer cells because of the low level of antioxidant defenses found in tumors. Recent research demonstrates that redox dysregulation originating from metabolic alterations and dependence on mitogenic and survival signaling through reactive oxygen species represents a specific vulnerability of malignant cells that can be selectively targeted by pro-oxidant non-genotoxic redox chemotherapeutics.
Photodynamic therapy is used to treat some cancers as well as other conditions. It involves administration of a photosensitizer followed by exposing the target to appropriate wavelengths of light. The light excites the photosensitizer, causing it to generate reactive oxygen species, which can damage or destroy diseased or unwanted tissue.
- Puglia CD, Powell SR (1984). "Inhibition of cellular antioxidants: a possible mechanism of toxic cell injury". Environ. Health Perspect. 57: 307–11. doi:10.2307/3429932. JSTOR 3429932. PMC 1568295. PMID 6094175.
- James LP, Mayeux PR, Hinson JA (2003). "Acetaminophen-induced hepatotoxicity". Drug Metab. Dispos. 31 (12): 1499–506. doi:10.1124/dmd.31.12.1499. PMID 14625346.
- Jaeschke H, Gores GJ, Cederbaum AI, Hinson JA, Pessayre D, Lemasters JJ (2002). "Mechanisms of hepatotoxicity". Toxicol. Sci. 65 (2): 166–76. doi:10.1093/toxsci/65.2.166. PMID 11812920.
- Herbert V (1996). "Prooxidant effects of antioxidant vitamins. Introduction" (PDF). J. Nutr. 126 (4 Suppl): 1197S–200S. doi:10.1093/jn/126.suppl_4.1197S. PMID 8642456. Archived from the original (PDF) on 6 April 2008. Retrieved 7 May 2007.
- Han SG, Kim Y, Kashon ML, Pack DL, Castranova V, Vallyathan V (December 2005). "Correlates of oxidative stress and free-radical activity in serum from asymptomatic shipyard welders". Am. J. Respir. Crit. Care Med. 172 (12): 1541–8. doi:10.1164/rccm.200409-1222OC. PMID 16166614.
- Duarte TL, Lunec J (2005). "Review: When is an antioxidant not an antioxidant? A review of novel actions and reactions of vitamin C". Free Radic. Res. 39 (7): 671–86. doi:10.1080/10715760500104025. PMID 16036346. S2CID 39962659.
- Carr A, Frei B (1 June 1999). "Does vitamin C act as a pro-oxidant under physiological conditions?". FASEB J. 13 (9): 1007–24. doi:10.1096/fasebj.13.9.1007. PMID 10336883. S2CID 15426564.
- Stohs SJ, Bagchi D (1995). "Oxidative mechanisms in the toxicity of metal ions". Free Radic. Biol. Med. 18 (2): 321–36. doi:10.1016/0891-5849(94)00159-H. PMID 7744317.
- Valko M, Morris H, Cronin MT (2005). "Metals, toxicity and oxidative stress". Curr. Med. Chem. 12 (10): 1161–208. doi:10.2174/0929867053764635. PMID 15892631.
- Halliwell B (2007). "Dietary polyphenols: good, bad, or indifferent for your health?". Cardiovasc. Res. 73 (2): 341–7. doi:10.1016/j.cardiores.2006.10.004. PMID 17141749.
- Hao Q, Maret W (2005). "Imbalance between pro-oxidant and pro-antioxidant functions of zinc in disease". J. Alzheimers Dis. 8 (2): 161–70, discussion 209–15. doi:10.3233/jad-2005-8209. PMID 16308485.
- Schneider C (2005). "Chemistry and biology of vitamin E". Mol Nutr Food Res. 49 (1): 7–30. doi:10.1002/mnfr.200400049. PMID 15580660.
- Wondrak GT (December 2009). "Redox-directed cancer therapeutics: molecular mechanisms and opportunities". Antioxid. Redox Signal. 11 (12): 3013–69. doi:10.1089/ARS.2009.2541. PMC 2824519. PMID 19496700.