Reactive oxygen species
Reactive oxygen species (ROS) are chemically reactive molecules containing oxygen. Examples include oxygen ions and peroxides. ROS form as a natural byproduct of the normal metabolism of oxygen and have important roles in cell signaling and homeostasis. However, during times of environmental stress (e.g., UV or heat exposure), ROS levels can increase dramatically. This may result in significant damage to cell structures. Cumulatively, this is known as oxidative stress. ROS are also generated by exogenous sources such as ionizing radiation.
Normally, cells defend themselves against ROS damage with enzymes such as alpha-1-microglobulin, superoxide dismutases, catalases, lactoperoxidases, glutathione peroxidases and peroxiredoxins. Small molecule antioxidants such as ascorbic acid (vitamin C), tocopherol (vitamin E), uric acid, and glutathione also play important roles as cellular antioxidants. In a similar manner, polyphenol antioxidants assist in preventing ROS damage by scavenging free radicals. In contrast, the antioxidant ability of the extracellular space is less - e.g., the most important plasma antioxidant in humans is uric acid.
Effects of ROS on cell metabolism are well documented in a variety of species. These include not only roles in apoptosis (programmed cell death) but also positive effects such as the induction of host defencegenes and mobilisation of ion transport systems. This implicates them in control of cellular function. In particular, platelets involved in wound repair and blood homeostasis release ROS to recruit additional platelets to sites of injury. These also provide a link to the adaptive immune system via the recruitment of leukocytes.
Reactive oxygen species are implicated in cellular activity to a variety of inflammatory responses including cardiovascular disease. They may also be involved in hearing impairment via cochlear damage induced by elevated sound levels, in ototoxicity of drugs such as cisplatin, and in congenital deafness in both animals and humans. ROS are also implicated in mediation of apoptosis or programmed cell death and ischaemic injury. Specific examples include stroke and heart attack.
In general, harmful effects of reactive oxygen species on the cell are most often:
- damage of DNA
- oxidations of polyunsaturated fatty acids in lipids (lipid peroxidation)
- oxidations of amino acids in proteins
- oxidatively inactivate specific enzymes by oxidation of co-factors
When a plant recognizes an attacking pathogen, one of the first induced reactions is to rapidly produce superoxide or hydrogen peroxide to strengthen the cell wall. This prevents the spread of the pathogen to other parts of the plant, essentially forming a net around the pathogen to restrict movement and reproduction.
In aerobic organisms the energy needed to fuel biological functions is produced in the mitochondria via the electron transport chain. In addition to energy, reactive oxygen species (ROS) with the potential to cause cellular damage are produced. ROS can damage DNA, RNA, and proteins, which, in theory, contributes to the physiology of ageing.
ROS are produced as a normal product of cellular metabolism. In particular, one major contributor to oxidative damage is hydrogen peroxide (H2O2), which is converted from superoxide that leaks from the mitochondria. Catalase and superoxide dismutase ameliorate the damaging effects of hydrogen peroxide and superoxide, respectively, by converting these compounds into oxygen and water, benign molecules. However, this conversion is not 100% efficient, and residual peroxides persist in the cell. While ROS are produced as a product of normal cellular functioning, excessive amounts can cause deleterious effects. Memory capabilities decline with age, evident in human degenerative diseases such as Alzheimer's disease, which is accompanied by an accumulation of oxidative damage. Current studies demonstrate that the accumulation of ROS can decrease an organism's fitness because oxidative damage is a contributor to senescence. In particular, the accumulation of oxidative damage may lead to cognitive dysfunction, as demonstrated in a study in which old rats were given mitochondrial metabolites and then given cognitive tests. Results showed that the rats performed better after receiving the metabolites, suggesting that the metabolites reduced oxidative damage and improved mitochondrial function. Accumulating oxidative damage can′ then affect the efficiency of mitochondria and further increase the rate of ROS production. The accumulation of oxidative damage and its implications for aging depends on the particular tissue type where the damage is occurring. Additional experimental results suggest that oxidative damage is responsible for age-related decline in brain functioning. Older gerbils were found to have higher levels of oxidized protein in comparison to younger gerbils. Treatment of old and young mice with a spin trapping compound caused a decrease in the level of oxidized proteins in older gerbils but did not have an effect on younger gerbils. In addition, older gerbils performed cognitive tasks better during treatment but ceased functional capacity when treatment was discontinued, causing oxidized protein levels to increase. This led researchers to conclude that oxidation of cellular proteins is potentially important for brain function (Carney, 1991).
Exogenous ROS can be produced from pollutants, tobacco, smoke, drugs, xenobiotics, or radiation.
Ionizing radiation can generate damaging intermediates through the interaction with water, a process termed radiolysis. Since water comprises 55-60% of the human body, the probability of radiolysis is quite high under the presence of ionizing radiation. In the process, water loses an electron and become highly reactive. Then through a three-step chain reaction, water is sequentially converted to hydroxyl radical (-OH), hydrogen peroxide (H2O2), superoxide radical (O2-) and ultimately oxygen (O2). The hydroxyl radical is extremely reactive that immediately removes electrons from any molecule in its path, turning that molecule into a free radical and so propagating a chain reaction. But hydrogen peroxide is actually more damaging to DNA than hydroxyl radical since the lower reactivity of hydrogen peroxide provides enough time for the molecule to travel into the nucleus of the cell, subsequently wreaking havoc on macromolecules such as DNA.
ROS are produced intracellularly through multiple mechanisms, the major sources being mitochondria, peroxisomes, endoplasmic reticulum, and the NADPH oxidase (NOX) complex in cell membranes. Mitochondria convert energy for the cell into a usable form, adenosine triphosphate (ATP). The process in which ATP is produced, called oxidative phosphorylation, involves the transport of protons (hydrogen ions) across the inner mitochondrial membrane by means of the electron transport chain. In the electron transport chain, electrons are passed through a series of proteins via oxidation-reduction reactions, with each acceptor protein along the chain having a greater reduction potential than the previous. The last destination for an electron along this chain is an oxygen molecule. In normal conditions, the oxygen is reduced to produce water; however, in about 0.1–2% of electrons passing through the chain (this number derives from studies in isolated mitochondria, though the exact rate in live organisms is yet to be fully agreed upon), oxygen is instead prematurely and incompletely reduced to give the superoxide radical (·O2-), most well documented for Complex I and Complex III. Superoxide is not particularly reactive by itself, but can inactivate specific enzymes or initiate lipid peroxidation in its protonated form, hydroperoxyl HO2·. The pKa of hydroperoxyl is 4.8. Thus, at physiological pH, the majority will exist as superoxide.
If too much damage is present in mitochondria, a cell undergoes apoptosis or programmed cell death. Bcl-2 proteins are layered on the surface of the mitochondria, detect damage, and activate a class of proteins called Bax, which punch holes in the mitochondrial membrane, causing cytochrome C to leak out. This cytochrome C binds to Apaf-1, or apoptotic protease activating factor-1, which is free-floating in the cell's cytoplasm. Using energy from the ATPs in the mitochondrion, the Apaf-1 and cytochrome C bind together to form apoptosomes. The apoptosomes bind to and activate caspase-9, another free-floating protein. The caspase-9 then cleaves the proteins of the mitochondrial membrane, causing it to break down and start a chain reaction of protein denaturation and eventually phagocytosis of the cell.
Superoxide dismutases (SOD) are a class of enzymes that catalyze the dismutation of superoxide into oxygen and hydrogen peroxide. As such, they are an important antioxidant defense in nearly all cells exposed to oxygen. In mammals and most chordates, three forms of superoxide dismutase are present. SOD1 is located primarily in the cytoplasm, SOD2 in the mitochondria and SOD3 is extracellular. The first is a dimer (consists of two units), while the others are tetramers (four subunits). SOD1 and SOD3 contain copper and zinc ions, while SOD2 has a manganese ion in its reactive centre. The genes are located on chromosomes 21, 6, and 4, respectively (21q22.1, 6q25.3 and 4p15.3-p15.1).
- M(n+1)+ − SOD + O2− → Mn+ − SOD + O2
- Mn+ − SOD + O2− + 2H+ → M(n+1)+ − SOD + H2O2.
In this reaction the oxidation state of the metal cation oscillates between n and n+1.
Catalase, which is concentrated in peroxisomes located next to mitochondria, reacts with the hydrogen peroxide to catalyze the formation of water and oxygen. Glutathione peroxidase reduces hydrogen peroxide by transferring the energy of the reactive peroxides to a very small sulfur-containing protein called glutathione. The selenium contained in these enzymes acts as the reactive center, carrying reactive electrons from the peroxide to the glutathione. Peroxiredoxins also degrade H2O2, within the mitochondria, cytosol, and nucleus.
- 2 H2O2 → 2 H2O + O2(catalase)
- 2GSH + H2O2 → GS–SG + 2H2O (glutathione peroxidase)
Cause of aging
According to the Free-radical theory, oxidative damage initiated by reactive oxygen species is a major contributor to the functional decline that is characteristic of aging. While studies in invertebrate models indicate that animals genetically engineered to lack specific antioxidant enzymes (such as SOD), in general, show a shortened lifespan (as one would expect from the theory), the converse manipulation, increasing the levels of antioxidant enzymes, has yielded inconsistent effects on lifespan (though some studies in Drosophila do show that lifespan can be increased by the overexpression of MnSOD or glutathione biosynthesizing enzymes). Also contrary to this theory, deletion of mitochondrial SOD2 can extend lifespan in Caenorhabditis elegans.
In mice, the story is somewhat similar. Deleting antioxidant enzymes, in general, yields shorter lifespan, though overexpression studies have not (with some recent exceptions) consistently extended lifespan.
ROS in the context of cancer
ROS are constantly generated and eliminated in the biological system and are required to drive regulatory pathways. Under normal physiologic conditions, cells control ROS levels by balancing the generation of ROS with their elimination by scavenging system. But under oxidative stress conditions, excessive ROS can damage cellular proteins, lipids and DNA, leading to fatal lesions in cell that contribute to carcinogenesis.
Cancer cells exhibit greater ROS stress than normal cells do, partly due to oncogenic stimulation, increased metabolic activity and mitochondrial malfunction. ROS is a double-edged sword. On one hand, at low levels, ROS facilitates cancer cell survival since cell-cycle progression driven by growth factors and receptor tyrosine kinases (RTK) require ROS for activation and chronic inflammation, a major mediator of cancer, is regulated by ROS. On the other hand, a high level of ROS can suppress tumor growth through the sustained activation of cell-cycle inhibitor and induction of cell death as well as senescence by damaging macromolecules. In fact, most of the chemotherapeutic and radiotherapeutic agents kill cancer cells by augmenting ROS stress. The ability of cancer cells to distinguish between ROS as a survival or apoptotic signal is controlled by the dosage, duration, type, and site of ROS production. Modest levels of ROS are required for cancer cells to survive, whereas excessive levels kill them.
Metabolic adaptation in tumours balances the cells' need for energy with equally important need for macromolecular building blocks and tighter control of redox balance. As a result, production of NADPH is greatly enhanced, which functions as a cofactor to provide reducing power in many enzymatic reactions for macromolecular biosynthesis and at the same time rescuing the cells from excessive ROS produced during rapid proliferation. Cells counterbalance the detrimental effects of ROS by producing antioxidant molecules, such as reduced glutathione (GSH) and thioredoxin (TRX), which rely on the reducing power of NADPH to maintain their activities.
Most risk factors associated with cancer interact with cells through the generation of ROS. ROS then activate various transcription factors such as nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB), activator protein-1 (AP-1), hypoxia-inducible factor-1α and signal transducer and activtor of transcription 3 (STAT3), leading to expression of proteins that control inflammation; cellular transformation; tumor cell survival; tumor cell proliferation; and invasion, agiogenesis as well as metastasis. And ROS also control the expression of various tumor supressor genes such as p53, retinoblastoma gene (Rb), and phosphatase and tensin homolog (PTEN).
ROS and carcinogenesis
ROS-related oxidation of DNA is one of the main causes of mutations, which can produce several types of DNA damage, including non-bulky (8-oxoguanine and formamidopyrimidine) and bulky (cyclopurine and etheno adducts) base modifications, abasic sites, non-conventional single-strand breaks, protein-DNA adducts, and intra/interstrand DNA crosslinks. It has been estimated that endogenous ROS produced via normal cell metabolism modify approximately 20,000 bases of DNA per day in a single cell. 8-oxoguanine is the most abundant among various oxidized nitrogeneous bases observed. During DNA replication, DNA polymerase mispairs 8-oxoguanine with adenin, leading to a G->T transition mutation. The resulting genomic instability directly contributes to carcinogenesis.
ROS and cell proliferation
Uncontrolled proliferation is a hallmark of cancer cells. Both exogenous and endogenous ROS have been shown to enhance proliferatin of cancer cells. The role of ROS in promoting tumor proliferation is further supported by the observation that agents with potential to inhibit ROS generation can also inhibit cancer cell proliferation. Although ROS can promote tumor cell proliferation, a great increase in ROS has been associated with reduced cancer cell proliferation by induction of G2/M cell cycle arrest; increased phosphorylation of ataxia telangiectasia mutated (ATM), checkpoint kinase 1 (Chk 1), Chk 2; and reduced cell division cycle 25 homolog c (CDC25).
ROS and cell death
A cancer cell can die in three ways: apoptosis, necrosis and authophagy. Excessive ROS can induce apoptosis through both the extrinsic and intrinsic pathways. In the extrinsic pathway of apoptosis, ROS are generated by Fas ligand as an upstream event for Fas activation via phosphorylation, which is necessary for subsequent recruitment of Fas-associated protein with death domain and caspase 8 as well as apoptosis induction. In the intrinsic pathway, ROS funciton to facilitate cytochrome c release by activating activating pore-stabilizing proteins (Bcl-2 and Bcl-xL) as well as inhibiting pore-destabilizing proteins (Bcl-2-associated X protein, Bcl-2 homologous antagonist/killer). An even higher ROS level can results in both apoptosis and necrosis in cancer cells. ROS can also induce cell death through autophagy, which is a self-catabolic process involving sequestration of cytoplasmic contents (exhausted organelles and protein aggregates) for degradation in lyposomes.
ROS, tumor cell invasion, angiogeneis and metastasis
After growth factor stimulation of RTKs, ROS can trigger activation of signaling pathways involved in cell migration and invasion such as members of the mitogen activated protein kinase (MAPK) family -extracellular regulated kinase (ERK), c-jun NH-2 terminal kinase (JNK) and p38 MAPK. ROS can also promote migration by augmenting phosphorylation of the focal adhesion kinase (FAK) p130Cas and paxilin.
Both in vitro and in vivo, ROS have been shown to induce transcription factors and modulate signaling molecules involved in angiogenesis (MMP, VEGF) and metastasis (upregulation of AP-1, CXCR4, AKT and downregulation of PTEN).
ROS, chronic inflammation and cancer
Experimental and epidemiologic research over the past several years has indicated close associations among ROS, chronic inflammation, and cancer. ROS induces chronic inflammation by the induction of COX-2, inflammatory cytokines (TNFα, interleukin 1 (IL-1), IL-6), chemokines (IL-8, CXCR4) and pro-inflammatory transcription factors (NF-κB). These chemokines and chemokine receptors, in turn, promote invasion and metastasis of various tumor types.
ROS and cancer therapy
Both ROS-elevating and ROS-eliminating strategies have been developed with the former being predominantly used.Cancer cells with elevated ROS levels depend heavily on the antioxidant defense system. ROS-elevating drugs further increase cellular ROS stress level, either by direct ROS-generation (e.g. motexafin gadolinium, elesclomol) or by agents that abrogate the inherent antioxidant system such as SOD inhibitor (e.g. ATN-224, 2-methoxyestradiol) and GSH inhibitor (e.g. PEITC, buthionine sulfoximine (BSO)). The result is an overall increase in endogenous ROS, which when above a cellular tolerability threshold, may induce cell death. On the other hand, normal cells appear to have, under lower basal stress and reserve, a higher capacity to cope with additional ROS-generating insults than cancer cells do. Therefore, preferentially accumulation ROS in cancer cells to achieve selective killing is possible.
Radiotherapy also relies on ROS toxicity to eradicate tumor cells. Radiotherapy uses X-rays, γ-rays as well as heavy particle radiation such as protons and neutrons to induce ROS-mediated cell death and mitotic failure.
Due to the dual role of ROS, both prooxidant and antioxidant-based anticancer agents have been developed. However, modulation of ROS signaling alone seems not to be an ideal approach due to adaptation of cancer cells to ROS stress, redundant pathways for supporting cancer growth and toxicity from ROS-generating anticancer drugs.Combinations of ROS-generating drugs with pharmaceuticals that can break the redox adaptation could be a better strategy for enhancing cancer cell cytotoxicity.
- Devasagayam, TPA; Tilak JC, Boloor KK, Sane Ketaki S, Ghaskadbi Saroj S, Lele RD (October 2004). "Free Radicals and Antioxidants in Human Health: Current Status and Future Prospects". Journal of Association of Physicians of India (JAPI) 52: 796.
- Rada B, Leto TL (2008). "Oxidative innate immune defenses by Nox/Duox family NADPH oxidases" (PDF). Contrib Microbiol. Contributions to Microbiology 15: 164–87. doi:10.1159/000136357. ISBN 978-3-8055-8548-4. PMC 2776633. PMID 18511861. — Review
- Conner GE, Salathe M, Forteza R (December 2002). "Lactoperoxidase and Hydrogen Peroxide Metabolism in the Airway". Am J Respir Crit Care Med 166 (12): S57–61. doi:10.1164/rccm.2206018. PMID 12471090.
- Brooker, Robert J. (2011). Genetics: analysis and principles (4th ed.). McGraw-Hill Science. ISBN 978-0-07-352528-0.
- Patel RP, T Cornwell, Darley-Usmar VM (1999). "The biochemistry of nitric oxide and peroxynitrite: implications for mitochondrial function". In Packer L, Cadenas E. Understanding the process of aging: the roles of mitochondria, free radicals, and antioxidants. New York, N.Y: Marcel Dekker. pp. 39–56. ISBN 0-8247-1723-6.
- Liu J, Head E, Gharib AM, Yuan W, Ingersoll RT, Hagen TM, Cotman CW, Ames BN (February 2002). "Memory loss in old rats is associated with brain mitochondrial decay and RNA/DNA oxidation: partial reversal by feeding acetyl-L-carnitine and/or R-alpha -lipoic acid". Proc. Natl. Acad. Sci. U.S.A. 99 (4): 2356–61. Bibcode:2002PNAS...99.2356L. doi:10.1073/pnas.261709299. PMC 122369. PMID 11854529.
- Stadtman ER (August 1992). "Protein oxidation and aging". Science 257 (5074): 1220–4. Bibcode:1992Sci...257.1220S. doi:10.1126/science.1355616. PMID 1355616.
- Muller, Florian (2000). "The nature and mechanism of superoxide production by the electron transport chain: Its relevance to aging". AGE 23 (4): 227–253. doi:10.1007/s11357-000-0022-9
- Han D, Williams E, Cadenas E (January 2001). "Mitochondrial respiratory chain-dependent generation of superoxide anion and its release into the intermembrane space". Biochem. J. 353 (Pt 2): 411–6. doi:10.1042/0264-6021:3530411. PMC 1221585. PMID 11139407.
- Li X, Fang P, Mai J, "et al." (February 2013). "Targeting mitochondrial reactive oxygen species as novel therapy for inflammatory diseases and cancers". J Hematol Oncol. 6 (19). doi:10.1186/1756-8722-6-19. PMID 23442817.
- Van Raamsdonk JM, Hekimi S (February 2009). "Deletion of the mitochondrial superoxide dismutase sod-2 extends lifespan in Caenorhabditis elegans". PLoS Genet. 5 (2): e1000361. doi:10.1371/journal.pgen.1000361. PMC 2628729. PMID 19197346.
- Muller FL, Lustgarten MS, Jang Y, Richardson A, Van Remmen H (August 2007). "Trends in oxidative aging theories". Free Radic. Biol. Med. 43 (4): 477–503. doi:10.1016/j.freeradbiomed.2007.03.034. PMID 17640558.
- Dickinson BC, Chang CJ (August 2011). "Chemistry and biology of reactive oxygen species in signaling or stress responses". Nat. Chem. Biol. 7 (8): 504–11. doi:10.1038/nchembio.607. PMC 3390228. PMID 21769097.
- Irani K, Xia Y, Zweier JL, et al. (March 1997). "Mitogenic signaling mediated by oxidants in Ras-transformed fibroblasts". [[Science (journal)|Science]] 275 (5306): 1649–52. PMID 9054359.
- Ramsey MR, Sharpless NE (November 2006). "ROS as a tumour suppressor?". Nat. Cell Biol. 8 (11): 1213–5. doi:10.1038/ncb1106-1213. PMID 17077852.
- Takahashi A, Ohtani N, Yamakoshi K, et al. (November 2006). "Mitogenic signalling and the p16INK4a-Rb pathway cooperate to enforce irreversible cellular senescence". Nat. Cell Biol. 8 (11): 1291–7. doi:10.1038/ncb1491. PMID 17028578.
- Renschler MF (September 2004). "The emerging role of reactive oxygen species in cancer therapy". Eur. J. Cancer 40 (13): 1934–40. doi:10.1016/j.ejca.2004.02.031. PMID 15315800.
- Toler SM, Noe D, Sharma A (2006). "Selective enhancement of cellular oxidative stress by chloroquine: implications for the treatment of glioblastoma multiforme". Neurosurg Focus 21 (6): E10. PMID 17341043.
- Cairns RA, Harris IS, Mak TW (February 2011). "Regulation of cancer cell metabolism". Nat. Rev. Cancer 11 (2): 85–95. doi:10.1038/nrc2981. PMID 21258394.
- Gupta SC, Hevia D, Patchva S, Park B, Koh W, Aggarwal BB (June 2012). "Upsides and downsides of reactive oxygen species for cancer: the roles of reactive oxygen species in tumorigenesis, prevention, and therapy". Antioxid. Redox Signal. 16 (11): 1295–322. doi:10.1089/ars.2011.4414. PMID 22117137.
- Waris G, Ahsan H (2006). "Reactive oxygen species: role in the development of cancer and various chronic conditions". J Carcinog 5: 14. doi:10.1186/1477-3163-5-14. PMC 1479806. PMID 16689993.
- Ames BN (September 1983). "Dietary carcinogens and anticarcinogens. Oxygen radicals and degenerative diseases". [[Science (journal)|Science]] 221 (4617): 1256–64. PMID 6351251.
- Ozben T (September 2007). "Oxidative stress and apoptosis: impact on cancer therapy". J Pharm Sci 96 (9): 2181–96. doi:10.1002/jps.20874. PMID 17593552.
- Martindale JL, Holbrook NJ (July 2002). "Cellular response to oxidative stress: signaling for suicide and survival". J. Cell. Physiol. 192 (1): 1–15. doi:10.1002/jcp.10119. PMID 12115731.
- Hampton MB, Orrenius S (September 1997). "Dual regulation of caspase activity by hydrogen peroxide: implications for apoptosis". FEBS Lett. 414 (3): 552–6. PMID 9323034.
- Shrivastava A, Kuzontkoski PM, Groopman JE, Prasad A (July 2011). "Cannabidiol induces programmed cell death in breast cancer cells by coordinating the cross-talk between apoptosis and autophagy". Mol. Cancer Ther. 10 (7): 1161–72. doi:10.1158/1535-7163.MCT-10-1100. PMID 21566064.
- Tochhawng L, Deng S, Pervaiz S, Yap CT (May 2013). "Redox regulation of cancer cell migration and invasion". Mitochondrion 13 (3): 246–53. doi:10.1016/j.mito.2012.08.002. PMID 22960576.
- Kong Q, Beel JA, Lillehei KO (July 2000). "A threshold concept for cancer therapy". Med. Hypotheses 55 (1): 29–35. doi:10.1054/mehy.1999.0982. PMID 11021322.
- Schumacker PT (September 2006). "Reactive oxygen species in cancer cells: live by the sword, die by the sword". Cancer Cell 10 (3): 175–6. doi:10.1016/j.ccr.2006.08.015. PMID 16959608.
- Trachootham D, Alexandre J, Huang P (July 2009). "Targeting cancer cells by ROS-mediated mechanisms: a radical therapeutic approach?". Nat Rev Drug Discov 8 (7): 579–91. doi:10.1038/nrd2803. PMID 19478820.
- Sen CK (2003). "The general case for redox control of wound repair". Wound Repair Regen 11 (6): 431–8. doi:10.1046/j.1524-475X.2003.11607.x. PMID 14617282.
- Krötz F, Sohn HY, Gloe T, Zahler S, Riexinger T, Schiele TM, Becker BF, Theisen K, Klauss V, Pohl U (August 2002). "NAD(P)H oxidase-dependent platelet superoxide anion release increases platelet recruitment". Blood 100 (3): 917–24. doi:10.1182/blood.V100.3.917. PMID 12130503.
- Pignatelli P, Pulcinelli FM, Lenti L, Gazzaniga PP, Violi F (January 1998). "Hydrogen peroxide is involved in collagen-induced platelet activation". Blood 91 (2): 484–90. PMID 9427701.
- Guzik TJ, Korbut R, Adamek-Guzik T (December 2003). "Nitric oxide and superoxide in inflammation and immune regulation". J. Physiol. Pharmacol. 54 (4): 469–87. PMID 14726604.