This Key Event Relationship is licensed under the Creative Commons BY-SA license. This license allows reusers to distribute, remix, adapt, and build upon the material in any medium or format, so long as attribution is given to the creator. The license allows for commercial use. If you remix, adapt, or build upon the material, you must license the modified material under identical terms.
Oxidative Stress leads to Modified Proteins
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding||Point of Contact||Author Status||OECD Status|
|Deposition of energy leading to occurrence of cataracts||adjacent||Moderate||Low||Arthur Author (send email)||Open for citation & comment|
Life Stage Applicability
|All life stages||Moderate|
Key Event Relationship Description
Oxidative stress refers to production of reactive oxygen species (ROS) and reactive nitrogen species (RNS), and a reduction/insufficiency in radical-clearing enzymes (Brennan et al., 2012; Engwa et al., 2022; Cabrera & Chihuailaf, 2011). Under normal conditions, radicals are kept at a sustainable level by the body’s antioxidant defense system but if the radicals exceed the defense threshold, it can lead to protein oxidation (Taylor & Davies, 1987; Cabrera & Chihuailaf, 2011; Engwa et al., 2022).ROS and RNS, collectively known was RONS, have subdivisions of radicals and non-radicals, with the former being the more reactive (Cabrera & Chihuailaf, 2011; Engwa et al., 2022). The superoxide ion radical works to oxidize biological structures such as proteins and DNA, as well as helping to generate other types of radicals. Superoxide ion can oxidize the amino acids arginine into glutamic semialdehyde and methionine into disulphides. Ozone, another ROS, specifically oxidizes proteins by reacting with their alcohol, amine, and sulfhydryl functional groups (Engwa et al., 2022). Furthermore, H2O2 is able to travel further than other ROS as it is more stable (Spector, 1990), it can also interact with transition metal ions (Cu+ or Fe2+) that are often bound to proteins such as ferritin and ceruloplasmin. This interaction oxidizes the protein, converting H2O2 into a hydroxyl radical. (Engwa et al., 2022). Another example of non-radical oxidation of proteins is peroxynitrite’s action on tryptophan and methionine. These amino acids are oxidized, tryptophan into nitrotryptophan and methionine into methionine sulfoxide or ethylene (Engwa et al., 2022; Perrin & Koppenol, 2000). There is also evidence to support H2O2 leading to protein modifications, however singlet oxygen or hydroxyl radicals seem to not be involved (Hightower, 1995). Targets of free radicals can include lipids, DNA, and proteins (Engwa et al., 2022).
Antioxidants stabilize radicals by facilitating an electron donation (Cabrera & Chihuailaf, 2011). This reduces the number of radicals available to oxidize other macromolecules like proteins, thus reducing the number of molecules sustaining modifications (Engwa et al., 2022). Proteins are particularly good targets of free radicals because of their abundance of amino acids containing sulfur and aromatics, as well as the fact that following proline oxidation, peptide bonds are at risk of free radical attack (Cabrera & Chihuailaf, 2011). Free radicals have an affinity for sulfur-containing amino acids, such as cysteine and methionine, due to their ability to readily react with most ROS, making the proteins containing them the most susceptible to oxidative modifications. This quality of the amino acids makes them act in an antioxidant capacity for the other structures in the area (Bin et al., 2017).
Proteins that interact with RONS will undergo bond alterations that can lead to aggregation. Free radicals can modify proteins in both reversible and irreversible ways. Redox-response proteins get oxidized as part of the protective mechanism against oxidizing radicals but will be repaired once the threat is over. In this instance, modifications are reversible, and homeostasis is maintained via antioxidative action. These proteins function as buffer, reducing free radicals before that can oxidize other proteins. Irreversible oxidation, on the other hand, occurs when there is oxidation on important functional or structural sites (Chen et al., 2013). These sites are important to a certain function of a protein or help maintain its specific structural configuration. This damage can result in loss of function and/or misfolding of proteins. The amino acids of proteins are very susceptible to ROS attacks, with methionine, tryptophan, histidine, and cysteine residues being the most at risk (Chen et al., 2013; Balasubramanian, 2000). Once the amino acids get oxidized by the ROS, they become oxidation products and are no longer useful for the originally intended function within the protein (Engwa et al., 2022).
Protein carbonyl level is changed by ROS exposure through the post-translational modification called carbonylation, where carbonyl groups are added to the protein (Grimsrud et al., 2008). ROS accomplishes this by interacting with amino acids such as proline and lysine, on the protein side chains, which tend to create carbonyl derivatives (Engwa et al., 2022). In proteins attacked by radicals, there is also a tendency to form cross-links between the proteins. These connections affect water solubility of the proteins. Normal proteins have a balance of protein-protein and protein-water interactions that maintain structure and solubility, however following the oxidation of the amino side chains of the proteins, they become thermodynamically preferred to have more protein-protein interactions. This causes an increase in cross-linking and aggregation, which leads to decreased water solubility (Xiong, 2000).
Evidence Collection Strategy
The strategy for collating the evidence to support the relationship is described in Kozbenko et al 2022. Briefly, a scoping review methodology was used to prioritize studies based on a population, exposure, outcome, endpoint statement.
Evidence Supporting this KER
Overall Weight of Evidence: Moderate
The biological plausibility of the relationship between increased oxidative stress leading to modified proteins is strongly supported by the literature. Studies show that increase of oxidative stress, leads to protein modifications (Uwineza et al., 2019; Taylor & Davies, 1987; Truscott, 2005; Brennan et al., 2012; Davies & Truscott, 2001). This relationship has been observed in rabbit and guinea pig models (Shang et al., 2001; Giblin et al., 2002). Excessive ROS generation can lead to oxidization of amino acid side chains, cross-link formation, and conformational changes (Uwineza et al., 2019). Radical oxygen species can modify protein molecules chemically, and act to increase proteolytic activity of cellular enzymes by inactivating proteolytic enzyme inhibitors (Balasubramanian, 2000; Stohs, 1995). Oxidation via radical species-protein interactions can also lead to increased insolubility of the proteins due to their modified structure and inability to interact with unmodified proteins (Kim et al., 2015).
Proteins aggregation can be exacerbated by protein oxidation. For example, in lens cells, the presence of free radicals can attack abundant proteins such as alpha crystalline. The thiol groups on the crystallin proteins then become oxidized and increase the number of disulfide adducts, increasing protein aggregates (Cabrera & Chihuailaf, 2011; Moreau et al., 2012). Amino acid side chains are particularly susceptible to damage from oxidative stress, resulting in cross-linking and conformational changes which can culminate in protein accumulation. The accumulation is a result of the cells being denucleated and therefore, unable to reverse the sustained damage via protein turnover (Uwineza et al., 2019). Oxidative conditions can contribute to the loss of protein function leading to the generation of high molecular weight aggregates. This change is hypothesised to be a result of methionine oxidation, which is more likely to happen when GSH levels are low resulting in an increase in hydroxyl radical formation (Brennan et al., 2012; Truscott, 2005). The hydroxyl radical also results in covalently bound protein aggregates, and alongside superoxide ion it leads to protein fragmentation (Stohs, 1995).
Uncertainties and Inconsistencies
Known modulating factors
|Modulating Factor (MF)||MF Specification||Effect(s) on the KER||Reference(s)|
|Free Radical Scavengers||Antioxidant supplementation has been linked to reduced oxidative damage. The scavengers work to reduce the reactivity of ROS in the cell by donating one of their own electrons, resulting in a matching pair on the radical.||Lower levels of free radical scavengers would result in a limited ability to reduce RONS-mediated damage. Reduced GSH levels are associated with protein modifications, including changes to water-solubility (29% decrease GSH ≥ 20% decrease soluble proteins) and protein carbonyl concentration (84% decrease GSH ≥ 367% increase carbonyl concentration).||Taylor & Davies, 1987; Cabrera & Chihuailaf, 2011; Giblin et al., 2002; Shang et al., 2001|
|Age||Older lenses have reduced antioxidant capacities (in humans >30 years old). This is due in part to the development of a chemical barrier between the cortex and the nucleus of the lens that prevents GSH from protecting the oldest lens cells from oxidative damage.||Antioxidants function to prevent RONS-mediated damage, so proteins in older lenses, with reduced antioxidant capacities, will be more likely to undergo oxidative modifications.||Taylor & Davies, 1987; Cabrera & Chihuailaf, 2011; Quinlan & Hogg, 2018; Sweeney & Truscott, 1998|
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
This KER is plausible in all life stages, sexes, and organisms. The majority of the evidence is from in vivo male adult guinea pigs and rabbit in vitro models that do not specify sex.
Balasubramanian, D. (2000), “Ultraviolet radiation and cataract”, Journal of Ocular Pharmacology and Therapeutics, Vol.16/3, Mary Ann Liebert Inc, Larchmont, https://doi.org/10.1089/jop.2000.16.285.
Bin, P., R. Huang and X. Zhou. (2017). “Oxidation Resistance of the Sulfur Amino Acids: Methionine and Cysteine”, BioMed Research International, Vol.2017, Hindawi Limited, London, https://doi.org/10.1155/2017/9584932.
Brennan, L., R. McGreal and M. Kantorow. (2012), “Oxidative stress defense and repair systems of the ocular lens”, Frontiers in Bioscience – Elite, Vol.4/E(1), Frontiers in Bioscience, Singapore, https://doi.org/10.2741/365.
Cabrera, M., & R. Chihuailaf. (2011), “Antioxidants and the integrity of ocular tissues”, Veterinary Medicine International, Vol.2011, Hindawi Limited, London, https://doi.org/10.4061/2011/905153.
Chen, Y. et al. (2013), “Ocular aldehyde dehydrogenases: Protection against ultraviolet damage and maintenance of transparency for vision”, Progress in Retinal and Eye Research, Vol.33/1, Elsevier Ltd, London, https://doi.org/10.1016/j.preteyeres.2012.10.001.
Davies, M. and R. Truscott. (2001), “Photo-oxidation of proteins and its role in cataractogenesis”, Journal of Photochemistry and Photobiology B: Biology, Vol.63/1-3, Elsevier, Amsterdam, https://doi.org/10.1016/s1011-1344(01)00208-1.
Engwa, G.A., F.N. Nweke and B.N. Nkeh-Chungag. (2022), “Free Radicals, Oxidative Stress-Related Diseases and Antioxidant Supplementation”, Alternative therapies in health and medicine, Vol.28/1, InnoVision Health Media, Eagan, pp.114-128.
Giblin, F. et al. (2002), “UVA Light In vivo Reaches the Nucleus of the Guinea Pig Lens and Produces Deleterious, Oxidative Effects”, Experimental Eye Research, Vol.75/4, Academic Press Inc, Cambridge, https://doi.org/10.1006/exer.2002.2039.
Grimsrud, P.A. et al. (2008), “Oxidative Stress and Covalent Modification of Protein with Bioactive Aldehydes”, Journal of Biological Chemistry, Vol.283/32, American Society for Biochemistry and Molecular Biology, Rockville, https://doi.org/10.1074/jbc.R700019200.
Hightower, K. (1995), “The role of the Lens epithelium in development of UV cataract”, Current Eye Research, Vol.14/1, Taylor & Francis, Oxfordshire, https://doi.org/10.3109/02713689508999916.
Kim, I. et al. (2015), “Site specific oxidation of amino acid residues in rat lens γ-crystallin induced by low-dose γ-irradiation”, Biochemical and Biophysical Research Communications, Vol.466/4, Elsevier, Amsterdam, https://doi.org/10.1016/j.bbrc.2015.09.075.
Kozbenko, T. et al. (2022), “Deploying elements of scoping review methods for adverse outcome pathway development: a space travel case example”, International Journal of Radiation Biology, 1–12. https://doi.org/10.1080/09553002.2022.2110306
Perrin, D. and W.H. Koppenol. (2000), “The Quantitative Oxidation of Methionine to Methionine Sulfoxide by Peroxynitrite”, Archives of Biochemistry and Biophysics, Vol.377/2, Elsevier, Amsterdam, https://doi.org/10.1006/abbi.2000.1787.
Quinlan, R.A., and P.J. Hogg. (2018), “γ-Crystallin redox–detox in the lens”, Journal of Biological Chemistry, Vol.293/46, American Society for Biochemistry and Molecular Biology, Rockville, https://doi.org/10.1074/jbc.H118.006240.
Shang, F., T. Nowell and A. Taylor. (2001), “Removal of oxidatively damaged proteins from lens cells by the ubiquitin-proteasome pathway”, Experimental Eye Research, Vol.73/2, Academic Press Inc, Cambridge, https://doi.org/10.1006/exer.2001.1029.
Spector, A. (1990), “Oxidation and Aspects of Ocular Pathology”. CLAO Journal, Vol.16/1, Lippincott, Williams and Wilkins Ltd, Philadelphia, S8-S10.
Stohs, S. (1995), “The role of free radicals in toxicity and disease”, Journal of Basic Clinical Physiology and Pharmacology, Vol.6/3-4, Walter de Gruyter GmbH, Berlin, https://doi.org/10.1515/jbcpp.1995.6.3-4.205.
Sweeney, M.H.J. and R.J.W. Truscott. (1998), “An Impediment to Glutathione Diffusion in Older Normal Human Lenses: a Possible Precondition for Nuclear Cataract”, Experimental Eye Research, Vol.67, Academic Press Inc, Cambridge, https://doi.org/10.1006/exer.1998.0549.
Taylor, A. and K. Davies. (1987), “Protein oxidation and loss of protease activity may lead to cataract formation in the aged lens”, Free Radical Biology and Medicine, Vol.3/6, Elsevier, Amsterdam, https://doi.org/10.1016/0891-5849(87)90015-3.
Truscott, R. (2005), “Age-related nuclear cataract - Oxidation is the key”, Experimental Eye Research, Vol.80/5, Academic Press Inc, Cambridge, https://doi.org/10.1016/j.exer.2004.12.007.
Uwineza, A. et al. (2019), “Cataractogenic load- A concept to study the contribution of ionizing radiation to accelerated aging in the eye lens”, Mutation Research - Reviews in Mutation Research, Vol.779, Elsevier, Amsterdam, https://doi.org/10.1016/j.mrrev.2019.02.004.
Xiong, Y.L. (2000), “Chapter 4: Protein Oxidation and Implications for Muscle Food Quality”, in Antioxidants in Muscle Foods: Nutritional Strategies to Improve Quality, Wiley, Hoboken, pp.85-111.