Free Radicals in the Human Body PDF

Free Radicals and the Human Body L. O’Connor: 21 Feb 2024 Oxygen is essential to life Oxygen can be toxic Essential? Oxidation reactions Generation of ATP (ox phos) Detoxification reactions Biosynthetic reactions Toxicity? When oxygen accepts single electrons it is transformed into highly reactive o...

Free Radicals and the Human Body L. O’Connor: 21 Feb 2024 Oxygen is essential to life Oxygen can be toxic Essential? Oxidation reactions Generation of ATP (ox phos) Detoxification reactions Biosynthetic reactions Toxicity? When oxygen accepts single electrons it is transformed into highly reactive oxygen radicals that damage Biradical – 2 unpaired electrons in separate orbitals Superoxide, Hydrogen peroxide, hydroxyl radical Cellular lipids Proteins DNA O2 /ROS The generation of ROS is a natural everyday process Formed as accidental products of enzymatic and non enzymatic reactions Also formed during the inflammatory process as a deliberate part of the process UV radiation and pollutants in the air can increase formation of toxic oxygen-containing compounds Major Sources of Primary ROS in the cell 1. CoQ generates superoxide in ETC 2. Oxidases , oxygenases and peroxidases 3. Ionizing radiation Some of the electrons escape when CoQH* accidentally interacts with O2 to form superoxide What do they do? React with lipids, proteins , carbohydrate and DNA to extract electrons Evidence of free radical damage has been described in > 100 disease states – As the primary cause of the disease – Or by enhancing complications Some diseases associated with free radical injury Atherogenesis Cerebrovascular disorders Emphysema bronchitis Ischemia/reperfusion injury Duchenne-type muscular dystrophy Neurodegenerative disorders Pregnancy/preeclampsia ALS Cervical cancer Alzheimer disease Alcohol –induced liver disease Down syndrome Hemodialysis Ischemia-reperfusion injury following stroke Diabetes OXPHOS diseases (Mit DNA disorders) Acute renal failure Multiple Sclerosis Aging Parkinson Disease Free radical-mediated Cellular Injury Free radical-mediated cellular injury Superoxide and the hydroxyl radical initiate lipid peroxidation in the cellular, mitochondrial, nuclear, and ER membranes The increase in permeability results in an influx of Ca2+, which causes further mitochondrial damage The cysteine sulfhydryl groups and other AA residues on proteins are oxidized and degraded Nuclear and Mit DNA can be oxidized, resulting in strand breaks and other damage RNOS have similar effects Membrane Attack Chain rxns to form lipid free radicals and lipid peroxides in membranes – major contribution to ROS-induced injury Membrane lipid damage leads to loss of structural integrity of membrane Disruption of mit membrane may augment free radical damage Malondialdehyde appears in the blood and urine – biochem marker of free radical damage 2 issues a) Peroxidation of lipids damage lipid molecular structure b) The aldehydes formed can cross-link proteins Proteins and peptides Damage Aas proline, histidine, arginine , cysteine and methionine – susceptible to hydroxyl-damage attack and oxidative damage Consequence – protein may fragment/residues cross-link with other residues evidence of protein damage in many diseases associated with aging e.g. cataracts Proteins in the lens of the eye exhibit ROS damage contain methionine sulphoxide residues and tryptophan degradation products Also – AMD _ age-related macular degeneration Visual loss is related to oxidative damage to the retinal pigment epithelium and choriocapillaris epithelium Lipofuscin granules (liver spots) heterogeneous mix of cross-linked polymerized lipids and proteins formed by reactions between aa residues and lipid peroxidation products such as malondialdehyde DNA damage Oxygen-derived free radicals –major source of DNA damage The non-specific binding of Fe2+ to DNA facilitates localized production of the hydroxyl radical – can cause base alterations Can also attack the deoxyribose backbone and cause strand breaks DNA repair mechanisms do exist – apoptosis can be protective Approx 20 types of oxidatively-altered DNA molecules have been identified NOTE: This type of Damage can be repaired to some extent by the cell or minimized by apoptosis of the cell Mitochondrial vs Nuclear Susceptibility Histones coat nuclear DNA – protect it from damage by radicals Mit DNA lacks histones ROS can diffuse across all membranes DNA in mitochondria –proximity to cellular membrane Most radical species are formed from coenzyme Q – found in mitochondria Nitric Oxide and RNOS (reactive nitrogen-oxygen species) NO – essential and toxic NO has a single electron and binds to other cmps with single electrons e.g. Fe3+ At low conc –functions as a) neurotransmitter and b) as a hormone that causes vasodilation Nitroglycerin: often given to patients with CAD who experience angina – nitroglycerin decomposes in the blood, forming nitric oxide (NO) a potent vasodilator – increases blood flow to the heart and relieves the angina RNOS High concentrations –NO combines with O2 or with superoxide to form additional reactive and toxic species that contain both nitrogen and oxygen (RNOS) RNOS – involved in a) neurodegenerative diseases, e.g. Parkinson disease b) in chronic inflammatory diseases e.g RA (rheumatoid arthritis) Direct Toxic effects of NO By combining with Fe-containing compounds that also have single electrons Major destructive sites of attack include Fe-S centres (e.g. ETC complexes 1 -111) Fe-Heme proteins (e.g. hemoglobin and ETC cytochromes) At low concs – little damage but damage significant to respiratory function in cells already damaged by OxPhos diseases or ischemia RNOS Toxicity Formation of RNOS from nitric oxide During inflammation NO is present in high concs Result 1. Oxidative and free radical damage 2. Nitrating and nitrosylating compounds Consequences: Inhibition of enzymes Mit lip peroxidation Inhibition of ETC and energy depletion SS or DS Breaks in DNA Modification of bases in DNA Formation of Free Radicals during Phagocytosis NADPH Oxidase: Transfers electrons from NADPH to O2 to form superoxide – effective against bacterial infection Myeloperoxidase and Hypochlorous Acid – bacteria lose membrane transport/ ability to synthesize ATP or ETC damage(ETC of bacteria resides in PM) Part of the human antimicrobial defence system Free radical damage and inflammation In several disease states, free radical release by neutrophils or macrophages, during an inflammatory event contributes to injury in the surrounding tissues During stroke or MI – phagocytic cells move into the ischemic area to remove the dead cells Reperfusion injury John had an MI – therefore ability of the heart to make ATP reduced. During ischemia- CoQ and other single electron components of the ETC become saturated with electrons When oxygen is reintroduced (reperfusion) e- donation to O2 to form superoxide is increased  of superoxides results in formation of hydrogen peroxide and the hydroxyl radical Macrophages in the area clean up cell debris and produce NO –further damage to mitochondria by generating RNOS that attack Fe-S centers and cyt in ETC membrane lipids RESULT – RNOS can increase the infarct size Antioxidant Scavenging Enzymes Superoxide dismutase Catalase Glutathione Peroxidase and Glutathione reductase Transfers efrom GSH to hydrogen peroxide GSH redox cycle Regeneration of Glutathione Nonenzymatic Antioxidants Free Radical Scavengers Vitamin E – lipid-soluble antioxidant Vitamin C (Ascorbic Acid)- water soluble Carotenoids (precursor of vit A) Dietary antioxidants e.g flavonoids (red wine, green tea, chocolate …) Note: important to be aware of the concentration necessary to offer antioxidant capability Endogenous Antioxidants Compound synthesized endogenously for other functions or/as urinary excretory products, also function as antioxidants e.g. Uric acid – degradation product of purines – along with protein thiols, it accounts for the major free radical –trapping capacity of plasma e.g. Melatonin –Receptor functions but also free radical scavenger Disease Environment /Genetic Comments Free radical disease Both Damage caused to proteins and lipids lead to cellular dysfunction Myocardial Infarction Both Further damage to the heart muscle can occur because of free radical generation after oxygen is reintroduced to the cells, which were temporarily ischemic – a process known as ischemic reperfusion injury ALS (Amyotrophic lateral sclerosis) Both Genetic form of ALS caused by mutations in superoxide dismutase, leading to difficulty in disposing of superoxide radicals, leading to cell damage caused by excessive ROS Epidemiologic evidence suggests that individuals with a higher intake of foods containing vit E, carotene, and vit C have a lower risk of cancer and other ROS-related diseases Caution: supplementing well nourished individual – either no effect or WARNING –potentially harmful effect Note –study on -carotene – withdrawn due to negative results (Finland 1980s – study the protective effect of carotene on lung cancer) Studies are ongoing