Reactive Oxygen Species (ROS)

• Oxygen is both essential and toxic to human life
• O2 is used for oxidation reactions important for adenosine triphosphate (ATP) generation, detoxification, and biosynthesis
• Molecular oxygen is reduced to water through accepting 4 electrons
• A small percentage, 1-5 %, of oxygen undergoes stepwise reduction producing free radical and non-radical intermediates
• These intermediates are highly reactive, causing damage to cellular lipids, proteins and DNA
• They contribute to cellular death and degeneration in many diseases

Reactive Oxygen Species (ROS)

• Highly reactive chemicals formed from oxygen

ROS Consist Of:

• Free radicals containing one, or more, unpaired electrons
• Examples include superoxide (O2), hydroxyl radical (HO-), and peroxyl radical (ROO-)
• Nonradical derivatives of O2, lack unpaired electrons
• Examples include hydrogen peroxide (H2O2) and singlet oxygen 1O2
• The term ROS broadly represents free radicals and non-free radicals, which are extremely reactive in biological systems

Why non-radicals are reactive

• Non-radicals contain oxygen
• Hydrogen peroxide (H2O2) contains an additional oxygen atom, making it reactive
• Singlet oxygen contains oxygen in an excited state

Sources of Free Radicals

• Normal biological processes known as cellular metabolism are endogenous sources
• Environmental influences are exogenous sources

Antioxidants

• Substances that significantly delay or inhibit oxidation of a substrate
• They reduce and detoxify ROS into harmless products in cells
• They mitigate harmful effects of free radicals, also known as Antioxidant defense mechanisms
• They are scavengers of free radicals
• Synthesized within the body or extracted from food like fruits, vegetables, seeds, nuts, meats and oil

Classes of Antioxidants Based on Nature and Action:

• Enzymatic antioxidants that include SOD, catalase, glutathione peroxidase, and glutathione reductase
• Non-enzymatic antioxidants that include nutrient; (β-carotene, ascorbic acid, selenium, ɑ-tocopherol)
• Non-enzymatic antioxidants that include metabolic (glutathione, transferrin, albumin, etc)

Enzymatic Antioxidants

• Enzymatic inactivation of ROS that include SOD, catalase, and glutathione peroxidase
• SOD or Superoxide Dismutase, acts as the first line of defense to protect cells
• Catalase
• Glutathione peroxidase

Non-Enzymatic Antioxidants That Utilize Nutrients

• Chemical inactivation of ROS
• Vitamin E
• Vitamin C
• Carotenoids
• Selenium, Se
• Glutathione, GSH

Classification of Antioxidants Based on Location:

• Plasma antioxidants that include β-carotene, ascorbic acid, bilirubin, etc
• Cell membrane antioxidants such as ɑ-tocopherol
• Intracellular antioxidants such as SOD, catalase, and glutathione peroxidase

Antioxidants in Relation to Lipid Peroxidation:

• Preventive antioxidants block initial production of free radicals, such as catalase and glutathione peroxidase
• Chain breaking antioxidants inhibit propagative phase of lipid peroxidation, such as SOD, Vitamin E, and Uric acid

Oxidative Stress

• Disturbance of balance between the production of ROS and the levels of antioxidants in the body

Common Conditions of Oxidative Stress

• Oxidation of sulfhydryl groups, modification of certain amino acids leads to loss of biological activity of proteins
• PUFA lipids are highly susceptible to damage
• Linkage of CHOs to proteins (glycation) increases the susceptibility of proteins attacked by free radicals
• DNA strand breaks, and fragmentation of bases and deoxyribose

Implications of Free Radicals and Oxidative Stress

• Cardiovascular diseases, or CVD, where oxidized LDL, promote atherosclerosis and CHD
• Cancer, resulting from DNA mutation and inactivation of tumor suppressor genes
• Inflammatory diseases like RA, Chronic glomerulonephritis, and Ulcerative colitis
• Diabetes as a result of destruction of islet of pancreas
• Aging

Lipid Peroxidation

• Lipid peroxidation is any oxidation reaction producing a peroxide
• Crucial step in pathogenesis of several disease states in adult and infant patients
• Destruction of membrane lipids can be dangerous for the viability of cells and tissues
• Lipid peroxidation is a self-propagating chain-reaction
• Oxidation of a few lipid molecules can cause significant tissue damage

Lipid Peroxidation Stages:

• Initiation begins with a hydroxyl radical that removes a hydrogen atom from PUFA, producing a lipid free radical
• Production of the radical
• Propagation is when a lipid free radical interacts with oxygen and produces a peroxyl radical, attacking another PUFA molecule, which causes chain reaction
• Termination is when the radical reaction stops by the reaction of 2 radicals and produces a non-radical species
• A peroxyl radical reacts with another peroxyl radical to form inactive products

Vitamin E

• The forms of vitamin E are ɑ, β, ɣ, δ -tocopherol and tocotrienol
• The most active form is ɑ-tocopherol
• It is a fat-soluble vitamin
• It is present in cellular membranes and protects against lipid peroxidation
• ɑ-Tocopherol can directly affect oxyradicals, thus serving as an important chain breaking antioxidant

Recommended Daily Allowance of Vitamin E

• 15 mg/day of ɑ-tocopherol
• Dietary intake of PUFA impacts the required amount of Vitamin E

Dietary Sources of Vitamin E

• Vegetable oils
• Liver
• Egg
• Whole grains
• Leafy vegetables
• Legumes

Functions of Vitamin E

• Antioxidant that prevents the cell of nonenzymic oxidation, like peroxidation of PUFAs
• Has synergistic antioxidant action with selenium
• Is not recommended for the prevention of chronic disease, such as coronary heart disease or cancer

Vitamin E Deficiency

• Newborns have low reserves of vitamin E.
• Breast milk and formulas contain vitamin E
• Very-low-birth-weight infants may be given supplements to prevent hemolysis and retinopathy
• Deficiency is associated with defective lipid absorption or transport in adults
• Patients are sensitive to peroxides and can present with hemolytic anemia