L16 Antioxidants Flashcards
What is oxidative stress?
What does ionizing radiation produce?
Oxidation when ROS are produced faster than can be removed by antioxidants.
Ionizing radiation produces ROS and is deadly. Diets rich in antioxidants mute the effects of radiation.
Studies have shown correlation between diet high in fruits and vegetables and a decreased incidence in disease.
It was hypothesized that the anyioxidants in fruits and vegetable were responsible for this, however, studies have failed to show andioxidants reduce the risk of disease
What are ROS?
What are free radicals?
What are nonradical pro-oxidants?
ROS are compounds that accelerate oxidation. Most are highly reactive and thus unstable
Free radicals are ROS that contain an unpaired electron and have a short half-life:
- Superoxide radical (O2.- ), aka oxygen free radical
- Hydroxyl radical (OH.) (Extremely reactive relative to the superoxide radical)
- Nitric oxide (NO.) (signaling molecule)
Nonradical pro-oxidants are ROS whose outer electrons are paired but are easily converted to a reactive radical
- Hydrogen peroxide (H2O2)
- Singlet oxygen (O2) (outer most pair of electrons have antiparallel spins)
What is the origen of ROS?
ROS are constantly generated by normal aerobic metabolism
- Mitochondria generate 90% of the body’s accidental production of ROS.
- Aerobic exercise increases ROS production by 2- to 4-fold
Cytochrome P450 mediated drug metabolism
- Superoxide is an actual intermediate in drug hydroxylation reactions and occasionally escapes from the enzyme’s active site
How do neutrophils/ macrophages, free iron andradiation therapy contribute to ROS generation?
- Activated neutrophils and macrophages use NADPH oxidase to generate superoxide radicals to kill bacteria
- Infection and/or inflammation results in increased levels of H2O2
- Free iron or copper will catalyze Fenton reaction (e.g. phagolysosome of neutrophil and macrophage)
Fe+2 + H2O2 → Fe+3 + OH- + OH.
- Radiation therapy for cancer and radiographic x-rays generate hydroxy radicals from water
X-Rays + H2O → H. + OH.
What is lipid peroxidation?
What lipids are most susceptible?
What are the consequences?
Hydroxyl free radicals can be damaging especially in membranes where they can trigger a chain reaction
Polyunsaturated fatty acids (PUFAs) in membrane lipids are especially sensitive to peroxidation
As a general rule, a free radical initiates a chain reaction by taking an electron from a molecule to pair up with its unpaired electron:
- The molecule that lost an electron to the free radical now has an unpaired electron and is itself a free radical
- Lipid radical binds oxygen to form lipid peroxyl radical. This radical in turn takes an electron from a neighboring fatty acid to form another lipid radical (propagation)
- This starts a self-perpetuating chain
- reaction and is termed lipid peroxidation
Lipid peroxidation leads to:
- Rancidity of food (fats gone bad)
- Tissue damage and possibly disease (e.g. aging, cancer and atherosclerosis)
What is the most potent ROS?
What are its effects on a cell?
The hydroxyl radical is the most potent of the ROS and can be thought of as a “biological bullet”
- Protein damage
- Membrane damage
- DNA damage
- Lipid peroxidation
- Mitochondrial damage
- Cell swelling from H2O influx
- Increased Na+ permeability
- Massive influx of Ca2+
What evidence suggests ROS’s might be beneficial?
C. elegans (round worms used in aging research) engineered to produce high levels of free radicals actually lived longer.
- If mutant worms were fed vitamin C, the longevity advantage disappeared
It is hypothesized that ROS may stimulate internal repair mechanisms
- ROS may explain in part why exercise is beneficial
- Researchers studied exercisers who took antioxidants with exercisers who did not.
- Exercisers who did not take antioxidants were healthier than those who did
What is the role of antioxidants?
Antioxidants decrease oxidant stress by:
- Directly neutralizing ROS
- Preventing formation of ROS
- Binding transition metals (e.g. ferrous iron)
- Preventing and/or terminating chain reactions
- Regenerating oxidized antioxidants
What are 3 antioxidant categories?
Antioxidant categories include:
- Exogenous antioxidants; vitamins C and E, carotenoids, flavonoids, tannins, lignans, etc, obtained from dietary vegetables and fruits
- Endogenous antioxidants; glutathione (GSH), bilirubin, monosodium urate
- Antioxidant enzymes
What are two key endogenous antioxidants?
What is important about the sulfhydryl group of glutathione (GSH)?
How does alcoholic liver disease contribute to ROS formation?
Monosodium urate is the primary antioxidant in blood, more than vitamin C (extracellular)
- Monosodium urate/ Vitamin C (~300 μM / 50 μM)
The sulfhydryl group of glutathione (GSH)
(1) keeps protein sulfhydryl groups in the reduced state (Protein sulfhydryl groups (-SH) are very sensitive to oxidation)
(2) directly scavenges free radicals
Alcoholic liver disease:
Daily ethanol consumption → ↑ cyp2E1 → ↑ ROS →
↓ GSH → Liver cell damage →
Cirrhosis of the liver in susceptible individuals
What phytochemicals increase (induce) GSH synthesis?
- Kaempferol
- Sulforaphane
- Flavonoid fraction
- Ellagic acid
- Curcumin
- Diallyl sulfide
(pretty much found in fruits and vegetables)
What does Superoxide Dismutase (SOD) Destroy?
How many types of Superoxide Dismutases are there?
Superoxide dismutase (SOD) converts superoxide radicals to oxygen and hydrogen peroxide which is itself, another ROS.
- There are three SOD isoenzymes, each “customized” for one of the bodily compartments where superoxide radicals are generated (mitochondria, cytosol and extracellular space)
How is the ROS product that SOD generates (H2O2) destroyed?
Catalase and glutathione peroxidase work in collaboration with SOD to destroy H2O2.
- Catalase has a high Km for hydrogen peroxide and is found in peroxisomes
- Mice genetically engineered to produce more catalase live longer.
- Converse is not true
- Catalase knockout mice appear to be healthy and do not exhibit a reduced lifespan
- Glutathione peroxidase is found in the cytosol and has a lower Km than catalase and is thus more effective at cleaning up hydrogen peroxide, uses two GSHs to reduce H2O2 creating water and oxidized GSSG
How is selenium used?
Selenium (Se) is a trace mineral required for the synthesis and activity of glutathione peroxidase
- Selenium supplements increase the levels of GSH peroxidase.
- Selenium is added to animal feed in the US resulting in increased livestock productivity plus increasing dietary selenium for human consumers of meat, milk and eggs
- Placebo-controlled trials have not found any health advantage to taking selenium supplements
What are some traits of Vitamin C?
What are the effects of heat on Vitamin C?
Vitamin C (aka ascorbic acid) is:
- A water-soluble antioxidant
- Found in fresh fruits and vegetables
- Required by primates and guinea pigs
Vitamin C is very labile to oxidation and heat:
- Frozen concentrate has 25% more vitamin C than ready-to-drink juice
- Once a container of ready-to-drink orange juice is opened, ~2% of the vitamin C is lost per day.
- Cooking vegetables decreases levels of vitamin C by 50 to 80%
- Baked potato contains 80% of its initial vitamin C levels and can supply 45% of our daily vitamin C needs
How effective is taking large doses of Vitamin C?
Mega does of vitamin C are unable to sustainably elevate blood vitamin C levels.
How does Vitamin E act on peroxidation chain reaction?
Once a chain reaction is started, 10 to 15 fatty acids are often destroyed before vitamin E has a chance to terminate the chain reaction then:
- Peroxyl lipid radical will extract an electron from vitamin E faster than it would extract an electron from another PUFA
- The vitamin E radical is fairly stable (vitamin E radical does not need to extract an electron from a neighboring lipid molecule) and thus the chain reaction is terminated.
- Oxidized vitamin E can be regenerated using vitamin C, GSH and ultimately NADPH
