Free Radicals and Vitamin E

FREE RADICALS AND VITAMIN E

BY: DR. MAYUR MAKADIARESIDENT,

BIOCHEMISTRY DEPARTMENTPRAMUKHSWAMI MEDICAL COLLEGE

KARAMSAD

Definition

• A free radical is a molcule or molecular fragment that contains one or more unpaired electrons in its outer orbital.

• Represented by a superscript dot, (R˙)

ROS

• Normally, 4e- are transferred to complete reduction of O2 to form H2O.

• O2 + 4H⁺ + 4e- ―› 2 H2O • The products of partial reduction of O2

are highly reactive and called Reactive Oxygen Species(ROS), Reactive Oxygen Intermediates, Oxygen free radicals(OFR).

ROS (reactive oxygen species)

Free radicals Superoxide =O2

·

Hydroxyl radical =OH ·

Lipid peroxyl =ROO ·

Hydroperoxyl =HOO

·

Particals, which are not free radicals

Hydrogen peroxide (H2O2)

Hypochlorous acid(HClO)Ozone (O3)

Singlet oxygen (1O2)

Reactive Oxygen Intermediates

• Superoxide Radical: single electron reduction product of oxygen

• Hydrogen Peroxide: two-electron reduction product of oxygen

Dismutation

• Hydroxy Radical: three-electron reduction product of oxygen

Fenton Reaction Haber- Weiss Reaction

Transition Metals

• Ferrous iron, and Cuprous copper can react with hydrogen peroxide to yield the hydroxy radical.

• They are referred to as the Transition Metals.

• They never occur in free, unbound form in the body.

Generation of Oxygen Free Radicals

• 1) Electron Leakage: major sourceConstantly produced due to leaks in

ETC.There is production of superoxide

radicals from interaction between CoQ and O2 in ETC.

Generation of Oxygen Free Radicals

• 2) Normal Oxidation- Reduction Reaction

• Auto-oxidation of certain compound: Adrenaline, thiols, ascorbic acid

• Falvin coenzymes present in peroxisomes are specially active in generating H2O2.

• Enzymes: xanthine oxidase, aldehyde oxidase.

Generation of Oxygen Free Radicals

• 3) Exogenous Agents:• Toxic compound, such as CCl4• Ionizing Radiation• Light of appropriate wavelengths• Cigarette smoke• Inhalation of air pollutants

Generation of Oxygen Free Radicals

• 4) Respiratory Burst:• Free radical production is

sometimes required in biological systems.

Damage Produced by Free Radicals

• Extremely reactive• Half life is only a few milliseconds• Almost all biological macromolecule are

damaged• Protein: fragmentation of proteins.• Oxidation of sulfhydryl group containing

enzymes, loss of function.• DNA: damaged by strand breaks, directly

cause inhibition of protein and enzyme synthesis and indirectly cell death, mutation & carcinogenesis.

Lipid Peroxidation

• Lipids are most susceptible• PUFA present in cell membrane

especially prone to damage.• Initiation phase: • Primary event is production of R˙

by interaction of a PUFA with free radicals generated by other means.

• RH + OH˙ ―› R˙ + H2O

Lipid Peroxidation• Propagation phase:• R˙ rapidly react with molecular O2 forming a

Peroxyl radical (ROO˙) which can attack another PUFA.

• R˙ + O2 ―› ROO˙• ROO˙ + RH ―› ROOH + R˙• Net result is the conversion of R˙ to ROOH

(Hydroperoxide).• But simultaneous conversion of R˙ to ROO˙

which lead to continuous production of Hydroperoxide.

• Chain reaction• “Death kiss” by free radicals.

Lipid Peroxidation

• Termination phase:• The chain reaction continue untill

a peroxyl radical react with another peroxyl radical to form inactive products.

• ROO˙ + ROO˙ ―› RO-OR + O2• R˙ + R˙ ―› R-R• ROO˙ + R˙ ―› RO-OR

Free radical scavenger systems

• Also called Antioxidant defence mechanism

Free radical scavenger systems

• A) Preventive mechanism: Prevent the generation of free

radicals

• B) Interceptive mechanism: Destroy the free radicals that are

generated

Preventive mechanism

• The efficacy of ETC• Sequestration of transition metals• Peroxide decomposing enzymes

– Glutathion peroxidase– Catalase

Interceptive mechanism

• A)Enzyme : Superoxide dismutase (SOD)

• Only enzyme that take a free radical as its substrate.

• Different isoenzymes• 1) mitochondrial SOD: Mg++

dependant• 2)cytosoplasmic SOD: copper-zinc

dependant

Interceptive mechanism

• B) Non- enzyme substance:• Vitamin E• Retinoids• Ascorbate• Ceruloplasmin• Uric acid

Vitamin E as antioxidant

• Most effective naturally occurring chain breaking antioxidant in tissue.

• Alpha tocopherol (T-OH) intercept the peroxyl free radical

• T-OH + ROO˙―› TO˙+ ROOH• The tocoperoxyl radical formed is stable

and will not propagate the cycle any further

• TO˙+ ROO˙ ―› inactive products

Vitamin E

• Only traces of tocopherol are required to protect considerable amounts of PUFA.

• 1 tocopherol per 1000 lipid molecule• But as antioxidant, alpha tocopherol

is consumed, so it has to be replenished by dietary supply

Vitamin E

• Isolated from wheat germ oil• Earlier known as Anti-Sterility Vitamin• Other name “tocopherol” derived from

greek word tokos (childbirth), pheros (to bear) and ol(alcohol)

• Eight naturallly occurring tocophrols• Most Important: Alpha tocopherol

Vitamin E

• Chemical structure:• A chromane ring(tocol), with an

isoprenoid side chain

Vitamin E

• Absorption : along with other fats and need bile salts

• Transport : as chylomicrons• Store : adipose tissue• Plasma concentration : 0.5-1 mg/dL• Excretion : in feces via hepato-biliary

route, after chromane ring oxidation followed by conjugation with glucuronic acid

Function of Vitamin E

• Most powerful natural antioxidant

Function of Vitamin E• Reduce risk of Atherosclerosis by

reducing oxidation of LDL• Boost immune response• Protect RBC from haemolysis• Keeps structural and functional integrity

of all cells• Slows ageing process• Protection against Alzheimer’s disease• Act synergistically with Selenium to

minimise lipid peroxidation

Deficiency of Vitamin E

• Uncommon, due to adequate level in average diet.

• Deficiency seen in :– Cannot absorb dietary fat– In premature infants– In abetalipoproteinemia– Mutation in the gene for tocopherol

trasfer protein

RDA• Male : 10 mg/day• Females : 8 mg/day• Pregnancy : 10 mg/day• Lactation: 12 mg/day• 15 mg Vit. E = 33 IU• Requirement increase with higher intake of

PUFA• Pharmacological dose= 200-400 IU/day• Hypervitaminosis E: rarely occur• Cause tendency to hemorrhage, bcz it is

mild anti-coagulant

Thank You