- A fat soluble vitamin
- Also called Anti infertility vitamin
- Vitamin E acts as a chain-breaking antioxidant and is an efficient free radical scavenger.
- It protects low-density lipoproteins (LDLs) and polyunsaturated fats in membranes from oxidation.
- A network of other antioxidants (e.g., vitamin C, glutathione) and enzymes maintains vitamin E in its reduced state.
Structure of vitamin E
- Vitamin E derivatives have a chromane ring (tocol) system.
- An isoprenoid side chain is attached to the chromane ring (Figure-1)
- There are eight naturally occurring tocopherols
- Vitamin E is a collective name for all stereoisomers of tocopherols and tocotrienols.
- The most biologically active is α-tocopherols, but β-, γ-, δ-tocopherols, 4 tocotrienols, and several stereoisomers may also have important biological activity.
Figure-1- Alpha tocopherol- 5,7,8-trimethyl tocol
Absorption and Metabolism
- Absorption requires the presence of bile salts
- After absorption, vitamin E is taken up by chylomicrons to the liver
- A hepatic α -tocopherol transport protein mediates intracellular vitamin E transport and incorporation into very low-density lipoprotein (VLDL).
- The transport protein has particular affinity for α -tocopherol; thus this natural isomer has the most biologic activity.
- Vitamin E is widely distributed in the food supply and is particularly high in sunflower oil, safflower oil, and wheat germ oil;
- γ tocotrienols are notably present in soybean and corn oils.
- Vitamin E is also found in meats, nuts, and cereal grains, and small amounts are present in fruits and vegetables.
- The RDA for vitamin E is 15 mg/d (34.9 μmol or 22.5 IU) for all adults.
- Diets high in polyunsaturated fats may necessitate a slightly higher requirement for vitamin E.
Functions of vitamin E
1) It acts as a lipid-soluble antioxidant in cell membranes, and is important in maintaining the fluidity of cell membranes.
Antioxidant role of vitamin E
- Reactive oxygen species (ROS) are molecular oxygen metabolites that are highly reactive with lipids, proteins, and DNA, causing oxidative damage to these cellular macromolecules.
- This damage, termed oxidative stress, accumulates over time and is thought to contribute to both disease pathology and the aging process
- Cellular mechanisms that exist to counteract ROS include stabilization by enzymes such as superoxide dismutase and Catalase,
- Direct scavenging by antioxidant molecules such as glutathione (GSH), a major intracellular antioxidant; cysteine, a precursor of GSH and a major extracellular antioxidant in plasma
- Vitamin E, is a major lipid soluble antioxidant; and
- Ascorbate, is an intracellular and extracellular antioxidant.
The main function of vitamin E is as a chain-breaking, free-radical trapping antioxidant in cell membranes and plasma lipoproteins.
- By reacting with the lipid peroxide radicals formed by peroxidation of polyunsaturated fatty acids, it gets converted to tocopheroxyl radical.
- The resultant radical (oxidized form) is relatively unreactive, and ultimately forms nonradical compounds.
- Commonly, the tocopheroxyl radical is reduced back to tocopherol by reaction with vitamin C from plasma. (See Figure-2)
Synergism between vitamin E, C, Selenium and Glutathione
- Ascorbate (Vitamin C) is essential for maintaining vitamin E in its reduced, active form.
- Ascorbate is oxidized to dehydroascorbate in plasma and that is recycled back to ascorbate by GSH as well as by several enzyme systems in erythrocytes, neutrophils, endothelial cells and hepatocytes (See figure-2).
- GSH itself gets oxidized during this process and is converted back to its reduced form by Glutathione reductase utilizing NADPH as the reductant.
- GSH is also required by Selenium containing Glutathione Peroxidase enzyme for decomposing H2O2.
- A synergism is observed between selenium and vitamin E .
- The synergism is related to the process of antioxidation, wherein tocopherols tend to prevent oxidative damage to polyunsaturated fats in cell membranes, selenium, as part of seleno-enzyme glutathione peroxidase,catalyzed the destruction of lipid hydro peroxides.
- This explains how these two nutrients play separate but interrelated role sin the cellular defense system against oxidative damage.
- In high concentration, the tocopheroxyl free radical can penetrate further into cells and, potentially, propagate a chain reaction.
- Therefore, vitamin E may also have pro-oxidant actions, especially at high concentrations. This explains the bleeding observed in vitamin E toxicity
Figure- 2- A synergism is observed between Vitamin E, C and G-SH dependent Glutathione peroxidase. Vitamin E during the process of breaking the lipid peroxidation chain gets converted to oxidized form (tocopheroxyl ) that is reconverted back to reduced form by ascorbic acid (vitamin C), which in turn itself gets converted to oxidized or dehydroascorbate form. The regeneration of reduced form takes place by reduced glutathione(G-SH), and that gets oxidized during this process and converted back to reduced form by glutathione reductase using NADPH as the hydrogen donor. Free radicals can also be quenched by Glutathione peroxidase, a Se containing Metalloenzymes, that requires the presence of reduced G-SH. Adequate activity of glutathione peroxidase reduces the amount of Vitamin E in diet and similarly adequate concentration of vitamin E in diet reduces the requirement of selenium.
2) Other functions of vitamin E
- It also has a (relatively poorly defined) role in cell signaling.
- Vitamin E also inhibits prostaglandin synthesis
- As an antioxidant, vitamin E plays a protective role in many organs and systems.
- Vitamin E is necessary for maintaining a healthy immune system, and it protects the thymus and circulating white blood cells from oxidative damage.
- Also, it may work synergistically with vitamin C in enhancing immune function.
- Recent research evidence indicates that the combined use of high doses of vitamin C and vitamin E helps prevent Alzheimer’s disease
- In eyes, vitamin E is needed for the development of the retina and protects against cataracts and macular degeneration.
- In experimental animals vitamin E deficiency causes infertility
Vitamin E Deficiency
- Dietary vitamin E deficiency is common in developing countries;
- deficiency among adults in developed countries is uncommon and is usually due to fat malabsorption.
- Absorption of vitamin E depends on normal pancreatic biliary function, biliary secretion, micelle formation, and penetration across intestinal membranes. Interference with any of these processes could result in a deficiency state.
- The main symptoms are hemolytic anemia and neurologic deficits.
- Vitamin E deficiency causes fragility of RBCs and degeneration of neurons, particularly peripheral axons and posterior column neurons.
- Low α-tocopherol level or low ratio of plasma α-tocopherol to plasma lipids
- Measuring the plasma α-tocopherol level is the most direct method of diagnosis.
- In adults, vitamin E deficiency is suggested if the α-tocopherol level is < 5 μg/mL (< 11.6 µmol/L).
- Because abnormal plasma lipid levels can affect vitamin E status, a low ratio of plasma α-tocopherol to plasma lipids (< 0.8 mg/g total lipid) is the most accurate indicator in adults with hyperlipidemia.
- If malabsorption causes clinically evident deficiency, α-tocopherol 15 to 25 mg/kg orally once/day should be given.
- However, larger doses given by injection are required to treat neuropathy during its early stages or to overcome the defect of absorption
Although premature neonates may require supplementation, human milk and commercial formulas have enough vitamin E for full-term neonates.
Vitamin E Toxicity
- Large amounts of vitamin E for months to years may not cause apparent harm.
- Occasionally, muscle weakness, fatigue, nausea, and diarrhea occur.
- The most significant risk is bleeding.
- However, bleeding is uncommon unless the dose is > 1000 mg/day.
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ATP synthase is embedded in the inner membrane, together with the respiratory chain complexes .
- Several subunits of the protein form a ball-like shape arranged around an axis known as F1, which projects into the matrix and contains the phosphorylation mechanism .
- F1 is attached to a membrane protein complex known as F0, which also consists of several protein subunits (Figure-1).
- F0 spans the membrane and forms a proton channel.
- The flow of protons through F0 causes it to rotate, driving the production of ATP in the F1 complex.
Figure-1-The enzyme complex consists of an F0 sub complex which is a disk of “C” protein sub units. Attached is a Υ subunit in the form of a “bent axle.” Protons passing through the disk of “C” units cause it and the attached Υ subunit to rotate. The Υ subunit fits inside the F1 sub complex of three α and three β subunits, which are fixed to the membrane and do not rotate.
- As the electrons are transferred, some electron energy is lost with each transfer.
- This energy is used to pump protons (H+) across the membrane from the matrix to the inner membrane space. A proton gradient is established (Figure-2)
- The higher negative charge in the matrix attracts the protons (H+) back from the intermembrane space to the matrix.
- The accumulation of protons in the intermembrane space drives protons into the matrix via diffusion.
- Most protons move back to the matrix through ATP synthase.
- ATP synthase uses the energy of the proton gradient to synthesize ATP from ADP + Pi
Figure-2- Showing electrochemical gradient across the inner mitochondrial membrane. The chemiosmotic theory, proposed by Peter Mitchell in 1961, postulates that the two processes are coupled by a proton gradient across the inner mitochondrial membrane so that the proton motive force caused by the electrochemical potential difference (negative on the matrix side) drives the mechanism of ATP synthesis.
P: O Ratio
- Defined as the number of inorganic phosphate molecules incorporated in to ATP for every atom of oxygen consumed.
- Oxidation of NADH yields 3 ATP molecules(P: O ratio 3, Latest concept 2.5)
- Oxidation of FADH2 yields 2 ATP molecules (P: O ratio 2, Latest concept 1.5)
Inhibition of Oxidative phosphorylation
Oxidative phosphorylation is susceptible to inhibition at all stages of the process.
A) Site specific inhibitors -Specific inhibitors of electron transport chain are (figure-3) for example-
1) Inhibitors of complex I– Rotenone and amobarbital block electron transfer in NADH-Q oxidoreductase and thereby prevent the utilization of NADH as a substrate. In contrast, electron flow resulting from the oxidation of succinate is unimpaired, because these electrons enter through QH2, beyond the block. The other inhibitors are Chlorpromazine, Piericidin A and Guanethidine
2) Inhibitors of Complex II
Malonate is a competitive inhibitor of Complex II. The other inhibitors are Carboxin and TTFA
Figure-3- Site specific inhibitors block the flow of electron at specific sites
3) Inhibitors of Complex III
BAL (British Anti Lewisite), Antimycin A, Naphthoquinone and hypoglycemic agents interfere with electron flow from cytochrome b in Q-cytochrome c oxidoreductase.
4) Inhibitors of Complex IV
Furthermore, electron flow in cytochrome c oxidase can be blocked by hydrogen sulphide (H2S), cyanide (CN-),azide (N3 -), and carbon monoxide (CO). Cyanide and azide react with the ferric form of heme a 3, whereas carbon monoxide inhibits the ferrous form. Inhibition of the electron-transport chain also inhibits ATP synthesis because the proton-motive force can no longer be generated.
B) Inhibitors of ATP synthase complex
Oligomycin and dicyclohexyl carbodiimide(DCCD) prevent the influx of protons through ATP synthase. If actively respiring mitochondria are exposed to an inhibitor of ATP synthase, the electron transport chain ceases to operate. Indeed, this observation clearly illustrates that electron transport and ATP synthesis are normally tightly coupled.
C) Inhibition of ATP-ADP translocase
ATP-ADP translocase is specifically inhibited by very low concentrations of Atractyloside (a plant glycoside) or Bongregate (an antibiotic from a mold).Unavailability of ADP also inhibits the process of ATP formation. This is because oxidation and phosphorylation are tightly coupled; ie, oxidation cannot proceed via the respiratory chain without concomitant phosphorylation of ADP.
Figure-4- ADP moves in to the mitochondrial matrix and newly synthesized ATP is transported out in to the cytoplasm to be used for cellular processes
D) Uncouplers of Oxidative phosphorylation
The tight coupling of electron transport and phosphorylation in mitochondria can be disrupted (uncoupled) by agents called uncouplers of oxidative phosphorylation. These substances carry protons across the inner mitochondrial membrane. In the presence of these uncouplers, electron transport from NADH to O2 proceeds in a normal fashion, but ATP is not formed by mitochondrial ATP synthase because the proton-motive force across the inner mitochondrial membrane is dissipated. This loss of respiratory control leads to increased oxygen consumption and oxidation of NADH. Indeed, in the accidental ingestion of uncouplers, large amounts of metabolic fuels are consumed, but no energy is stored as ATP. Rather, energy is released as heat.
- Long chain fatty acids
- Brown Adipose tissue-Thermogenin (or the uncoupling protein) is a physiological uncoupler found in brown adipose tissue that functions to generate body heat, particularly for the newborn and during hibernation in animals
- Calcium ions.
The regulated uncoupling of oxidative phosphorylation is a biologically useful means of generating heat. The uncoupling of oxidative phosphorylation is a means of generating heat to maintain body temperature in hibernating animals, in some newborn animals (including human beings), and in mammals adapted to cold. Brown adipose tissue, which is very rich in mitochondria (often referred to as brown fat mitochondria), is specialized for this process of non shivering thermogenesis. The inner mitochondrial membrane of these mitochondria contains a large amount of uncoupling protein (UCP), here UCP-1, or thermogenin,a dimer of 33-kd subunits that resembles ATP-ADP translocase. UCP-1 forms a pathway for the flow of protons from the cytosol to the matrix. In essence, UCP-1 generates heat by short-circuiting the mitochondrial proton battery(figure-5).
Thus, the manner in which biologic oxidative processes allow the free energy resulting from the oxidation of foodstuffs to become available and to be captured is stepwise, efficient, and controlled—rather than explosive, inefficient, and uncontrolled, as in many non biologic processes. The remaining free energy that is not captured as high-energy phosphate is liberated as heat. This need not be considered “wasted,” since it ensures that the respiratory system as a whole is sufficiently exergonic, allowing continuous unidirectional flow and constant provision of ATP. It also contributes to maintenance of body temperature.
Figure-5- UCP-1 forms a pathway for the flow of protons from the cytosol to the matrix. The proton gradient is dissipated, oxidation proceeds without phosphorylation.
These compounds are toxic in vivo, causing respiration to become uncontrolled, since the rate is no longer limited by the concentration of ADP or Pi.
- 2, 4- dinitrocresol
- CCCP( chloro carbonyl cyanide phenyl hydrazone)
- High dose of Aspirin
- The antibiotic Oligomycin completely blocks oxidation and phosphorylation by blocking the flow of protons through ATP synthase.
EXPLANATION: In order to understand this concept, take the examples of D-Glucose, D-Mannose and D-Galactose .
As can be seen in the image, D-Mannose differs from D-Glucose ONLY at the 2nd Carbon atom. Hence, it is a C-2 epimer of D-Glucose.
D-Galactose, on the other hand, differs from D-Glucose ONLY at the 4th Carbon atom. Hence, it is a C-4 epimer of D-Glucose.
D-Mannose and D-Galactose. Are they epimers of each other? Are they isomers of each other?
These are not epimers since they differ in configuration around two carbon atoms, they are simple isomers.
For complete description of Isomers, anomers and epimers follow the link