Beta-Carotene as an Antioxidant

IINTRODUCTION

        Antioxidants catches  interests among researchers for its many contributory factor in maintaining a healthy life.  It is said to be a indispensable against many diseases.  It prevents the process of oxidation which destroys the inner and outer cell wall by preventing the formation of peroxy free radicals thus maintaining the   the integrity of epithelial tissues.  Antioxidant vitamins, Vitamin A, Vitamin C, and b-carotene, plays an important role in preventing these reactions.

        The important determinant of the immune cell function is its oxidant-antioxidant balance, not only maintaining integrity and functionality of membrane lipids, cellular proteins, and nucleic acids, but for control of signal transduction and gene expression in immune cells (Meydani et. al., Wu et. al., Santos et. al. and Hayek et. al., 1995.).  Oxidative stress reflects a shift toward the prooxidants in the oxidant-antioxidant balance that characterizes the normal aerobic steady state, leading to damage products.  Antioxidant functions are associated with lowered DNA damage, diminished lipid peroxidation, or inhibited malignant transformation in vitro ( Sies and Stahl, 1995.).    b-carotene is known to be among the antioxidants,  and the focus of this review article.

PROPERTIES AND STRUCTURE

             beta-carotene is one among the major carotenoids; which are naturally occurring compounds in plants and in human plasma and tissues but only in limited number.  Carotenoids are known for its positive effect on health as antioxidants. b-carotene is a precursor of Vitamin A that’s why it is also known as a provitamin A.  Among the precursors of Vitamin A, it is the most prominent, has the highest activity yielding two moles of retinol.  It is a yellowish-orange pigment that plays a major role in keeping our bodies healthy by preventing a wide a variety of diseases, from heart disease, cancer, cataracts, and some skin disorders. It has two b-ionone ring and no alcohol groups such that of Vitamin A, which is a complex primary  alcohol.   

            It is currently  one of the few yellow pigments approved by the Food and Drug Administration for  the artificial coloring of food.  It is used extensively in gelatin, margarine, soft drinks, cake mixes, and cereal products.

            b-carotene as precursors, or provitamin A is soluble in fat or fat solvents.  It is apolar hydrophobic molecules which is isoprene derivative.  To be absorbed efficiently, It requires a normal fat absorption.  It is stable to heat and less likely to be lost during cooking and food processing.  It is usually excreted in the feces and not in the urine since it is insoluble in water.  Because intakes in excess of need are stored in the body, there is as much concern about toxicity from the ingestion of large amounts as there is about a deficient intake or failure to absorb this nutrient (Guthrie, 1979)

SOURCES

            Reference:  Vitamin Nutrition Information Service

ABSORPTION, TRANSPORT, METABOLISM AND EXCRETION

            Preformed Vitamin A in food appears in combination with the fatty acid palmitic acid as retinyl palmitate.  Digestion of Vitamin A accompanies that of lipids.

1.      Substances that aid absorption

a.      Fat, an effective carrier

Vitamin A, b-carotene, and other provitamin A are lipid-soluble (fat-soluble) vitamins, which are apolar hydrophobic molecules.  They can only be absorbed efficiently when normal fat absorption occurs and factors that promote or depress the absorption of fat also enhance or depress Vitamin A absorption respectively.  Retinyl esters are dissolved in the fat diet.

b.      Bile salts act as vehicles of transport

The first step in digestion is emulsification.  The fat globules is broken into small sizes to facilitate action of the water-soluble enzymes on the globule surfaces.  Agitation in the stomach with the products of stomach digestion and then into the duodenum under the influence of bile, liver secretion which does not contain digestive enzymes but does contain bile salts and lecithin which are extremely important in the emulsification process.  Both contains polar part and non-polar part, wherein the fat-soluble portion dissolves in the globule surface while the polar portion projects outward into the surrounding watery fluids which greatly diminishes the interfacial tension and increases the surface area.  This would now facilitate the breaking up of the fat globules into very minute particles known as micelles.  Bile salt micelles would now transport the retinal to the brush borders of the intestinal epithelium.

c.      Pancreatic lipase

Pancreatic lipase is present in pancreatic juices.  They are water-soluble compounds that attack the fat globules on their surfaces.  Its action on the emulsified fat would produce the corresponding retinal.

2.      Carotene conversion

b-carotenes ingested are/may be oxidatively cleaved by b-carotene dioxygenase.  The presence of bile salts enhances the cleavage which utilizes molecular oxygen forming two molecules of retinaldehyde (retinal).  It is absorbed intact in the presence of bile salts from the intestine after having been released from the plant digestion.  They are converted in the intestinal mucosa to retinol through reduction by retinaldehyde reductase utilizing NADPH.  Conversion of carotene to retinol is enhanced by both thyroxin and Vitamin E.  Most of the retinol is esterified with saturated fatty acids and enters the lymphatic circulation being incorporated into lymph chylomicrons, which eventually enters the regular blood system to be carried to the liver.  They are taken up by the liver in the form of chylomicron remnants together with their content of retinol.  A small part (less than 10%) of the retinal is oxidized to retinoic acid.  It attaches to the protein albumin to increase its solubility in blood and enters the general circulation through the portal vein.

The conversion of  carotene to Vitamin A, which involves splitting the carotene molecule into two parts, is not complete, with some unchanged carotene being absorbed via the lymph entering the circulation, where normal carotene levels approximate 150 IU/dl of blood.  Blood carotene levels reflect dietary carotene and not the storage of Vitamin A; therefore in decreased intakes a rather rapid drop in carotene values occurs.  Unconverted carotene is stored in the fat depots and adrenals rather than in the liver.

3.    Storage

Most of the retinol and carotene absorbed goes to the liver where it is stored.  It is stored as an ester in the lipocytes (perisinusoidal stellate cells), Probably as a lipoglycoprotein complex.  To be transported to tissues, retinol bound to aporetinol-binding protein (RBP).  The resulting holo-RBP is processed in the golgi apparatus, secreted in the plasma and taken up into tissues via cell surface receptors.  Retinoic acid binds to albumin to be transported in the plasma.  Cellular retinol-binding protein (CRBP) binds retinol once it reaches the extrahepatic cells.  Unconverted carotene is stored in the fat depots and adrenals rather than in the liver.  A sufficient amount can be hoarded or stored to last an estimated several months in humans.

4.      Excretion and Secretion

It is stated earlier that neither Vitamin A nor provitamin A or the b-carotene is excreted in the urine but in the feces because of its property as lipid-soluble (fat-soluble) vitamin making it  insoluble to water.  In the feces, it is probably as an unabsorbed portion.  Bacteria destroy the unused material and any excess of the normal storage capacity or requirement.  Both carotene and Vitamin A are secreted by the mammary gland. 

MODE OF ACTION

           b-carotene help protect us from heart disease, cancer, cataracts and some skin disorders and also known to maintain our immune system.  The antioxidant activity of  b-carotene relates to its anticarcinogenic activity which attracted the principal interest of researchers.

 Beating Cancer

            Beta-carotene physically quenches the singlet oxygen (¹O₂) thus preventing damage and protecting our biological systems.  It is also capable of inhibiting free radical (chemically unstable, naturally occurring molecules that can damage inner and outer cell walls) reactions.  It acts as radical-scavenging antioxidants and suppresses lipid peroxidation by addition to the double bond to give a conjugated polyene, carbon-centered radical and not by hydrogen atom donation.  It scavenge radicals by inhibiting chain initiation and break chain propagation.  Antioxidant property of  Beta-carotene is higher in lower oxygen pressures.  The carbon-centered radical is resonance-stabilized while the peroxyl radical formed from it by interaction with the oxygen molecule is not stable but capable of inducing oxidation.  b-carotene together with Vitamin E, which is  b-carotene sparer, may exert a synergistic effect on different portions of the membrane and LDL-a-tocopherol at the surface and   b-carotene in the interior-and a-tococpherol scavenges the     b-carotene-derived peroxyl radicals (Niki et. al., Noguchi et. al., Tsuchihashi et. al., and Gotoh et al., 1995).  This would explain the the physiologic function of Vitamin E as to spare or protect Vitamin A and carotenes from oxidative destruction.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                

            According to the studies done by Meydani, Wu, Santos and Hayek; the actions of antioxidants in maintaining immune response, b-carotene supplementation variable doses (0, 15, 30, 45, and 60 mg/d) in elderly persons resulted to a significant increase in the percentage of T helper (CD4⁺) cells, natural killer cells (CD16⁺), and cells expressing IL-2 receptor (CD25⁺) and transferring receptor in subjects taking 30, 45, and 60 mg b-carotene/d compared with placebo.  While it did not have an significant effect on total T cells, nor on T suppressor-cytotoxic (CD8⁺) cells.  An efficient consumption of antioxidant nutrients, which exhibited by b-carotene and other vitamins as well (vit A and vit C), and/or efficient concentration of antioxidants contributes to the enhancement of the immune response in aged persons.    Beta-carotene helps the body in inhibiting precancerous changes by promoting  the release of killer cells, lymphocytes, and monocytes.

            Enhancing the body’s immune system and fighting damage caused by free radicals helps in combating cancer.

  Protecting sensitive skin

Beta-carotene also prevents skin disorders by protecting it from photo-oxidative and radical-induced damage of ultraviolet radiation.  Ultraviolet light from the sun prevents disease-fighting cells from working thus impairing the immune systems of light-sensitive individuals.  It breaks down vital nutrients in their blood.  This could be suppressed by taking a daily b-carotene supplement of 30 mg.

Preventing heart disease

            Oxidation reaction in our cells causes heart disease.    Beta-carotene, an antioxidant may slow the rate of the destructive oxidation process.  The low density lipids (LDL)  which are bad cholesterols, are prevented by combining with oxygen molecules.  The LDL particles when combined with oxygen, they would likely settle on artery walls and cause the blockages that leads to stroke and heart disease (Carnell, 1992).

Cutting cataract risk

            Cataract is a common eye abnormality that occurs maily in older people.  It is a cloudy or opaque area/s in the lens which obscures light transmission greatly that it seriously impairs vision.  Antioxidants prevents the damage of the lens that can lead to cataract.  A study was established that the presence of high blood levels of antioxidants would less likely develop a cataract (Carnell, 1992). 

 SAFETY FACTORS

            A lot of   b-carotene can be consumed safely, not like Vitamin A which can be toxic in large amounts.  Studies show that about 150 to 180 milligrams of b-carotene can be ingested a day with no ill effects.  But these equivalents can’t be reached by eating produce alone.  b-carotene at high levels may turn your skin yellow termed as carotenemia.  When the amount of b-carotene in the diet decreases, skin color returns to normal.

CONCLUSION

                b-carotene is one among the major carotenoids; which are naturally occurring compounds in plants and in human plasma and tissues but only in limited number.  Carotenoids are known for its positive effect on health as antioxidants.  It is a yellowish-orange pigment that plays a major role in keeping our bodies healthy by preventing a wide a variety of diseases, from heart disease, cancer, cataracts, and some skin disorders.  The antioxidant activity of  b-carotene relates to its anticarcinogenic activity which attracted the principal interest of researchers.

                 Beta-carotene physically quenches the singlet oxygen (¹O₂) thus preventing damage and protecting our biological systems.  It is also capable of inhibiting free radical (chemically unstable, naturally occurring molecules that can damage inner and outer cell walls) reactions.   Beta-carotene helps the body in inhibiting precancerous changes by promoting  the release of killer cells, lymphocytes, and monocytes.  Enhancing the body’s immune system and fighting damage caused by free radicals helps in combating cancer.  Beta-carotene, an antioxidant may slow the rate of the destructive oxidation process that may cause heart diseases and stroke.  Beta-carotene also prevents skin disorders by protecting it from photo-oxidative and radical-induced damage of ultraviolet radiation.  A study was established that the presence of high blood levels of antioxidants would less likely develop a cataract.

                      Beta-carotene do has variety of functions that posts a positive effect on our helth.  It is present in all variety of dark green leafy vegetables and fruits that contains the yellow pigment carotene which makes it easily available aside from the Beta-carotene supplementations.

REFERENCES

        Carnell, Marilyn. Beta-carotane: A Nugget of Nutritional Gold.  Better Homes and Gardens, October, 1992.

        Guthrie, Helen A. Introductory Nutrition.  Missouri: C.V. Mosby Company, 1979

        Guyton, Arthur C., Hall, John E. Textbook of Medical Physiology 10th Edition. WB Saunders Company, 2000

        Murray, Robert K., Granner, Daryl K., Mayes, Peter A.,Rodwell, Victor W. Harper's Biochemistry 25th Edition.

Connecticut: Appleton & Lange, 2000.

         Meydani, Simin Nikbin,. Wu, Dayong,. Santos, Michelle S and Hayek, Michael G. Antioxidants and immune response in aged persons: overview of present evidence, American Journal of Clinical Nutrition, USA: December, 1995.

         Niki, Etsuo , Noguchi, Noriko , Tsuchihashi, Hideyasu and Gotoh, Naohiro. Interaction among vitamin C, vitamin E, and Beta-carotene,  American Journal of Clinical Nutrition, USA: December, 1995.

        Sies, Helmut and Stahl, Wilhelm. Vitamin E and C, Beta-carotene, and other carotenoids as antioxidants. American Journal of Clinical Nutrition, USA: December, 1995.