ANTIOXIDANTS IN OPHTHALMOLOGY EVOLVING CONCEPTS
ABSTRACT
As one ophthalmologist had mentioned way back a decade ago: ‘advocating antioxidants is like shooting in the dark'. It is no more now. Today the pathophysiology of free radical mediated eye degenerative diseases like age-related macular degeneration and cataract are well established, and so is the definite role of antioxidants, particularly carotenoids. As far as eye is concerned, lutein and zeaxanthin have a vital role to play, and these two carotenoids are a must for the eye to be well protected form developing macular degeneration as well as cataract. In addition, lycopene, another carotenoids has a special place in eye defense since it is the best quencher of singlet oxygen - a reactive oxygen species which causes havoc particularly in the eye.
HOW FREE RADICALS ARE FORMED?
The various tissues in the human body are formed by innumerable molecules. Each molecule consists of two or more atoms joined together by chemical bonds. An atom, the smallest particle of an element, consists of a core which contains positively charged protons (or positrons) as well as neutral neutrons. In the orbit of each atom (referred to as orbital) are present the electrons (or negatrons). Each orbital can accommodate a maximum of two electrons both of which spin in opposite directions.
Most molecules are non-radical since they contain a paired set of electrons. But oxygen is always electronegative. As a consequence, it pulls electrons away from other atoms (including oxygen itself) and renders these as free radicals.
Oxygen-derived free radicals have a lifespan of only a few microseconds. Their concentration at any single site is miniscule. However the danger lies in their ability to combine with another nonradical to render the latter as free radical. Normally, bonds don't split in a way that leaves a molecule with an odd, unpaired electron. But when weak bonds split, free radicals are formed. Free radicals are very unstable and react quickly with other compounds, trying to capture the needed electron to gain stability.
Generally, free radicals attack the nearest stable molecule, thereby "stealing" its electron. When the "attacked" molecule loses its electron, it becomes a free radical itself. This leads to a process of chain reaction. Once such a process has started, it can cascade, finally resulting in the disruption of a living cell.
Radicals can react with other molecules in a number of ways. If two radicals meet, they can combine their unpaired electrons symbolized byand join to form a covalent bond (a shared pair of electrons). The hydrogen atom, with one unpaired electron, is a radical and two atoms of hydrogen easily combine to form the diatomic hydrogen molecule. Radicals react with non radicals in several ways. A radical may donate its unpaired electron to a non-radical (a reducing radical) or it might take an electron from another molecule in order to form a pair (an oxidizing radical). A radical may also join onto a non radical. Whichever of these three types of reaction occurs, the non radical species becomes a radical. A feature of the reactions of free radicals with non radicals is that they tend to proceed as chain reactions, where one radical begets another.
SOURCES OF FREE RADICALS
Some free radicals arise normally during metabolism. Sometimes the body's cells or its immune system purposefully create them to neutralize viruses and bacteria. However, environmental factors such as pollution, radiation, cigarette smoke and herbicides can also generate free radicals. Free radicals causing structural damage (to proteins) resulting in aging changes such as cataract and ARMD.
An adult utilizes 3.5 ml oxygen per kg body weight per minute. Assuming a body weight of 70 kgs, this works out to 352.8 liters per day. Even if 1% of oxygen is converted to free radicals, this amounts to 1.72 kg of free oxygen radicals per year!
Some free radicals well studied free radicals are:
¨ SUPEROXIDE ANION (O2.)
¨ HYDROXYL RADICAL (OH.)
It is important to note that free radicals such as hydroxyl radical differ from hydroxyl ions in their content of electrons.
REACTIVE OXYGEN SPECIES
These are partially reduced oxygen species which do not contain any unpaired electron.Examples of reactive oxygen species are:
¨ HYDROGEN PEROXIDE (H2O2)
¨ HYDROPEROXY RADICAL (HOO-)
¨ HYPOCHLOROUS ACID RADICAL (HOCl)
Under certain conditions reactive oxygen species have potential to enter free radical reactions to form the more toxic free radicals. Another reactive oxygen species, which is not a free radical, is singlet oxygen (O). In this, a rearrangement of electrons has occurred which allows it to react faster with biological molecules - as compared to ‘normal' oxygen.
ANTIOXIDANTS
HOW ANTIOXIDANTS WORK?
Antioxidants serve as natural protectors in the body, mopping up free radicals and reactive oxygen species, which are potentially damaging. Antioxidants protect the tissues in 4 ways:
¨ Physically separating the free radicals / reactive oxygen species from the susceptible molecules of the human body.
¨ Providing molecules which effectively compete for oxygen.
¨ Rapidly repair the damage caused by free radicals / reactive oxygen species.
¨ Lyse the free radicals / reactive oxygen species and rapidly remove these.
CLASSIFICATION OF ANTIOXIDANTS
ANTIOXIDANT ENZYMES
¨ Superoxide dismutase
¨ Catalase
¨ Glutathione peroxidase
PREVENTIVE ANTIOXIDANTS
¨ Ceruloplasmin
¨ Transferrin
¨ Albumin
CHAIN-BREAKING ANTIOXIDANTS
Water-soluble*
¨ Uric acid (200-400 mmol/L)
¨ Ascorbate (25-100 mmol/L)
¨ Thiols (400-500 mmol/L)
¨ Bilirubin (10-20 mmol/L)
¨ Flavanoids
r Fat-soluble*
¨ Tocopherols (20-30 mmol/L)
¨ Ubiquinol-10 (
¨ Beta-carotene (1-2 mmol/L)
¨ Estrogens
* optimal blood level given in brackets
The most important antioxidants are three vitamins and three minerals.
ANTIOXIDANT VITAMINS
¨ CAROTENOIDS
¨ VITAMIN E
¨ VITAMIN C
ANTIOXIDANT MINERALS
¨ SELENIUM
¨ ZINC
¨ MANGANESE
¨ COPPER
carotenoids
Carotenoids of human importance are:
¨ Lutein
¨ Zeaxanthin
¨ Lycopene
¨ Alpha carotene
¨ Beta cryptoxanthin
As far as the eye is concerned, Lutein and zeaxanthin are exclusively concentrated in the macula, lens and iris. The retina and choroids additionally contain lycopene, alpha- and beta carotene. In the ciliary body, all the carotenoids taken in foodstuff or as dietary supplement get accumulated.
VITAMIN E
Being a fat soluble vitamin, alpha tocopherol is abundant in all cell membranes as well as in lipoproteins. In the eye, vitamin E is present in retina and choroids, and balance in iris and ciliary body. It is important in protection of rods and cones in retina, and also for preventing free radical damage to lens. Vitamin E acts synergistically with vitamin C, beta carotene and selenium for better functioning of glutathione.
Vitamin C is protective for the cytoplasm & is also most important for plasma defense. It also occurs in certain cells like muscle, adrenals and eye. Vitamin C has the capacity to regenerate vitamin E. It is more significant in combating free radicals formed due to pollution and cigarette smoke. Vitamin C especially concentrates in ocular tissues and is the first antioxidant to tackle free radicals.
ZINC, MANGANESE & COPPER
Zinc (Zn), manganese (Mn) and copper (Cu) are constituents of superoxide dismutase (SOD) antioxidant enzyme. SOD is widely distributed in tissues as well as fluid compartments. CuZnSOD is present in cytoplasm and nucleus, MnSOD in operates mitochondria whilst CuSOD is most distributed in plasma. SOD attacks free radicals like hydroxyl radical to convert these into hydrogen peroxide.
Besides, Zn serves an important structural role, whilst Cu is necessary for functioning of another antioxidant enzyme called as catalases. Hydrogen peroxide is converted by catalases into harmless water and molecular oxygen.
In the retina, SOD plays an important role by scavenging free radicals to prevent the oxidative damage which plays a role in the development of drusen, an early sign of ARMD. Catalases, on the other hand, are vital for lens protection.
SELENIUM
Selenium (Se) is the most important dictator of glutathione peroxidase activity. Glutathione peroxidase is concentrated in various tissues, besides blood and synovial fluid. In tisues, it operates in the cytoplasm and mitochondria principally. Like catalases, glutathione peroxidase breaks down hydrogen peroxide, besides reducing lipid peroxidation like vitamin E and beta carotene.
Glutathione peroxidase and related enzymes in the retina, plus the precursor amino acids (N-acetylcysteine, L-glycine, and glutamine and selenium) are protective against damage to human retinal pigment epithelium cells. Glutathione peroxidase prevents free radical-induced apoptosis (cell suicide) and helps prevent or treat ARMD.
FOCUS ON CAROTENOIDS IN ARMD
ARMD - PATHOPHYSIOLOGY
The light must pass the macular pigment, which contains abundance of zeaxanthin and lutein before striking the photoreceptors. If any damage to the rods and cones is to be prevented the short wave length of light rays (
¨ 5-286 nm wavelength (ultraviolet C rays): filtered by the earth's ozone layer.
¨ 286-320 nm wavelength (ultraviolet B rays): filtered by cornea.
¨ 320-400 nm wavelength (ultraviolet C rays): filtered by lens.
¨ 400-500 nm wavelength (visible blue light): filtered by lutein / zeaxanthin in macula.
The light entering the retina is between the wavelengths of 400 to 700 nm. The eye would be in perfect focus for daylight only at 560 nm, and even at night 500 nm wavelength of light is optimal for functioning of rods. Hence, filtering out 400-500 nm wavelength of light prevents damage to macula without affecting vision.
Thus macular pigments represent a significant filtering element and hence protect against the light–initiated cumulative oxidative damage. The macular pigment also removes much of the blurry, short wave blue and blue-green light that results from the eye's chromatic aberration. Apart from this the earth's atmosphere through which we view objects almost always contain small-suspended particles, which scatters short wave length light more than other wavelengths and results in a bluish veiling luminance.
The eye and skin are the only structures which have dual exposure to oxygen and light. In presence of blue light (400-500 nm wavelength) the oxygen will be split into singlet oxygen which is one of the most deadly reactive oxygen species as far as the eye is concerned. The blue light has potential to split molecular oxygen due to the high energy contained in it.
The singlet oxygen and other free radicals formed inside the eye initiate lipid peroxidation of photoreceptors. The polyunsaturated fatty acids in the outer membrane of rods and cones are attacked by free radicals and singlet oxygen species to result in damage of these photoreceptors. As a consequence, there is accumulation of lipofuscin by retinal pigment epithelium which then contributes in druse formation.
ARMD - MEDICAL MANAGEMENT
The damage to macula and formation of druse can be prevented by filtering out the damaging blue light of the visible spectrum. This is possible by the macula, if its content of lutein and zeaxanthin are adequate. The additional available of lycopene in adequate amounts is of paramount importance in tackling the singlet oxygen single this carotenoids is the best antioxidant known for quenching this reactive oxygen species. In addition, glutathione peroxidase and SOD too have been shown to have preventive benefit in ARMD.
CATARACT - INTRODUCTION
Cataract is a multifactorial disease. Oxidative stress together with weakened antioxidant defense mechanism is attributed to the changes observed in human diabetic cataract. Oxidative damage to the lens has been recognized as a primary event in the pathogenesis of many forms of cataract. Consistent with this view, epidemiological reports have identified factors related to oxidative process that both increase (eg smoking and light exposure) and decrease (eg antioxidant intake) cataract risk.
Epidemiological studies provide evidence that nutritional antioxidants slow down the progression of cataract.
CATARACT - PATHOPHYSIOLOGY
Oxidative stress is high in the eye due to ultraviolet rays which promote liberation of free radicals and singlet oxygen. The epidemiological evidence to support the possibility that lutein and zeaxanthin have an important role in reducing the risk of cataract is somewhat consistent, and justifies the belief in free radical & reactive oxygen species mediated damage to the lens.
Few of the recent studies have stressed the significance of vitamin C, E and selenium in the etiology of cataract. Role of vitamin E has been more specifically stressed by several workers. Low blood levels of vitamin E are associated with approximately twice the risk of both cortical and nuclear cataracts, compared to median or high levels. Smokers are 2.6 times likely to develop posterior subcapsular cataracts more than nonsmokers. Patients with senile cataracts were found to have significantly lower blood and intraocular levels of the mineral selenium than control.
CATARACT - MEDICAL MANAGEMENT
Lower prevalence of nuclear cataract in women or men was associated with intake of lutein and zeaxanthin in high doses. Furthermore, in prospective cohort studies it was noted that people who consumed diet rich in lutein and zeaxanthin, had 20-25 percent lower risk of cataract extraction and 70 percent lower risk of cataract extraction under the age of 65 years.
Experimental study in human lens epithelial cells (HLEC) in culture was evaluated and it was concluded that addition of lycopene had a protective effect to prevent vacuolization of epithelial cells. It was observed that there was as positive effect of retardation of lens opacities due to lutein and zeaxanthin in the aging lenses.
ANTIOXIDANTS IN RETINITS PIGMENTOSA
There is possibility that lutein may slow degeneration of vision in retinitis pigmentosa, a heterogeneous group of slow retinal degenerations. However, only preliminary data in a very small number of patients has been published in which lutein slowed vision loss associated with retinitis pigmentosa in one.
CONCLUSION
The overwhelming body of evidence points to significant beneficial effects of nutritional supplementation for most degenerative eye conditions. Important to remember is that most of the above studies used blood levels and food intakes associated with a normal diet. Taking supplements, specifically containing zeaxanthin, lutein and lycopene in adequate doses, which are theorized to provide protection to macula and lens with adequate doses, may have a much more protective effect than dietary levels alone. With so little risk, and the other potential health benefits from taking nutritional supplements, it would certainly seem prudent to try them, especially for macular degeneration where there are no real options.
Once the damage is done it cannot be reversed (except to a small degree), so prevention and early intervention is essential, especially if we have a family history of the disease. Of course, it's important to slow further progression at any stage of development. Prevention of lens and macula from the ultraviolet rays and hazard of smoking, however, needs to be over stressed.
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