In vitro damage to rat lens by xanthine-xanthine oxidase: protection by ascorbate

Role of ultraviolet irradiation and oxidative stress in cataract formation-medical prevention by nutritional antioxidants and metabolic agonists

PURPOSE

Cataract is a significant cause of visual disability with relatively high incidence. It has been proposed that such high incidence is related to oxidative stress induced by continued intraocular penetration of light and consequent photochemical generation of reactive oxygen species, such as superoxide and singlet oxygen and their derivatization to other oxidants, such as hydrogen peroxide and hydroxyl radical. The latter two can also interact to generate singlet oxygen by Haber-Weiss reaction. It has been proposed that in addition to the endogenous enzymatic antioxidant enzymes, the process can be inhibited by many nutritional and metabolic oxyradical scavengers, such as ascorbate, vitamin E, pyruvate, and xanthine alkaloids, such as caffeine.

METHODS

Initial verification of the hypothesis has been done primarily by rat and mouse lens organ culture studies under ambient as well as ultraviolet (UV) light irradiation and determining the effect of such irradiation on its physiology in terms of its efficiency of active membrane transport activity and the levels of certain metabolites such as glutathione and adenosine triphosphate as well as in terms of apoptotic cell death. In vivo studies on the possible prevention of oxidative stress and cataract formation have been conducted by administering pyruvate and caffeine orally in drinking water and by their topical application using diabetic and galactosemic animal models.

RESULTS

Photosensitized damage to lens caused by exposure to visible light and UVA has been found to be significantly prevented by ascorbate and pyruvate. Caffeine has been found be effective against UVA and UVB. Oral or topical application of pyruvate has been found to inhibit the formation of cataracts induced by diabetes and galactosemia. Caffeine has also been found to inhibit cataract induced by sodium selenite and high levels of galactose. Studies with diabetes are in progress.

CONCLUSIONS

Various in vitro and in vivo studies summarized in this review strongly support the hypothesis that light penetration into the eye is a significant contributory factor in the genesis of cataracts. The major effect is through photochemical generation of reactive oxygen species and consequent oxidative stress to the tissue. The results demonstrate that this can be averted by the use of various antioxidants administered preferably by topical route. That they will be so effective is strongly suggested by the effectiveness of pyruvate and caffeine administered topically to diabetic and galactosemic animals.

Oxidative damage to mouse lens in culture. Protective effect of pyruvate

Studies have been conducted to examine the feasibility of preventing oxyradical-dependent oxidative stress to mouse lens in culture, using pyruvate as an antioxidant. The extent of oxidative damage to the tissue was assessed by measurement of the status of Na(+)-K(+) ATPase dependent active transport of rubidium 86Rb(+). The tissue levels of adenosine triphosphate (ATP), glutathione (GSH), malonaldehyde (MDA) and catalase were also determined. While the measurement of 86Rb(+) uptake provides an assessment of the integrity of the primary active transport system, measurement of the other components reflects the status of intracellular oxidative stress. ATP measurement also reflected on the overall status of metabolic integrity. Incubation of the lens with xanthine (XA)/xanthine oxidase (XO) system had an adverse effect on all these parameters. Incorporation of pyruvate was strikingly protective. The protective effect of pyruvate is apparently due to its ability to scavenge ROS generated in the medium with the possibility of its action on tissue metabolism as well. The findings are hence considered useful for further studies on the prevention of oxidative stress to tissues by exogenous supplementation with pyruvate, specially the human lens where the biochemistry of its antioxidant mechanisms is similar to the mouse lens, contrary to the rat lens.

Quercetin inhibits hydrogen peroxide-induced oxidation of the rat lens

Cataract results from oxidative damage to the lens. The mechanism involves disruption of the redox system, membrane damage, proteolysis, protein aggregation and a loss of lens transparency. Diet has a significant impact on cataract development, but the individual dietary components responsible for this effect are not known. We show that low micromolar concentrations of the naturally-occurring flavonoid, quercetin, inhibit cataractogenesis in a rat lens organ cultured model exposed to the endogenous oxidant hydrogen peroxide. Other phenolic antioxidants, (+)epicatechin and chlorogenic acid, are much less effective. Quercetin was active both when incubated in the culture medium together with hydrogen peroxide, and was also active when the lenses were pre-treated with quercetin prior to oxidative insult. Quercetin protected the lens from calcium and sodium influx, which are early events leading to lens opacity, and this implies that the non-selective cation channel is protected by this phenolic. It did not, however, protect against formation of oxidized glutathione resulting from H2O2 treatment. The results demonstrate that quercetin helps to maintain lens transparency after an oxidative insult. The lens organ culture/hydrogen peroxide (LOCH) model is also suitable for examining the effect of other dietary antioxidants.

Oxyradical scavenging effects of dehydroascorbate

Dehydroascorbate (DHA) has been shown to possess vitamin C like activities as well as to protect the lens against oxidative stress and cataract formation. The results presented here suggest that some of the beneficial effects of DHA can be attributed to its property of undergoing peroxidative decarboxylation and of O2-. scavenging. Incubation of 1-14C-DHA with peroxide at physiological pH has been found to liberate 14CO2 in quantitative yields, with recovery of 79-94%. The recovery increases with the increase in the amount of DHA used. Its O2-. scavenging activity was apparent by inhibition of O2-. dependent reduction of ferricytochrome c and nitroblue tetrazolium. The authenticity of the latter mechanism was proved by inhibition of the O2-. dependent reactions also by superoxide dismutase.

Prevention of cataracts by nutritional and metabolic antioxidants

Among aging disabilities, the one associated with the progressive decline of vision is functionally most disadvantageous. Cataracts are one of the more common causes of such visual disability. Several predisposing factors have been identified in the genesis of this disease. While it is perhaps a multifactorial process, significant developments have taken place in recent years suggesting that oxygen radicals are involved in the development of this aging manifestation. Antioxidant enzymes, such as catalase and superoxide dismutase, have been demonstrated to protect the lens cell membrane from oxidative stress as reflected by the prevention of the Na(+)-K(+)-ATPase-dependent pump deterioration due to oxyradical-dependent oxidation of its proteins and lipids. From the nutritional point of view, antioxidants such as ascorbate and vitamin E also offer significant protection to the lens against damage due to oxidative stress. Evidence regarding the protective effect of these nutrients has been based on lens organ culture studies in the presence of active oxygen, generated photochemically as well as enzymatically. The experiment involving photochemical environs simulate the status of the eye during the photopic vision. In vivo, the effectiveness of ascorbate against cataracts has been tested in rat pups developing cataracts under the oxidative influence of sodium selenite. Certain antioxidants produced metabolically also may be useful in protecting against cataracts. Pyruvate produced in glucose metabolism seems to be an important antioxidant. The efficacy of this compound has been tested within in vitro organ culture as well as in vivo, the latter experiments being done with selenite-treated rats. There is a hope that these and other nutritional and metabolic antioxidants may one day be useful in delaying or even preventing cataract formation in human beings.

Pharmacological treatment strategies in age-related cataracts

Cataract is the major cause of blindness worldwide and at present the only approved treatment in many countries including the UK and USA is surgical removal of the lens. In other countries various anti-cataract drugs are available without proof of their efficacy. Research is continuing into the possible benefits of several groups of drugs and some vitamins. The first to be studied were sorbitol-lowering agents (aldose reductase inhibitors) based on the sorbitol hypothesis for diabetic cataract. Sorbitol-lowering agents have distinct effects in vitro and many of them delay the development of cataract in galactose-fed rats. A few delay cataract in diabetic rats but none have been proved effective in clinical trials, although these continue. Aspirin, paracetamol (acetaminophen) and ibuprofen delay diabetic cataract in rats, and have been shown to delay other experimental cataracts. Case-control studies from 3 continents indicate that these drugs, or at least aspirin, protect against cataract. Results of studies on all 3 drugs indicate a benefit even at low doses. Population-based studies did not identify any protection against early lens opacities but tiny opacities that do not impair vision are not a problem. Bendazac protects lens proteins in vitro and delays cataractogenesis in x-irradiated rats. In humans, it reached the clinical trial stage but most trials have been small and with subjective criteria of opacification. One objectively monitored trial suffered from a high drop-out rate. Other preparations studied less extensively include vitamins, aminoguanidine to prevent protein cross-linking in diabetes and agents designed to boost glutathione levels. It is probable that some agents which may delay or prevent cataract will be proved effective soon, and in the end there may be different drugs to delay cataract in different high risk groups. This is what might be expected of a multifactorial disease, although compounds that intervene in the final common pathways to cataract could have a broad efficacy.

Ocular ascorbate transport and metabolism

1. The concept is reviewed that the eye is subject to photo-oxidative damage through chemical free radical species that interact with sensitive tissue components. 2. The role of ascorbic acid may be to protect the eye by scavenging free radicals. 3. Ascorbic acid is present at a high concentration in various ocular compartments of diurnal animals, regardless of whether the animal synthesizes the compound or extracts it from the diet. 4. Ascorbic acid accumulates in the eye by active transport through the iris-ciliary body into aqueous humor, and subsequent transport into the lens and cornea. 5. Conservation of ascorbic acid occurs by reduction of dehydro-L-ascorbic acid and the ascorbate free radical through processes that appear to be enzymatic.

Photoinduction of cataracts in rat lens in vitro. Preventive effect of pyruvate

Intact rat lenses were incubated in riboflavin-containing Tyrode solution or medium-199, generating photochemically active species of oxygen and the oxidative stress measured in terms of the decrease in active accumulation of rubidium, and the fall in the levels of glutathione and ATP. Addition of pyruvate to the medium prevented the tissue against oxidative damage as evidenced by a greater accumulation of rubidium and higher levels of glutathione and ATP. Pyruvate was thus found to be effective against the toxicity of oxygen derivatives, particularly the hydrogen peroxide. In dark experiments also, conducted in glucose-free medium, the uptake of rubidium was substantially greater in the presence of pyruvate. The levels of ATP were also higher. These results, therefore, suggest that this ketoacid is beneficial to the tissue through its ability to decompose H2O2 as well through providing a metabolic support. The development of in vitro cataract under the photochemical effects of riboflavin and oxygen was also effectively thwarted by pyruvate. The results are thus potentially useful from the point of view of developing pyruvate and similar compounds as effective anticataract agents.

The protective effects of L-ascorbic acid and DL-alpha-tocopherol on cultured rat embryos treated with xanthine/xanthine oxidase

Abnormalities of the neural suture were observed in cultured rat embryos exposed to oxygen radicals generated by xanthine and xanthine oxidase. The distribution of the severity of these abnormalities was altered by the addition of L-ascorbic acid (AA) or DL-alpha-tocopherol (AT). The antioxidant effect of AA and AT were probably responsible for the protection of the embryos from the damaging effects of oxygen radicals.

Clinical signs of acute ocular inflammatory response to endotoxin are not altered by increasing antioxidant potency of intraocular fluids

Plasma antioxidant activity is due in large part to the ferroxidase activity of the copper (Cu) transport protein, ceruloplasmin. Implantation of osmotic pumps containing copper into rabbits resulted in a doubling of Cu concentration, ferroxidase activity, and antioxidant activity in plasma. Blood-ocular barriers essentially prevent the entry of large molecules such as proteins from plasma into the intraocular fluid compartments. However, during ocular inflammation, when these barriers are disrupted, plasma proteins can enter. Twenty-four hours after the induction of ocular inflammation by intraocular injection of endotoxin, the Cu concentration and antioxidant activity of intraocular fluids from the Cu pump animals was twice that of control groups, reflecting the differences in plasma levels. This is the first direct demonstration that alterations in plasma levels of ceruloplasmin can influence the antioxidant potency of the extracellular fluids. Increased intraocular fluid antioxidant activity did not affect the acute anterior segment response to endotoxin. However, it is possible that the time course and resolution of the response is altered by changes in extracellular fluid antioxidant activity. This possibility is currently under investigation.

Effects of age and dietary vitamin C on the contents of ascorbic acid and acid-soluble thiol in lens and aqueous humour of guinea-pigs

Variations in ascorbate and thiol concentration in lens and aqueous humour, with age and vitamin C nutrition, are of potential biological importance. To study these relationships, Dunkin-Hartley guinea-pigs were maintained for periods of up to 1.3 yr on diets containing either high or low (marginal) vitamin C. Ascorbate contents of liver, spleen, adrenals, lens and aqueous humour, and acid-soluble thiol of lens and aqueous humour were measured. High vitamin C intake maintained ascorbate levels in the internal organs between five and 30 times the level attained by the low vitamin C intake and aqueous humour vitamin C was 10-20 times higher at high vitamin C intake. Lens ascorbate, however, was only about twice as high at high vitamin C intake than at low intake, and at both intake levels it declined steadily to about half its initial value, after 1.3 yr. Thus an animal aged 1.3 yr on the low intake had about one-quarter to one-fifth of the lens ascorbate level of a young animal receiving a generous intake. Acid-soluble thiol in the lens, in contrast to ascorbic acid, increased significantly with age but was not significantly affected by dietary vitamin C intake. Acid-soluble thiol in the aqueous humour was only 0.5-2% of the concentration found in lens, and unlike the lens thiol level, it declined with age. No sex differences were observed for ascorbate or thiol levels either in lens or in aqueous humour.

Peroxide damage to the eye lens in vitro prevention by pyruvate

The ability of pyruvate to protect the eye lens against physiological damage by hydrogen peroxide has been studied. The physiological damage was estimated in terms of a decrease in the ability of the lens to transport rubidium against an electrochemical gradient under organ culture conditions. Peroxide was either added directly to the culture medium or generated therein by incorporation of xanthine and xanthine oxidase. In both these cases, addition of pyruvate to the medium led to a greater accumulation of rubidium by the lens. The net accumulation of this cation in the presence of 1 to 5 mM pyruvate from the medium containing peroxide (0.2 to 0.45 mM) was very close to that observed in the absence of peroxide. The protective effect was thus substantial. The mechanism of the pyruvate effect has been discussed, and seems to be related to the scavenging of peroxide by pyruvate.