Oxidative stress on lens and cataract formation: role of light and oxygen

A comparative study on the peroxidase activity within the aqueous humor of the rabbit, guinea-pig and rat

Aqueous humor of the rabbit, guinea-pig and rat has been examined to determine if non-specific peroxidase activity is present within this ocular fluid. Substantial peroxidase activity was found within the aqueous humor of the rat. In contrast, no peroxidase activity could be detected in the aqueous humor of the rabbit or guinea-pig. Partial purification by gel chromatography revealed that the peroxidase within the aqueous humor of the rat has a mol. wt of approx. 45,000.

The alteration of lipid peroxide in glucocorticoid-induced cataract of developing chick embryos and the effect of ascorbic acid

The level of lipid peroxide (LPO) in glucocorticoid-induced cataractous lenses was measured by using thiobarbituric acid. When 0.25 mumol of hydrocortisone hemisuccinate sodium (HC) were administered to 15-day-old chick embryos, the level of LPO in the lens increased to approximately 1.4-fold of the control level at 24 to 48 hr after HC treatment. However, the level returned to the control level by 96 hr with the disappearance of opacity in the lens. A triple application of ascorbic acid (20 mumol/egg) at 3, 10 and 20 hr after HC treatment prevented cataract formation and elevation in the level of LPO in 60% of the lenses. In 40% of the eggs treated with HC plus ascorbic acid, the results were no different than HC alone. Treatment with prednisolone hemisuccinate sodium (0.25 mumol/egg) produced an elevation in the level of LPO in the lens but that with cortisone hemisuccinate sodium or cortexolone hemisuccinate sodium did not change the level of LPO in the lens. The phenomena of the cataract formation and the elevation of LPO in the lens caused by HC seemed to be related to each other and due to its glucocorticoid activities.

Superoxide in ocular inflammation: human and experimental uveitis

A possible involvement of superoxide in the pathogenesis of uveal inflammation in man and experimental animals was investigated. Superoxide production by the leukocytes of Behcet patients was significantly higher in the attack phase than in the remission phase. Leukocyte superoxide generation was also enhanced in guinea pigs with S-antigen-induced experimental autoimmune uveoretinitis (EAU). If the animals were treated with superoxide dismutase (SOD) at the onset of EAU, aqueous humor cell count was significantly lower than that of control (i.e., without SOD treatment). Infiltration of the inflammatory cells in the anterior retina was markedly reduced in SOD-treated animals. A similar protective effect of SOD against tissue damage was also observed in a bovine serum albumin-induced passive Arthus type uveitis in rabbits. These results suggest that superoxide may play a role in causing tissue damage in animal models of ocular inflammation and possibly in Behcet disease.

Hypertrophic cardiomyopathy associated with a mitochondrial myopathy of voluntary muscles and congenital cataract

Structurally abnormal mitochondria were found in skeletal muscle cells from a woman with hypertrophic cardiomyopathy and myopathy of voluntary muscles associated with congenital cataracts. Moderate exercise resulted in lactic acidosis. Oxidation of pyruvate and other substrates and the production of adenosine triphosphate were normal in vitro. A younger brother of the patient had had congenital cataract and had died from hypertrophic obstructive cardiomyopathy.

The effects of "oxygen radicals" generated in the medium on lenses in organ culture: inhibition of damage by chelated iron

Rat lenses in organ culture were exposed to activated species of oxygen generated in the culture medium either by xanthine oxidase and hypoxanthine or by riboflavin and visible light, two systems which have been shown to produce superoxide and H2O2. In each case there was marked damage to carrier-mediated transport systems of the lens. Under standard culture conditions this damage was strongly inhibited by catalase, but not by superoxide dismutase (SOD). By the addition to the medium of chelated iron, hydroxyl radicals were produced in a Fenton reaction with a concomitant decrease in H2O2 levels. With both oxygen radical-generating systems, the addition of chelated iron strongly inhibited lens damage. This inhibitory effect could be reversed by the addition of SOD with the chelated iron. Under such conditions SOD converts superoxide anion to H2O2, thereby preventing reduction of the chelated iron and thus stopping the generation of hydroxyl radicals. Increased lens damage following addition of SOD to the iron-containing systems correlated with higher H2O2 concentrations, and was inhibited by catalase. These findings suggest that, when generated in the fluids surrounding the lens, H2O2 poses a much greater oxidative stress for the lens than do the superoxide or hydroxyl free radicals.

Role of hydrogen peroxide in riboflavin-sensitized photodynamic damage to cultured rat lenses

Previous reports have differed on whether hydrogen peroxide or free radicals are the principal toxic species responsible for photodynamic damage to intact lenses cultured in medium with riboflavin, light and oxygen. To differentiate between hydrogen peroxide and free radicals, TC-199 medium containing 50 microM riboflavin was incubated in visible light prior to adding the lenses. After 4 hr the medium was placed in the dark, rat lenses, [3H]-choline and 86Rb were added, and the lenses were cultured for 4 hr. Damage to the lenses was evaluated by measuring the accumulation of choline and Rb. Lenses cultured in previously irradiated medium accumulated less choline and Rb than lenses in control medium, indicating that in the presence of light, riboflavin generates a stable factor which is toxic to lenses. The lens damage was similar to that caused by direct exposure of lenses to riboflavin and light for 4 hr. This toxicity was blocked by catalase, and was presumed to be caused by hydrogen peroxide. Because of their short lifetime in solution, photochemically generated free radicals could not have been present in the dark after the lenses were added. Rose bengal, a photosensitizer which is thought to form singlet oxygen, did not generate a stable toxic factor in the culture medium and damaged lenses only if they were present during the exposure to light. The hydrogen peroxide concentration in riboflavin-containing medium after 4 hr exposure to light was approximately 200 microM, whereas rose bengal generated less than 10 microM hydrogen peroxide.(ABSTRACT TRUNCATED AT 250 WORDS)

Detoxification of hydrogen peroxide by the rabbit iris-ciliary body

When the intact rabbit iris-ciliary body was incubated in Tyrode's solution containing 10(-4) M hydrogen peroxide, the concentration of hydrogen peroxide remaining in solution diminished rapidly. The iris-ciliary body was homogenized and centrifuged at 100 000 g. It was observed that the hydrogen peroxide detoxification activity was resident primarily in the 100 000 g supernatant and not in the pellet. The hydrogen peroxide detoxification activity was found to be inactivated by heat and to be non-dialyzable. Gel filtration chromatography experiments revealed that breakdown of hydrogen peroxide by the ciliary body was not a 'bulk effect' due to generalized oxidation of tissue constituents. In fact, the active principle, localized by gel filtration chromatography, identified closely with catalase. These observations indicate that catalase within the iris-ciliary body enables the tissue to detoxify hydrogen peroxide from solution. Catalase might protect the iris-ciliary body from damage by hydrogen peroxide generated by normal physiological or pathophysiological conditions.

Lens lipids

Lens cells can synthesize, degrade, and remodel lipids. Endogenous lipid synthesis, in conjunction with uptake of exogenous cholesterol and certain fatty acids, leads to the formation of a plasma membrane that is especially rich in sphingomyelin, cholesterol, and long-chain saturated fatty acids. As a result of this unusual lipid composition, lens membranes have very low fluidity, which is restricted even further by lipid-protein interactions. The composition and metabolism of membrane lipids may affect the formation of various types of cataracts. Diets rich in vegetable oils offer some protection against the formation of osmotic cataracts and the hereditary cataract of the RCS rat, although the mechanism of this effect is not clear. Vitamin E also protects against the formation of several types of cataract in vivo and in vitro, suggesting that lipid peroxidation may play a role in cataractogenesis. Certain drugs which inhibit lipid synthesis or degradation are cataractogenic, and a deficiency in cataractogenic, and a deficiency in phosphatidylserine is associated with a loss of Na+/K+ ATPase activity in several types of cataract. Human senile cataracts show a marked loss of protein-lipid interactions, although the overall lipid composition is normal. This loss of protein-lipid interactions may be related to oxidative damage to membrane-associated proteins. Interestingly, the decrease in the fluidity of lens membranes with age would counteract the formation of aqueous pores in the membrane, which can result from the oxidative cross-linking of membrane-associated proteins. Certain pathways of lipid metabolism seem to have regulatory functions. Among these are phosphatidylinositol turnover, phosphatidylethanolamine methylation, and arachidonic acid metabolism. All of these pathways function in the lens. Phosphatidylinositol turnover is correlated with the rate of lens epithelial cell division, while phosphatidylethanolamine methylation seems to be related to the initiation of lens fiber cell formation. Both pathways are associated with the release and metabolism of arachidonic acid in other cell types. While it is not known whether phosphatidylinositol turnover or phosphatidylethanolamine methylation result in the release of arachidonic acid in the lens, recent work has shown that lens cells from a variety of species can metabolize arachidonic acid by both the cyclooxygenase and lipoxygenase pathways. The possible physiological significance of these metabolites to the lens is yet to be determined.

Hydrogen peroxide and hydroxyl radical formation by methylene blue in the presence of ascorbic acid

Using ESR we have demonstrated the formation of the ascorbate free radical from sodium ascorbate, methylene blue and light. In oxygen uptake experiments we have observed the production of hydrogen peroxide while spin trapping experiments have revealed the iron catalyzed production of the hydroxyl free radical in this system. The presence of this highly reactive radical suggests that it could be the radical that initiates free radical damage in this photodynamic system.