Cysteine and glutathione metabolism in organ-cultured rat corneas

Novel roles for the lens in preserving overall ocular health

Outside the traditional roles of the lens as an important refractive element and a UV filter, it was David Beebe's group that first demonstrated that the lens acts an oxygen sink that protects the tissues of the anterior segment of the eye from oxygen or oxygen metabolites. In this review, we follow on from this work, and present new evidence from our laboratory to demonstrate that the lens serves as a reservoir for the release of the antioxidant glutathione (GSH) into the aqueous humor to provide a source of GSH and/or its precursor amino acids to nearby tissues that interface with the aqueous humor, or to remove toxic metabolites from the eye via the aqueous outflow pathway. In addition to GSH release, our laboratory and others have shown that ATP is released from the lens under hyposmotic conditions to activate purinergic signalling pathways in an autocrine manner to alter lens function. In this review, we raise the idea that ATP and/or its subsequent degradation product adenosine may exert a paracrine function and influence purinergic signalling systems in other tissues to alter aqueous humor outflow. These new secondary roles indicate that the lens is not just a passive optical element, but a highly dynamic and active tissue that interacts with its neighbouring tissues, through modifying the environments in which these tissues function. We believe that the lens actively contributes to the ocular environment and as a consequence, removal of the lens would alter the functionality of neighbouring tissues. We speculate that a long term effect of lens removal may be to inadvertently increase the exposure of anterior tissues of the eye to oxidative stress due to elevated oxygen levels and a reduction in the availability of GSH and purinergic signalling molecules in the aqueous humor. Since cataract surgery is now being performed on younger patients due to our increasing diabetic population, over time, we predict these changes may increase the susceptibility of these tissues to oxidative stress and the incidence of subsequent ocular pathologies. If our view of the lens is correct, the actual loss of the biological lens may have longer term consequences for overall ocular health than currently appreciated.

Changes of the thiol levels in the corneas of the diabetic rats: effect of carnosine, aspirin and a combination eye drops

AIM

To investigate the effect of carnosine (Car), aspirin (Asp) and a combination of Car and Asp eye drops on the change of the thiol contents from glutathione (GSH) and protein in the corneas of the diabetic rats.

METHODS

All the animals were randomly divided into five groups. The normal control group received injections of vehicle only. Diabetes was induced by injection of streptozotocin (STZ) in the Sprague-Dawley (SD) rats. The untreated group rats received only the vehicle solution (the placebo). One treated group rats were treated by instillation of one drop of 10g/L Car eye drops, another were treated by 0.5g/L Asp eye drops and the last group were treated alternately by Car 10g/L and Asp 0.5g/L eye drops for a period of 8 weeks. At the end of 8 weeks, the animals were killed and the thiols contents in the corneas were investigated.

RESULTS

About 15.6% of the rats (blood glucose measured <14mmol/L) were rejected. In the corneas, the levels of thiols were declined in the untreated, Asp-treated group and combination-treated group, but they went up in Car-treated group. The levels of thiols in the Car-treated groups were much higher than that in the untreated group, and there was statistically significant difference between them (P<0.05).

CONCLUSION

The results indicated that diabetes decreases the levels of thiols (from GSH and proteins) in the cornea. The Car eye drops in our study may protect the cornea against the oxidative damage caused by diabetes. And the combination eye drops also may have a certain protection for the diabetic corneas.

Glutathione-Related Enzymes and the Eye

Glutathione and the related enzymes belong to the defence system protecting the eye against chemical and oxidative stress. This review focuses on GSH and two key enzymes, glutathione reductase and glucose-6-phosphate dehydrogenase in lens, cornea, and retina. Lens contains a high concentration of reduced glutathione, which maintains the thiol groups in the reduced form. These contribute to lens complete transparency as well as to the transparent and refractive properties of the mammalian cornea, which are essential for proper image formation on the retina. In cornea, gluthatione also plays an important role in maintaining normal hydration level, and in protecting cellular membrane integrity. In retina, glutathione is distributed in the different types of retinal cells. Intracellular enzyme, glutathione reductase, involved in reducing the oxidized glutathione has been found at highest activity in human and primate lenses, as compared to other species. Besides the enzymes directly involved in maintaining the normal redox status of the cell, glucose-6-phosphate dehydrogenase which catalyzes the first reaction of the pentose phosphate pathway, plays a key role in protection of the eye against reactive oxygen species. Cornea has a high activity of the pentose phosphate pathway and glucose-6-phosphate dehydrogenase activity. Glycation, the non-enzymic reaction between a free amino group in proteins and a reducing sugar, slowly inactivates gluthathione-related and other enzymes. In addition, glutathione can be also glycated. The presence of glutathione, and of the related enzymes has been also reported in other parts of the eye, such as ciliary body and trabecular meshwork, suggesting that the same enzyme systems are present in all tissues of the eye to generate NADPH and to maintain gluthatione in the reduced form. Changes of glutathione and related enzymes activity in lens, cornea, retina and other eye tissues, occur with ageing, cataract, diabetes, irradiation and administration of some drugs.

An HPLC radiotracer method for assessing the ability of L-cysteine prodrugs to maintain glutathione levels in the cultured rat lens

PURPOSE

To apply a high performance liquid chromatographic radiotracer method to test a variety of L-cysteine prodrugs and one dipeptide prodrug for their ability to synthesize glutathione in cultured rat lenses.

METHODS

Rat lenses were incubated for 48 h in a medium containing [14C(U)]-glycine and prodrugs. Following homogenization and derivatization, lens extracts were analyzed to determine the extent of biosynthetic incorporation of this labeled amino acid into [14C]-glutathione using high performance liquid chromatography with radioisotope and ultraviolet absorption detection. All of the thiazolidine prodrugs contained masked sulfhydryl groups to stabilize them against air oxidation. L-buthionine-(S,R)-sulfoximine-an inhibitor of the first step in glutathione biosynthesis-was present in media containing the dipeptide prodrug.

RESULTS

In all cases, a large [14C]-labeled peak eluted just prior to [14C]-glutathione. This peak had some characteristics of the mixed disulfide of glutathione and L-cysteine, viz., L-cysteine/glutathione disulfide, but requires further investigation in order to be positively identified. Of the eleven L-cysteine prodrugs investigated, the most effective was 2(R,S)-methylthiazolidine-4(R)-carboxylic acid, which increased the rate of [14C]-glutathione biosynthesis 35% over that of the controls. A number of other L-cysteine prodrugs were somewhat effective, increasing glutathione synthesis 5-30% over the controls, while several L-cysteine prodrugs were totally ineffective. The only dipeptide prodrug investigated, viz., gamma-L-glutamyl-L-cysteine ethyl ester, increased the biosynthesis of [14C]-glutathione 18% over control. Biosynthetic rates based on ultraviolet absorption of the derivatized glutathione demonstrated a similar pattern, the compounds most effective in synthesizing [14C]-glutathione generally yielding the highest ultraviolet glutathione concentrations and the ineffective compounds showing the lowest concentrations.

CONCLUSIONS

2(R,S)-methylthiazolidine-4(R)-carboxylic acid, 2(R,S)-n-propylthiazolidine-4(R)-carboxylic acid and N-acetyl-L-cysteine were the only compounds that were statistically significant in yielding higher levels of both ultraviolet and radioactive glutathione as compared to their respective controls. Thus, these prodrugs have very promising anti-cataract potential.

Age-related cysteine uptake as rate-limiting in glutathione synthesis and glutathione half-life in the cultured human lens

The study included human lenses of ages ranging from newborn to 92 years. Protein-free reduced glutathione decreased 14-fold, whereas protein-free oxidized glutathione increased 2.6-fold with increasing age. L-Cyst(e)ine uptake g-1 lens of very old cultured lenses decreased 70% from that exhibited in newborn lenses, demonstrating a marked decline of L-cyst(e)ine uptake as a function of age. In these same lenses the synthesis of reduced glutathione (mumol g-1 lens) decreased 73% with age. It was concluded that the glutathione decrease observed in the aging human lens was associated with decreased uptake of L-cyst(e)ine, decreased glutathione synthesis and possibly an increase in protein-free oxidized glutathione. The high correlation of L-cyst(e)ine uptake and glutathione synthesis supports the hypothesis that L-cyst(e)ine uptake is a rate-limiting factor of glutathione synthesis in the intact human lens. By the use of buthionine sulfoximine, the half-life of glutathione was estimated to be 90 hr in the cultured human lens.