Delivery of Glutathione to the Lens Nucleus

Glutathione (GSH), an anionic tripeptide, is the principal antioxidant in the lens where it functions to maintain protein thiols in a reduced state. Levels of GSH are high in both the epithelium and the outer cortex of the lens where the anti-oxidant is synthesized from the amino acids cysteine, glutamate and glycine by g-glutamylcysteinesynthetase and glutathione synthetase. A reducing environment in the outer fiber cells is also maintained by regeneration of GSH from GSSG by the enzyme glutathione reductase which utilizes NADPH produced via the hexokinase shunt pathway as a cofactor. In contrast, levels of GSH are 80-90% lower in the nucleus of the lens owing to the low specific activity of enzymes involved in its synthesis and regeneration. This regional difference in GSH levels worsens with age. The age-dependent reduction in GSH levels in the nucleus and the resulting increase in GSSG lead to oxidation of cysteine and methionine residues in proteins and formation of protein mixed disulfides (PSSG) and eventually, loss of transparency. This decline in GSH levels is believed to be the key initiating factor in the formation of age-related nuclear cataracts, the leading cause of blindness in the world. How GSH is transported to the lens nucleus and why this process is disrupted with age is not clear. According to one view, GSH is transported by an extracellular pathway driven by the lens microcirculation. 5 All vertebrate lenses studied to date have a circulating ionic current that enters at the poles and exits at the equator of the lens. 6 This internal circulating current is primarily carried by sodium ions, and enters the lens along the extracellular spaces between cells. After crossing the fiber cell membranes in the lens interior, it flows from cell to cell towards the surface through gap junction mediated pathways. The high concentration of gap junction channels at the equator allows the intracellular current to be directed to surface cells where Na + /K + pumps are located in epithelial cells transporting sodium out of the lens. 6 The circulating ionic current generates fluid flow through the lens, as indicated by recent measurements of intracellular hydrostatic pressure in vertebrate lenses. 7 Glucose and other nutrients are convected into extracellular spaces surrounding fiber cells, where uptake transporters allow their delivery into fiber cells. Anti-oxidants such as GSH (and ascorbic acid) are believed to be carried to the lens core in a similar fashion. …