Thiol Regulation in the Lens

Abstract

The high content of glutathione (GSH) in the lens is believed to protect the thiols in structural proteins and enzymes for proper biological functions. The lens has both biosynthetic and regenerating systems for GSH to maintain its large pool size (4-6 mM). However, we have observed that, in aging lenses or lenses under oxidative stress, the size of GSH pool is diminished; and some protein thiols are being S-thiolated by oxidized nonprotein thiols to form protein-thiol mixed disulfides, either as protein-S-S-glutathione (PSSG) or protein-S-S-cysteine (PSSC). We have shown in an H₂0₂-induced cataract model that PSSG formation precedes a cascade of events starting with protein disulfide crosslinks, protein solubility loss, and eventual lens opacification. Recently, we discovered that this early oxidative damage in protein thiols could be spontaneously reversed in H₂0₂ pretreated lenses if the oxidant was removed in time. This dethiolation process is likely mediated through a redox regulating enzyme, thioltransferase (TTase), which has been discovered recently in the lens. To understand if the role of oxidative defense and repair is the physiological function of TTase in the lens, we cloned the TTase gene and purified the recombinant human lens TTase. Although TTase required GSH for its activity, TTase was far more efficient in dethiolating lens proteins than GSH alone. It favored PSSG over PSSC and dethiolated γ-crystallin-S-S-G better than the α-crystallin counterparts. Furthermore, TTase showed a remarkable resistance to oxidation (H₂0₂) in cultured rabbit lens epithelial cells when GSH peroxidase, GSH reductase, and glyceraldehyde-3-phosphate dehydrogenase were severely inactivated. We further showed that activity loss in those SH sensitive enzymes could be attributed to S-thiolation, but reactivation via dethiolation could be attributed to TTase. We conclude that TTase can regulate and repair the thiols in lens proteins and enzymes through its dethiolase activity, thus contributing to the maintenance of the function of the lens.