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 H2O2-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 H2O2 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
gamma-crystallin-S-S-G better than the
alpha-crystallin counterparts. Furthermore, TTase showed a remarkable resistance to oxidation (H2O2) 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.