OBJECTIVE To compare levels of S-glutathiolation and S-cysteinylation occurring at more than 60 cysteine residues of 12 different guinea pig lens water-soluble nuclear crystallins following treatment of the animals with hyperbaric oxygen (HBO). METHODS Guinea pigs (initially 18 months old) were treated 30X (3X per week for 10 weeks) with HBO (2.5 atm 100% O(2) for 2.5 h) as a model to study the formation of nuclear cataract. This treatment produces a moderate increase in lens nuclear light scatter (compared to denser scatter occurring after 80 HBO treatments), with five- to sixfold increases in levels of protein-bound glutathione (PSSG) and protein-bound cysteine (PSSC). Trypsin digests of lens nuclear water-soluble proteins were analyzed with two-dimensional liquid chromatography and mass spectrometry to identify specific cysteine residues binding either glutathione or cysteine. Lens nuclei of age-matched untreated animals were used as controls. RESULTS All major crystallins, except αB, were modified to some extent by either S-glutathiolation or S-cysteinylation. Overall, 72% of the cysteine residues of guinea pig lens nuclear crystallins were shown to be capable of binding glutathione, cysteine, or both molecules. The crystallin with the highest level of modification was βA1/A3 (six of eight -SH groups), and that with the lowest (two of five -SH groups) was βA2. O(2)-induced increases in PSSG levels were 2.8, 2.4, and 4.1 times control for γA-, γB-, and γC-crystallins, respectively. Comparable increases in PSSC levels for the three γ-crystallins were 2.3, 2.7, and 2.4 times control, respectively. βB2-crystallin showed the highest amount of O(2)-induced PSSG formation of any of the crystallins, as well as a substantial level of control PSSG, and nearly all of this was due to a single residue, C67, a site also present in human βB2-crystallin. Overall, 32 of the 44 modified cysteine residues were homologous with the human. CONCLUSIONS This large-scale study successfully identified lens crystallin cysteine residues that bound glutathione and/or cysteine under normal or oxidative stress conditions. The high percentage of protein -SH groups that are modified by S-thiolation in the guinea pig lens nucleus demonstrates the substantial protein sulfhydryl redox buffer capability present in the center of the lens. The results suggest that PSSG and PSSC formation may act to delay O(2)-induced insolubilization of γA-, γB-, and γC-crystallins, and β-crystallins, but with a greater effect on the γ-crystallins at an early stage of oxidative stress. The study has shown that technological approaches are now available to investigate in considerable detail the role of specific lens -SH groups in nuclear cataractogenesis.