Protein-thiol mixed disulfides in lenses have been implicated in the mechanism of protein-protein disulfide and other cross-linking leading to protein aggregation. The methodology for the detection and quantitation of protein-thiol mixed disulfides has been successfully established in our laboratory. Examination of mixed disulfides at different stages during development of a cataract may give relevant information on the mechanism of cataractogenesis, and whether oxidation is a part of that mechanism. In this study we investigated the involvement of mixed disulfides in cataract formation by using the H2O2-exposed lens as a model. Rat lenses, after being exposed to 0.5 mM H2O2 in culture showed an inverse relationship between the GSH loss and the protein-GSH formation with no effect on the protein-cysteine level. The H2O2-induced protein modification was also demonstrated indirectly by isoelectric focusing. The rate of protein-GSH production is dependent on the time of exposure and the concentration of H2O2. Age also plays some role as the lens GSH level decreases and the protein-thiol mixed disulfides increase as the animal becomes older. Lenses of older rats did not display more susceptibility to H2O2-induced mixed disulfide formation. The two protein-thiol mixed disulfides have a well-defined pattern of distribution in the rat lens. Most of the protein-GSH was found in the cortex and the water-soluble protein fraction whereas more protein-cysteine was found in the nucleus and water-insoluble protein fraction. Lens of older rat has more protein-cysteine as well as more water-insoluble proteins.(ABSTRACT TRUNCATED AT 250 WORDS)