Protein-thiol mixed disulfides in human lens

Protein-thiol mixed disulfide formation has been implicated as a possible mechanism for the protein-protein aggregation in cataractogenesis. Previously we have found that two species of thiols are bound to proteins: GSH (PSSG) and cysteine (PSSC). In this study we found these molecules are ubiquitous in animal lenses with the highest levels in human, dog and rat, and lowest in monkey. However, the relative amount of PSSG to PSSC is quite different in each animal species. The ratio of PSSG/PSSC was 1/10 in rat lens, 4/1 in human and dog lenses and 2/1 in monkey lens. We also studied the effect of aging on the protein-thiol mixed disulfide levels in human donor lenses between 3 months and 88 years. Lens GSH levels were inversely related to age, similar to earlier reports, but PSSC levels increased linearly with age. PSSG levels showed a triphasic pattern with an initial sharp and linear increase from a low content in infants to a highest level at age 20; fell back about 50% to a new steady state level that was maintained for four more decades; finally, above 60 years, the levels in some lenses were two to three-fold higher while some lenses remained at the same low value. PSSC in human lens appeared to concentrate in the nuclear region and in the water insoluble proteins while PSSG was more evenly distributed. Besides the aging effect on the protein-thiol mixed disulfides, oxidative stress also potentiated protein modification in the human lens.(ABSTRACT TRUNCATED AT 250 WORDS)