Age-related increase in concentration and aggregation of degraded polypeptides in human lenses

Proteins from human lenses of various ages were separated into three fractions based on their solubility; water-soluble (WS), water-insoluble-urea-soluble (WI-US) and water-insoluble-urea-insoluble (WI-UI). The SDS-PAGE of these fractions showed increasing quantities of degraded polypeptides (mol. wt; Mr less than 18 kDa) with aging. On separation of degraded polypeptides from the rest of lens crystallins by gel-filtration chromatography under denaturing conditions, the polypeptides showed an age-related concentration increase in the WS and WI-US fractions, which constituted 12-14% and 17-18% of the total proteins respectively. The degraded polypeptides from WI-US fractions of lenses from 50-yr-old donors exhibited five polypeptides ranging in Mr between 3- and 18 kDa. The degraded polypeptides isolated from WS and WI-US fractions of lenses of various ages self-aggregated on storage to protein moieties with Mr ranging between 18- and greater than 1500 kDa. An antiserum was raised against degraded polypeptides isolated from WI-US protein fraction of 50-yr-old human lenses. During the Western blot analyses, the antiserum exhibited specific immuno-reaction with alpha-crystallin and 20-kDa gamma-crystallin species and with degraded polypeptides among various human lens crystallins tested. Similarly, when the self-aggregated degraded polypeptides of WS and WI-US fractions were reacted with the antiserum, several polymeric proteins of increasing Mrs were observed. In addition, when the heavy molecular weight (HMW) proteins from WS proteins and the total WI proteins of lenses of various ages were probed with the antiserum, several polymeric proteins, similar to the in vitro self-aggregated polymers of degraded polypeptides as previously described, were seen. Such polymers were in relatively lower quantities in the total WS proteins of lenses of various ages. These results suggest an apparent age-related polymerization of degraded polypeptides into HMW proteins leading to their insolubilization.