Age-dependent protein modifications and declining proteasome activity in the human lens

The proteasome is known to be the main enzymatic complex responsible for the intracellular degradation of altered proteins, and the age-related accumulation of modified lens proteins is associated to the formation of cataracts. The aim of this study was to determine whether the human lens proteasome becomes functionally impaired with age. The soluble and insoluble protein fractions of human lenses corresponding to various age-groups were characterized in terms of their levels of glyco-oxidative damage and found to show increasing anti-carboxymethyl-lysine immunoreactivity with age. Concomitantly, decreasing proteasome contents and peptidase activities were observed in the water-soluble fraction. The fact that peptidylglutamyl-peptide hydrolase activity is most severely affected with age suggests that specific changes are undergone by the proteasome itself. In particular, increasing levels of carboxymethylation were observed with age in the proteasome. It was concluded that the lower levels of soluble active enzymatic complex present in elderly lenses and the post-translational modifications affecting the proteasome may at least partly explain the decrease in proteasome activity and the concomitant accumulation of carboxymethylated and ubiquitinated proteins which occur with age.