Oxidative stress increases ubiquitin-protein conjugates in synaptosomes

SYNAPTOSOMES were incubated in the presence of FeSO₄ to test the hypothesis that iron-catalyzed oxidative damage causes an increase in the ubiquitination of synaptosomal proteins. Incubation with 10 or 50μM FeSO₄ caused concentration-dependent increases in carbonyl groups (an indication of protein oxidation) and ubiquitinated proteins (determined by probing Western blots with a monoclonal antibody to ubiquitin). Differences in protein ubiquitination occurred within 5 min of incubation, indicating a rapid response to oxidative stress. Results of experiments with MG132, an inhibitor of the degradation of ubiquitinated proteins, suggested that oxidative damage stimulated ubiquitination rather than inhibited degradation of ubiquitinated proteins. The data are consistent with the hypothesis that synaptic terminals utilize the ubiquitin/proteasome proteolytic pathway to degrade oxidatively damaged proteins.