Experiments in the cell-free system by Western blot analysis confirm direct interaction of PSA/NPEPPS and SOD1. A. Purified human SOD1 was incubated with post-microsomal protein fractions of SH-SY5Y cells transfected with PSA/NPEPPS overexpressing vector (PSA↑), blank vector (Cntr) or Stealth-RNAi specific for hPSA/NPEPPS (PSA↓); and the exhibited SOD1 levels are reversely proportional to the levels of PSA/NPEPPS protein. Independent experimental triplicates are shown. B. Incubation of purified human SOD1 with purified rat PSA/NPEPPS for 6 hours demonstrates up to 80% proteolysis of human SOD1. Controls contained human SOD1 only. The gel bands in B demonstrate the results from two independent co-incubation experiments. Only representative images are shown. Error bars represent standard deviations.