White solid. A potent, reversible, and cell-permeable proteasome inhibitor (Ki = 4 nM). Reduces the degradation of ubiquitin-conjugated proteins in mammalian cells and permeable strains of yeast by the 26S complex without affecting its ATPase or isopeptidase activities. Activates c-Jun N-terminal kinase (JNK1), which initiates apoptosis. Inhibits NF-kB activation (IC50 = 3 µM). Prevents b-secretase cleavage. Purity: ≥98% by HPLC. CAS 133407-82-6.