A potent, reversible, and cell permeable proteasome inhibitor; a more effective inhibitor of chymotrypsin-like, trypsin-like, and peptidylglutamyl peptide hydrolyzing proteasome activities compared to (S)-MG132 (IC50s = 0.22 versus 0.89 µM (ChTL); 34.4 versus 104.43 µM (TL); 2.95 versus 5.70 µM (PGPH), respectively).