Amino Acid Sequence Z-Leu-Leu-Nva Biochem/physiol Actions Potent inhibitor of the 20S proteasome (Ki = 21 nM) and 26S proteasome (Ki = 35 nM). Cells treated with 54 μM N-CBZ-Leu-Leu-Norvalinal showed complete block of the G1/S and metaphase transitions and delayed passage through S-phase. Induces apoptosis in Rat-1 and PC12 cells, perhaps by accumulation of stabilized p53.