PSI is a synthetic peptide proteasome inhibitor.^1,2,3It inhibits the acidic and neutral chymotrypsin-like activities of the 20S proteasome (IC₅₀s = 0.25 and 6.5 µM, respectively).³PSI is cytotoxic to HL-60, KG-1a, RWLeu-4, AR230, and K562 human leukemia cells (IC₅₀s = 4.9, 17.5, 7.8, 7.3, and 10 nM, respectively).⁴It selectively induces apoptosis in subconfluent bovine aortic endothelial cells (BAECs) and human umbilical vein endothelial cells (HUVECs; EC₅₀s = 24 and 7 nM, respectively) over confluent BAECs and HUVECs (EC₅₀s = 8,150 and 7,830 nM, respectively).⁵PSI induces accumulation of ubiquitin-protein conjugates in HT4 mouse neuronal cells.⁶

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