Clasto-Lactacystin β-lactone, a natural active metabolite of lactacystin, which is a metabolite of Streptomyces, acts as an irreversible 20S proteasome inhibitor. IC50 & Target: 20S proteasome^[1] In Vitro: Clasto-Lactacystin β-lactone is a natural active metabolite of lactacystin, which is a metabolite of Streptomyces. Clasto-Lactacystin β-lactone is the cell cycle progression in an osteosarcoma cell line^[1]. Clasto-Lactacystin β-lactone is an irreversible 20S proteasome inhibitor, and also acts as a partial inhibitor of branched amino acid preferring peptidase (BrAAP). β-Lactone inactivates approximately 60% of the BrAAP activity rapidly, and the remaining 40% of the BrAAP activity is inactivated by β-lactone at a 50-fold slower rate^[2].