This buffer contains the optimal formulation for the general inhibition of enzyme activities that deconjugate ubiquitin and ubiquitin-like (UBL) protein (SUMO, NEDD8, ISG15) from protein substrates. Useful during the preparation of crude lysate samples to enrich for protein conjugates or to quench in vitro assays. This agent will also quench the activities of other non-related cycsteine proteases. More specific deconjugating enzyme inhibitor include aldehyde and vinyl sulfone derivatives such as ubiquitin (U-201, U-202), SUMO (UL-701, UL-702), NEDD8 (UL- 801, UL-802) and ISG15 (UL-602, UL-603).