Purified 20S proteasome inhibition by Cd1, Cd2 and Cd3, supported by nucleophilic susceptibity analysis. A: Purified human proteasome (35 ng) was incubated with DMSO or various concentrations of Cd1, Cd2 and Cd3 for 2 h, followed by proteasomal chymotrypsin-like activity assay. Points, mean+/−SE of three independent experiments; bars, SD. B, C, D: The nucleophilic susceptibility of Cd1, Cd2 and Cd3 was analyzed by using CAChe software. Molecular orbital energy analysis is shown by drawing an electron density isosurface and the highest susceptibility towards nuceophilic attack was signified by white center.