Purity >96% Heavy Metals: Less than 20 ppm . An antitumour agent that inhibits the Ubiquitin-ATP-dependent proteolytic activity of rabbit reticulocytes, but not the Ubiquitinylation process. More recently, it has been shown that it only affects the Chymotrypsin-like activity of the 20S Proteasome. It has no effect on Cathepsin B, stimulated Trypsin, and inhibited Chymotrypsin and, to a lesser extent, Calpain.