derivatives 3α-O-pimeloyl-lithocholic acid methylester (2) and its isosteric isomer (6) were found to activate the chymotrypsin-like activity of the proteasome at an EC50 of 7.8 and 4.3 μM, respectively. Replacing the C24 methylester in 2 with methylamide resulted in a complete devoid of proteasome activating activity. Epimerizing the C3 substituent from an α to β orientation transformed the activator