['The proteasome is the proteolytically active core of the ubiquitin-proteasome system, which regulates vital processes and which can cause various diseases when it malfunctions. Therefore, the proteasome has become an attractive target for pharmaceutical interventions. Inhibition of the cardiac proteasome by specific proteasome inhibitors has been shown to attenuate cardiac hypertrophy and ischaemia reperfusion injury of the heart. We have resolved the cardiac proteasome into its subtypes and have addressed the key question of how proteasome inhibitors affect single cardiac proteasomal subtypes.', 'The 20S proteasome from rat heart was dissected into three different subpopulations (groups I-III), each comprising 4-7 different subtypes. The major group (group II) comprises standard proteasome subtypes; the two minor subpopulations (groups I and III) contain intermediate proteasome subtypes. All subtypes exhibit chymotrypsin-, trypsin-, and caspase-like activity but to different degrees. We have tested the effect of two common proteasome inhibitors on the chymotrypsin-like activity of all subtypes: 20-30 nmol/L MG132 caused 50% inhibition of all subtypes from groups I and II, whereas 100 nmol/L was necessary to affect group III subtypes to the same extent. However, another inhibitor, bortezomib (VELCADE), already used clinically, inhibited 50% of the activity of group III proteasome subtypes even below 20 nmol/L, a concentration showing almost no effect on group I and II proteasome subtypes. The caspase-like activity of group II proteasome subtypes was not affected by MG132 and was inhibited by bortezomib only at concentrations above 100 nmol/L.'] Do multiple cardiac proteasome subtypes differ in their susceptibility to proteasome inhibitors?