Distribution of Proteasomes and of the Five Proteolytic Activities in Rat Tissues

Five peptidase activities (ChT-L, T-L, PGPH, BrAAP, and SNAAP) of the proteasome, and its caseinolytic activity, were measured in crude extracts of 10 rat tissues under experimental conditions simulating those found in vivo, thereby eliminating the alterations observed with the purified enzyme. The total and individual peptidase activities varied considerably from one tissue to another, whereas the proteolytic activity measured with [14C]methylcasein varied no more than twofold. The tissue-specific variations in individual peptidase activities may reflect tissue-specific differences in proteasome subunit composition, or the presence of regulators. Immunological assay using an antibody directed against the iota (α1) subunit showed that there was no correlation between protein abundance and peptidase activity. The results also show that the different peptidase activities are not representative of proteasome distribution in the different tissues.