The age dependence of intracellular proteolysis: changes of the substrate proteins.

Liver cytosol proteins of young (4--6 months) and old (18--27 months) rats were degraded in vitro by papain, pronase, trypsin, pepsin, cathepsin D from rat liver and a soluble lysosomal enzyme mixture from rat liver. We could demonstrate the capability of the latter enzyme mixture to degrade proteolytically the cytosol proteins of young animals about 20% faster than those of the older animal group. Digesting radioactive labelled "young" cytosol in the presence of unlabelled "old" cytosol the possibility could be excluded, that this effect was due to an inhibitor of macromolecular size present in the "old" cytosol.