Localization of Degradative Pathways

Although there have been numerous and comprehensive studies on intracellular proteinases, they have been carried out predominantly with the purified enzymes. This work has provided information on catalytic mechanisms, susceptibility of proteinases to different classes of inhibitors, specificities of the enzymes toward a variety of artificial and natural substrates, and details on optimal conditions for each proteinase (Barrett, 1969). Unfortunately, these experiments provide little if any direct evidence about whether a particular enzyme is involved in the breakdown of intracellular enzymes or other proteins. 

Proteinases are found in all subcellular fractions of liver, but most of the activity associated with an acid pH optimum is localized in lyso-somes. Although neutral proteinases occur in various cell membranes, it is not yet clear whether any selective enrichment occurs in a particular membrane type. The total neutral proteinase activity found in the microsomal fraction, which includes the Golgi and plasma membranes as well as the smooth and rough endoplasmic reticulum, is adequate to explain the observed breakdown of proteins in these fractions (Bohley, 1968 Bohley et al., 1971). Of course this does not imply self-proteolysis, only the capacity to degrade the membrane proteins. 

Whereas proteolytic activities in mitochondria, nuclei, and the microsomal fraction are relatively high, very little proteolytic capacity is found in the cell cytosol. In part this may reflect technical difficulties, because the highly efficient proteolytic inhibitors found in serum, especially a2-macroglobulin and aj-antitrypsin, would occur in the cytosol fraction of an homogenized tissue. Nevertheless, separation of cytosol proteins from serum proteolytic inhibitors by Sephadex chromatography does not give rise to any fractions with substantial proteolytic capacity (Bohley et al., 1971). 

---