Cells contain multiple mechanisms for proteolysis.
Lysosomes contain acid hydrolases and other enzymes the degrade endocytosed proteins from the extracellular space, cell surfaces and some intracellular components.
The ubiquitin-proteasome pathway degrades many nuclear and cytoplasmic proteins.
In the ubiquitin proteasome pathway proteins are first conjugated to ubiquitin and then degraded within a cytoplasmic proteolytic organelle, the proteasome.
Hormones, particularly glucocorticosteroids and thyroid hormone, stimulate proteasome mediated protein degradation, while insulin opposes such activities.
Cytokines, such as TNF (tumor necrosis factor) increase muscle proteolysis.