# Cathepsins (lysosomal proteases)

Cathepsins are a family of protein-chewing enzymes (proteases) that work inside your lysosomes, the cell's acidic recycling chambers. Most are cysteine proteases (cathepsins B, C, H, K, L, S, V, and X/Z), with a few aspartyl (D, E) and serine (A, G) types. Their main job is the final breakdown of proteins delivered by autophagy, endocytosis, and phagocytosis. Some do more than digest, though. A few get secreted to remodel the tissue scaffold outside cells (cathepsin K is the main bone-collagen cutter). And if cathepsins leak into the main cell body, especially cathepsin B, they can trigger the NLRP3 inflammasome and start cell death, a process called lysosomal membrane permeabilization. Cathepsin activity tends to fall with age, partly because lysosomes get less acidic and their natural inhibitors (cystatins) fall out of balance. That weakens protein quality control and lets undigested gunk pile up in long-lived cells like neurons.

## Sources

- Turk et al.. (2001). Lysosomal cysteine proteases: facts and opportunities. EMBO Journal. https://doi.org/10.1093/emboj/20.17.4629
- López-Otín et al.. (2023). Hallmarks of aging: An expanding universe. Cell. https://doi.org/10.1016/j.cell.2022.11.001

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_Canonical: https://longevity-switzerland.com/en/glossary/cathepsins · Part of Longevity Cities · Updated 2026-06-22_