The word "lysosome" is Latin for "kill body." Lysosomes are organelles produced by the Golgi apparatus that contain powerful protein digesting enzymes. Lysosomes are responsible for the breakdown and absorption of materials taken in by the cell.

Lysosomes are key components of the "intracellular digestive tract." They contain a battery of hydrolytic enzymes, which have two special properties. First, they can function in the acid milieu of the lysosomes. Second, these enzymes constitute a special category of secretory proteins that, in contrast to most others, are destined for secretion not into the extracellular fluids but into an intracellular organelle. This latter characteristic requires special processing within the Golgi apparatus, which is reviewed briefly.

Similar to all other secretory proteins, lysosomal enzymes (or acid hydrolases, as they are sometimes called) are synthesized in the endoplasmic reticulum and transported to the Golgi apparatus. Within the Golgi complex, they undergo a variety of post-translational modifications, of which one is worthy of special note. This modification involves the attachment of terminal mannose-6-phosphate groups to some of the oligosaccharide side chains.

The phosphorylated mannose residues may be viewed as an "address label" that is recognized by specific receptors found on the inner surface of the Golgi membrane. Lysosomal enzymes bind to these receptors and are thereby segregated from the numerous other secretory proteins within the Golgi.

Subsequently, small transport vesicles containing the receptor-bound enzymes are pinched off from the Golgi and proceed to fuse with the lysosomes. Thus, the enzymes are targeted to their intracellular abode, and the vesicles are shuttled back to the Golgi.

The lysosomal acid hydrolases catalyze the breakdown of a variety of complex macromolecules. These large molecules may be derived from the metabolic turnover of intracellular organelles (autophagy), or they may be acquired from outside the cells by phagocytosis (heterophagy).

Types

 secretory lysosomes
 lysosomal membrane
 lysosomal membrane proteins
 lysosome biogenesis

Lesions

 lipolysosomes

Pathology: lysosomal storage diseases

 Mutations in the gene that encodes the lysosomal exoglycohydrolase alpha-galactosidase A (alpha-GalA) cause Fabry's disease
 Wolman disease

See also

 secretory lysosomes
 lipolysosomes
 phagolysosomes

References

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 Fehrenbacher N, Jaattela M. Lysosomes as targets for cancer therapy. Cancer Res. 2005 Apr 15;65(8):2993-5. PMID: #15833821#

 Eskelinen EL, Tanaka Y, Saftig P. At the acidic edge : emerging functions for lysosomal membrane proteins. Trends Cell Biol. 2003 Mar ;13(3):137-45. PMID : #12628346#

 Huizing M, Gahl WA. Disorders of vesicles of lysosomal lineage: the Hermansky-Pudlak syndromes. Curr Mol Med. 2002 Aug;2(5):451-67. PMID: #12125811#

 Simons K, Gruenberg J. Jamming the endosomal system : lipid rafts and lysosomal storage diseases. Trends Cell Biol. 2000 Nov ;10(11):459-62. PMID : #11050411#

 Andrews NW. Regulated secretion of conventional lysosomes. Trends Cell Biol. 2000 Aug;10(8):316-21. PMID: #10884683#