Lysosomes Structure and Function - "suicidal bags"

Waste disposal is a major problem in our day today life. The processes involving recovering and recycling is the priority for efficient waste management.  Inside the cell there is an efficient compartment that digest waste material produced by metabolic activities of the cell and that is lysosome, the incinerator of the cell.

 

Definition:

A single membrane bound organelle involved in intracellular digestion, present only in animal cells. Lysosome lumen has a slightly acidic internal pH (pH 4-5) and pH is maintained by an ATP driven proton pump in the membrane.

 A range of digestive enzymes (called hydrolases) are present in the lumen and these enzymes works only in acidic pH (like proteases, lipases etc) and thus inactive in cytosolic pH. Lysosomal membrane is rich in sialic acid.

Why lysosomes are absent in plants?

In plants, Digestive enzymes are not localised in an organelle with the specific function of digestion and usually occurs in vacuole. 

Other structures in plants with hydrolytic enzymes include spherosomes (a membrane bound spherical particle involved in lipid synthesis and storage) and aleurone grains (proteinaceous bodies found in a membrane bound structure present in endosperm and seed cotyledons).

History:

 Initially called as “perinuclear dense bodies” C. de Duve (1955) based on electron microscopic studies. Later renamed as lysosomes (Gr., Lyso=digestive soma=body) indicating its digestive activity. C. de Duve got Nobel Prize (1974) in physiology for the discovery and further works on lysosome.

Location: are common in plant cells and very rare in plant cells and are absent in mature RBC and prokaryotic cells. Phagocytic cells and granulocytes are rich in lysosomes whereas very few in muscle cells and acinar cells of pancreas. 

Isolation and staining properties:

Lysosomes are isolated by sucrose density centrifugation. It can be located inside the cell using toluidine blue or Schiffs reagent. Lysosomes tend to accumulate drugs like Chloroquine (anti-malarial drug) and vital stains such as Evans blue and Neutral red. Enzyme marker for lysosome is acid phosphatase and the technique used is Gomori’s staining technique.

How the cell is protected from the dangerous digestive enzymes inside the lysosome?

Lysosome is a site where approximately 40 different hydrolytic enzymes are packaged (proteases, lipases, nucleases etc), that is why it is called as “suicidal bags of the cell”. These enzymes can be fatal to the cell once outside the lysosomal membrane. But cell has a wonderful mechanism. All these enzymes are active only at pH 5 maintained within lysosomes. The cytoplasmic pH is ~7.2. So even if there is a leakage, the rest of the cell is elegantly protected from these devastating enzyme troops.  

Functions:

• Digestion of extracellular substances taken up by phagosomes or pinosomes and intracellular substances like proteins, glycogen etc to supply energy to the cell
• Autolysis or cellular autophagy:  is a pathological condition where lysosome digests organelles of the cell and ultimately digesting the entire cell. This occurs when a cell dies upon infection.

• Digestion of extracellular substances outside the cell:  Sperm cells discharge lysosomal enzymes that digest the outer membrane of ovum and facilitate its entry. Similarly during bone formation, Cathepsin D, a lysosomal enzyme is released by fibroblast to breakdown the connective tissue.

Steps in lysosome formation

Steps Involved in Lysosome Formation

1.     The ER and Golgi apparatus make a lysosome

2.     The lysosome fuses with a digestive vacuole

3.     Activated acid hydrolases digest the contents

      Lysosomes associated diseases include:

 Inborn diseases like (I-cell disease, Tay-Sach’s disease), silicosis.

Extra points:

Lysosomal proteins are tagged with mannose -6-phosphate (M-6-P) in the cis golgi apparatus.  M-6-P groups are identified by the M-6-P receptors (transmembrane protein) in the trans golgi apparatus and targeted to the lysosome efficiently.