The lysosomes have sometimes been likened to "The Police Force of the Cell". Even in a place as small as a cell, we need someone to keep things in order. But unlike the police, these lysosomes literally eat things which disturb the natural order of the cell. Lysosomes pick up foreign invaders such as bacteria, food and old organelles and break them into small pieces that can hopefully be used again. If they pick up a really harmful invader, they will eat it up and expel what is left of it out of the cell so that the debris can be removed from the body.

The lysosome is able to do this because it is filled with enzymes. These enzymes are specially made for the lysosome by the rough endoplasmic reticulum and work only at low pH (highly acidic) levels. The reason for this is that the enzymes are so strong that they could eat the whole cell if the lysosome ever let them out. However because they can only work at low pH levels and the rest of the cell has a neutral pH level, they can be neutralized if they accidentally escape from the lysosome.

Electron micrograph of lysosomes.

Lysosomes are produced in the Golgi apparatus

From here they are dispersed throughout the cytosol. Despite the fact that the range of enzymes found in lysosomes are fairly uniform throughout many types of cells they take many forms. For example some are comparatively large (up to 1 um in diameter) whereas other present as small vesicles (25 to 50 nm in diameter). This variation is most evident in macrophages which are cells specialised in ingesting particles and organisms which have invaded the body. Along with similar organelles called peroxisomes they form what is known as the acid vesicle system .

Within the human acid vesicle system there are more than 30 defined acid hydrolases present in various types of lysosome.

Another function of lysosome in the human occurs during fertilization of the egg by the sperm. The head of the sperm cell contains a package of lysosomal material called the acrosome. The enzymes from this are released when the sperm makes contact with an egg and they effectively bore a hole through the cell membrane of the egg allowing the sperm to enter.

Lysosomes also play a key role in destroying old organelles within the cell and thus allow them to be replaced with fresher, more effective ones.
This process is known as autophagy and is accomplished in two stages. Firstly a membrane is donated by the endoplasmic reticulum. This membrane then surrounds the old organelle. Secondly a lysosome fuses with this membrane to form an autophagic vacuole. The lysosome can safely enter it's deadly enzyme contents into this vacuole and destroy the old organelle.
In human liver cells it is calculated that at least one mitochondria undergoes autophagy every 10 minutes. This reflects the highly active nature of the liver as an organ and it's need for a constant supply of fresh organelles.
The electron micrograph below shows a lysosome in the process of destroying a membrane bound mitochondria.

Genetic defects in the production of specific acid hydrolases lead the sufferer to an inability to degrade specific classes of molecule. These molecules which would normally be destroyed then accumulate within the individual. Most of these disorders currently known are passed on as single gene autosomal recessive traits within families.

Examples of these lysosomal storage disorders include:

Acid Maltase Deficiency (Lysosomal Glycogen Storage Disease) which leads to the accumulation of glycogen in muscle tissue.

Tay-Sachs Disease is due to a deficiency in one of the enzymes which breaks down certain types of fat (lipid) called hexosaminidase A. As a result of this deficiency huge amounts of lipids are deposited in neuronal (nerve) tissue and leads to severe brain damage and nervous degeneration. The disease is progressive and terminal resulting in early death around 3 years of age.

Adrenoleukodystrophy is due to a disorder of the peroxisomes which also results in the deposition of huge amounts of lipid material in the brain and spinal cord and the adrenal glands which results in dementia and adrenal failure.

Gaucher's Disease is due to the deficiency of the lysosomal enzyme glucocerebrosidase. The disease results in liver and spleen enlargement and erosion of the long bones such as the femur. If the disease manifests itself in infancy there is also brain damage causing learning disability.

In the past few years the prospects for treatment of lysosomal storage disorders has improved due to the demonstration that the symptoms of Gaucher's disease can be partially alleviated by enzyme replacement using suitably modified forms of the deficient enzyme (glucocerebrosidase). However, how effective the approach will become in the long term remains to be seen.

The miner's disease silicosis results from the uptake of silica fibres from the dusty atmosphere of a coal mine by macrophages and other phagocytic cells in the lungs. These fibres then become enclosed in the lysosomes of these cells but they cannot be digested by the enzymes. Instead they cause the lysosome to leak it's contents in quantities which cannot be neutralized resulting in damage to the tissue of the lungs. The same process occurs in asbestos workers resulting in the disease asbestosis.
Both conditions can be severely debilitating or even fatal.
It is also thought that as we age lysosomes become "leaky" and can cause damage to our own tissues. Rheumatoid Arthritis is thought to occur partly due to damage caused to cartilage cells in the joints by enzymes leaked from lysosomes.

This page last updated Tue Aug 17 16:42:42 BST 2004

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