To lyse is to break apart a larger particle into smaller pieces. Lysis, or the process of lysing, can occur both inside and outside of the cell. While localized lysis can result in a tiny puncture of a cell wall or cell membrane, harsher chemical lyses result in the expulsion of all cellular contents and cell death. A variety of lytic mechanisms exist, and can result in the disintegration of either cellular or extra-cellular components.
Lysis is an important process in many biological contexts, including the digestion of food by an organism, viral replication, immune defense against bacteria and viruses, and biofilm development. It is also used by bacteria, viruses and animal cells to invade other cells.
In humans, the enzyme lysozyme is found in saliva and lyses bacteria. Additionally, antibiotics such as penicillin synthesize β-lactamase, an enzyme that attacks the β-lactam ring of bacteria. This causes destruction of the bacteria cell wall and results in the bacteria’s death. Bacteriophages, viruses that infect bacteria, produce lytic enzymes which break open bacterial cells and permit the transfer of viral particles into new bacteria host cells.
In the laboratory, protocols that aim to isolate specific cellular components, such as organelles, proteins, RNA, and DNA, lyse cells and to destroy all components that are not the desired target. Mechanical disruption via freeze-thaw cycles, filtration, pressure, sonication, and vibration, all result in the breaking open of cells. In order to prevent the destruction of the target during the lytic processes, these methods are used in combination with detergents, chelating agents and enzyme inhibitors that protect the target molecule of interest from destruction. The resulting suspension is referred to as the lysate. In order to extract the target molecule, lysates often undergo additional purification steps.
Types of Lysis
Autolytic or self-destruction of a cell usually occurs in injured or dying cells. Autolysis begins with the cell’s own lysosomes releasing digestive enzymes into the cytoplasm. Having literally ‘eaten itself,’ the cell dies as a result.
Cytolysis, also called osmotic lysis, occurs when the concentration of solute in the extra-cellular space is hypotonic, or lower than the concentration of solute in the intra-cellular space. The difference in the concentrations of solutes results in an osmotic pressure gradient. Unlike firm plant cell walls, the delicate phospholipid bilayer of animal and protozoan cell membranes cannot withstand high pressure gradients. As a result, the osmotic imbalance causes water to rush into the cell causing the cell to burst open and die.
Cytolysis is used by the immune system to target pathogenic bacteria. For example, when a cell is resisting a bacterial infection, it can activate its innate immune response and form transmembrane channels on the surface of pathogenic bacterial cells. This causes osmotic lysis of bacteria, allowing the cell to clear the infection.
Hemolysis is the lysis of red blood cells and the expulsion of their contents into the surrounding fluid. Autoimmune disorders, genetic disorders, Gram-positive bacteria, hypotonic plasma, and parasites can all cause hemolysis.
Oncolysis is the destruction of tumor cells. A person undergoing chemotherapy for the treatment of cancer will be treated with drugs that result in the destruction or lysis of tumor cells.
Ionizing radiation breaks chemical bonds such as DNA and results in the formation of highly reactive free radicals. Depending on the amount of exposure, radiolysis can produce an abundance of toxic free radicals inside the cell, which results in lysis.
Plasmolysis is the reverse of cytolysis and often occurs in plant cells, as they are more sensitive to plasmolysis and more resistant to cytolysis. While cytolysis causes the cell to burst, plasmolysis causes the cell to shrink. Plasmolysis occurs when the concentration of solute in the extra-cellular space is hypertonic or higher than the concentration of solute in the intra-cellular space. The osmotic imbalance causes water to rush out of the cell. If the extracellular space remains hypertonic, the difference in pressure leads to a collapse of the cell wall. A wilted plant is an example of extensive plasmolysis in its cells.
Examples of Lysis Disorders
Hemolytic Disease of the Newborn
HDN as it is sometimes called, is caused by the mother’s immune response to a fetal blood antigen. Sometimes, if the baby’s blood contains an antigen that the mother’s blood does not, her immune system will see the baby’s antigen as foreign and respond by trying to eradicate it as if it were a dangerous pathogen. Some immunoglobulins pass from her blood stream, through the placenta to the baby’s bloodstream and attack the antigen of the baby’s red cells, resulting in hemolysis.
Tumor Lysis Syndrome
Tumor lysis syndrome is a potentially lethal complication of chemotherapy (anti-cancer treatment). It occurs when many cancerous cells are killed off at one time. This leads to the massive dumping of intracellular ions and metabolic derivatives into the bloodstream, which can result in kidney failure.
Lysosomal Storage Disease
Lysosomes organelles filled with critical enzymes that digest bulky molecules. Once digested, lysosomes transfer these sacks of broken-up fragments on to other cellular compartments for recycling or excretion. If one of these critical enzymes is absent or malfunctioning due to a genetic mutation, large undigested molecules accumulate inside the cell, which leads to cellular stress and autolysis. The proper functioning of the lysosome is critical. Hence, people suffering from lysosomal storage disorders have very short life spans.