When treated with a cytotoxic compound, living cells may face one of two fates. They could either stop growing and dividing, or die through either of two distinct processes - necrosis or apoptosis. Basically, cells undergoing necrosis (accidental cell death) swell and lose membrane integrity before shutting down and releasing their intracellular contents into the surrounding environment. This type of cell death is usually triggered by external factors such as toxic chemical or traumatic physical events.
On the other hand, cells undergoing apoptosis (normal or programmed cell death) go through a series of well-defined events such as the shrinking of the cytoplasm, cleavage of DNA into smaller fragments, etc. before being engulfed by white blood cells.
When the cell membranes are compromised or damaged in any way, lactate dehydrogenase (LDH), a soluble yet stable enzyme found inside every living cell, is released into the surrounding extracellular space. Since this only happens when cell membrane integrity is compromised, the presence of this enzyme in the culture medium can be used as a cell death marker. The relative amounts of live and dead cells within the medium can then be quantitated by measuring the amount of released LDH using a colorimetric or fluorometric LDH cytotoxicity assay.
Other enzymes such as adenylate kinase and glucose-6-phosphate may also be used to measure cytotoxicity but these enzymes are not stable and lose their activity during cell death assays.
