Granzymes are serine protease

Serine protease

Serine proteases are enzymes that cleave peptide bonds in proteins, in which serine serves as the nucleophilic amino acid at the active site.They are found ubiquitously in both eukaryotes and prokaryotes...

s that are released by cytoplasmic granules within cytotoxic T cell

Cytotoxic T cell

A cytotoxic T cell belongs to a sub-group of T lymphocytes that are capable of inducing the death of infected somatic or tumor cells; they kill cells that are infected with viruses , or are otherwise damaged or...

s and natural killer cell

Natural killer cell

Natural killer cells are a type of cytotoxic lymphocyte that constitute a major component of the innate immune system. NK cells play a major role in the rejection of tumors and cells infected by viruses...

s. Their purpose is to induce apoptosis

Apoptosis

Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

 within virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

-infected cells, thus destroying them.

Cytotoxic T cells and natural killer cells release a protein called perforin

Perforin

Perforin-1 is a protein that in humans is encoded by the PRF1 gene.- Function :Perforin is a cytolytic protein found in the granules of CD8 T-cells and NK cells. Upon degranulation, perforin inserts itself into the target cell's plasma membrane, forming a pore. The lytic membrane-inserting part...

, which attacks the target cells. Researchers used to think that perforin creates pores within the cell membranes, through which the granzymes can enter, inducing apoptosis. However, new evidence indicates that a multimeric complex (Granzyme B

GZMB

Granzyme B is a serine protease that in humans is encoded by the GZMB gene. Granzyme B is expressed by cytotoxic T lymphocytes and natural killer cells.-External Links:* The MEROPS online database for peptidases and their inhibitors:...

, perforin, and granulysin

Granulysin

Granulysin is a substance released by cytotoxic T cells when they are attached to infected body cells. It functions to create holes in the target cell membrane and destroy it...

) can enter a cell through the mannose 6-phosphate receptor

Mannose 6-phosphate receptor

In the fields of biochemistry and cell biology, mannose 6-phosphate receptors are proteins that bind newly synthesized lysosomal hydrolases in the trans-Golgi network and deliver them to pre-lysosomal compartments. There are two different MPRs, one of ~300kDa and a smaller, dimeric receptor of...

 (or another receptor found in tumor cells) and is enclosed in a vesicle (a sac). Perforin then allows GrB to pass through the vesicle surface and into the cell, causing apoptosis by various pathways.

They do so by cleaving caspase

Caspase

Caspases, or cysteine-aspartic proteases or cysteine-dependent aspartate-directed proteases are a family of cysteine proteases that play essential roles in apoptosis , necrosis, and inflammation....

s (especially caspase-3), which in turn activates caspase-activated DNase. This enzyme degrades DNA, thus inducing apoptotic cascades. Also, GrB cleaves the protein Bid, which recruits the protein Bax and Bak to change the membrane permeability of the mitochondria, causing the release of cytochrome c

Cytochrome c

The Cytochrome complex, or cyt c is a small heme protein found loosely associated with the inner membrane of the mitochondrion. It belongs to the cytochrome c family of proteins. Cytochrome c is a highly soluble protein, unlike other cytochromes, with a solubility of about 100 g/L and is an...

 (which is one of the parts needed to activate caspase-9 via the Apoptosome), Smac/Diablo and Omi/HtrA2 (which suppress the inhibitor of apoptosis proteins(IAPs)), among other proteins. As well, GrB is shown to cleave many of the chemicals responsible for apoptosis without the aid of caspase, as proven by experiments on caspase knockout mice CTL cells incubated with other cells.

Granzyme secretion can be detected and measured using Western Blot

Western blot

The western blot is a widely used analytical technique used to detect specific proteins in the given sample of tissue homogenate or extract. It uses gel electrophoresis to separate native proteins by 3-D structure or denatured proteins by the length of the polypeptide...

 or ELISA

ELISA

Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...

 techniques. Granzyme secreting cells can be identified and quantified by flow cytometry

Flow cytometry

Flow cytometry is a technique for counting and examining microscopic particles, such as cells and chromosomes, by suspending them in a stream of fluid and passing them by an electronic detection apparatus. It allows simultaneous multiparametric analysis of the physical and/or chemical...

 or ELISPOT

ELISPOT

The Enzyme-linked immunosorbent spot assay is a common method for monitoring immune responses in humans and animals. It was developed by Cecil Czerkinsky in 1983....

. Alternatively, granzyme activity can be assayed by virtue of their protease activity.