Thymidine is present in the body fluids as a result of degradation of DNA from food and from dead cells.

[6][7] Two different classes of thymidine kinases have been identified[8][9] and are included in this super family: one family groups together thymidine kinase from herpesvirus as well as cellular thymidylate kinases, the second family groups TK from various sources that include, vertebrates, bacteria, the bacteriophage T4, poxviruses, African swine fever virus (ASFV) and Fish lymphocystis disease virus (FLDV).

This complex is more stable and has a higher specific activity than any of the lower molecular weight forms.

[21][22] Recombinant TK1 cannot be activated and converted to a tetramer in this way, showing that the enzyme occurring in cells has been modified after synthesis.

[23] This serves to maintain a balanced amount of TTP available for nucleic acid synthesis, not oversaturating the system.

Mutations in the gene for TK2 lead to a myopathic form of mitochondrial DNA depletion syndrome.

Another reason for TK 2 deficiency may be oxidative stress induced S-glutathionylation and proteolytic degradation of mitochondrial thymidine kinase 2.

The formation of tetramer after modification of thymidine kinase 1 after synthesis enhances the enzyme activity.

Its use for fine regulation of DNA synthesis is suggested to have been established in warm blooded animals after they branched out from the vertebrates.

[79] In spite of errors in the technique, it is still used to determine the growth rate of malignant cells and to study the activation of lymphocytes in immunology.

Its uptake is regulated by thymidine kinase 1, and it is therefore taken up preferentially by rapidly proliferating tumor tissue.

The fluorine-18 radiolabeled fluorothymidine F-18 is therefore useful for PET imaging of active tumor proliferation, and compares favorably with the more commonly used marker fludeoxyglucose (18F).

Hybridomas can be expanded to produce large quantities of immunoglobulins with a given unique specificity (monoclonal antibodies).

One common way to solve this is to use thymidine kinase negative (TK−) tumor cell lines for the fusion.

After fusion, the cells are grown in a medium with methotrexate[87] or aminopterin[88] that inhibit the enzyme dihydrofolate reductase thus blocking the de novo synthesis of thymidine monophosphate.

[89][90][91][92][93] Hybridoma cells can also be isolated using the same principle as described with respect to another gene the HGPRT, which synthesizes IMP necessary for GMP nucleotide synthesis in the salvage pathway.

Molecular combing of DNA fibers can be used to monitor the structure of chromosomes in the budding yeast Saccharomyces cerevisiae.

Other thymidine analogues, for instance Idoxuridine (ATC: J05AB02) act by blocking base pairing during subsequent replication cycles, thereby making the resulting DNA chain defective.

Human thymidine kinase, in contrast, with its more narrow specificity, is unable to phosphorylate and activate the prodrug.

One possible approach would be to use the specificity of the thymidine kinase of poxvirus for the purpose, in a similar way that it is used for drugs against herpesvirus.

The structure of poxvirus thymidine kinases has therefore been determined to find potential antiviral drugs.

This is desirable in case the recombinant gene causes a mutation leading to uncontrolled cell growth (insertional mutagenesis).

This approach has been used to treat cancer in animal models, and is advantageous in that the tumor may be killed with as few as 10% of malignant cells expressing the gene.

Such tumor markers are, for instance, CEA (carcinoembryonic antigen) and AFP (alpha fetoprotein).

An alternative approach tested in Plasmodium falciparum (causing malaria) is the introduction of a TK gene in the parasite genome.

[150] Thymidine kinase levels in serum or plasma have been mostly measured using enzyme activity assays.

In commercial assays, this is done by incubation of a serum sample with a substrate analog and measurement of the amount of product formed.

[168][169][170] Certain non-malignant diseases also give rise to dramatic elevation of TK values in cells and tissue: in peripheric lymphocytes during monocytosis[171] and in bone marrow during pernicious anemia.

[174] Staining for thymidine kinase was found to be a reliable technique for identification of patients with stage 2 breast carcinoma.

The highest number of patients identified was obtained by combination of thymidine kinase and Ki-67 staining.