A genetically distinct thymidine kinase in mammalian mitochondria. Exclusive labeling of mitochondrial deoxyribonucleic acid.

A thymidine kinase (EC 2.7.1.21) associated with mitochondria has been demonstrated in tissue culture both in wild type mouse cells and in cells which lack the cytoplasmic thymidine kinase. The mitochondrial thymidine kinase represents approximately 1% of the total cell thymidine kinase activity in wild type cells grown in culture and all of the activity of the mutant cells. The mitochondrial enzyme can be distinguished from the cytoplasmic enzyme on the basis of its K, for thymidine and its localization on the mitochondrial membrane. Furthermore, the presence of the enzyme in the mitochondria of the mutant cells results in exclusive labeling of mitochondrial DNA when the cells are grown in the presence of t3H]thymidine. Labeling studies in the presence and absence of methotrexate (an inhibitor of dihydrofolate reductase) demonstrate that, in the presence of exogenously added thymidine, both the product of the mitochondrial thymidine kinase and thymidine nucleotides synthesized in the cell cytoplasm are incorporated into mitochondrial DNA.