Thymidine kinase in rat tissues during growth and differentiation

Among normal adult tissues, bone marrow, thymus, small intestine and spleen had significant thymidine kinase activity (61, 16.4, 4.0 and 3.8 units/g); in kidney, brain, lung, pancreas, testes and lactating mammary gland the activity was less than 1.0 unit/g. In nine transplanted tumors of different (non-hepatic) tissues of origin there was a direct correlation between thymidine kinase content (ranging from 28 to 1.1 units/g) and growth rate (doubling times ranging from 1 to 23 days). Fetal tissues (with the exception of liver) had similar thymidine kinase activities per gram, and lower activities per cell, than equally fast growing neoplasias. Study of normal organs in the course of development revealed that in spleen the level of thymidine kinase rises swiftly after birth (to 100 units/g) and declines to its low adult level by the 3rd postnatal week, whereas in fetal liver it reaches a peak (86.5 units/g) between the 17th and 19th days of gestation and then decreases rapidly. Thus, immature liver and spleen, at the time when engaged in erythropoietic activity, contain more thymidine kinase than any of the normal or neoplastic tissues tested.