Thymidine kinase in rat liver during development.

1. The activity of thymidine kinase in rat liver supernatant decreased with development to a value in the adult that was 1% of that in the 17-day foetus. 2. The foetal enzyme was more stable than the adult to gel filtration on Sephadex G-25 at 0 degrees . 3. The greater stability of the foetal enzyme to incubation at 45 degrees was attributable to the presence of higher concentrations of nucleotides in foetal liver supernatant. 4. The K(m) values for foetal and adult enzymes were approx. 2.5mum- and 2.1mum-thymidine respectively. 5. The foetal enzyme was more sensitive to inhibition by thymidine triphosphate. 6. The decline in enzyme activity during the neonatal period was correlated with a shift in the enzyme properties from the foetal to the adult type, and may reflect the decrease in the proportion of haemopoietic tissue in the liver.