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°. 3. The greater stability of the foetal enzyme to incubation at 45° 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. 2·5μm- and 2·1μm-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.