98 J. N. DAVIDSON 



pronounced 48 hr. after operation [3, 4, 6, 7, 11, 13, 19]. The polymerase 

 activity is also increased. This behaviour of the thymidylate kinase system 

 is in striking contrast to that of the kinases phosphorylating dAMP, 

 dGMP and dCMP which are very little elevated in regenerating, as 

 compared with normal, liver [3, 14]. 



A detailed study of the three kinases in regenerating rat liver at various 

 times after partial hepatectomy shows that the appearance of the enzymes 

 tends to be sequential, TdR and TMP kinases appearing before TDP 

 kinase [13]. 



Fig. 2. Per cent conversion of thymidine to TTP by kinases present in 

 extracts of various rabbit tissues. 



By 24 hr. the TdR and TMP kinases have begun to increase sharply, 

 reaching maximum activity after about 30 hr. The activity of TDP kinase 

 develops more slowly, reaching its peak at about 48 hr., which is also the 

 time of maximum incorporation of pH-]TdR into DNA in the overall 

 reaction. Subsequently all kinases decline in activity. 



A similar sequential pattern of kinase appearance is also found in 

 cultures of the L strain of mouse subcutaneous fibroblasts [11, 13]. Such 

 cells in the resting phase after exhaustion of the medium show either no 

 kinase activity or only TdR kinase activity. On inoculation into fresh 

 medium a period of rapid growth occurs after a lag phase. The TdR 

 kinase appears early, rises sharply and remains elevated. The TMP kinase 



