PROBLEMS IN POLYNUCLEOTIDE BIOSYNTHESIS 99 



activity rises later but before the TDP kinase, and both enzymes reach 

 their peak of activity during the early part of the growth phase and decline 

 before growth (increase in cell number) has stopped. This pattern can be 

 greatly modified by adding thymidine to the cultures without affecting the 

 growth rates. While there is little effect on the enzymes during the early 

 period of growth, the TdR kinase activity remains elevated for a much 

 longer period in the test cultures and TAIP and TDP kinases rise to a 

 second peak during the period when these activities in the control cultures 

 decline [ii, 13]. These elevations on the kinase activities can be produced 

 only during the growth phase and are apparently due to true enzyme 

 induction. 



Although extracts of normal liver tissue show very little activity in 

 synthesizing DNA they nevertheless contain an active polymerase which 

 can be separated by ammonium sulphate fractionation [14]. The low 



TABLE III 



Effect of Liver Extract on Partly Purified Polymerase and Kinases from 

 Ascites Tumour Extracts [14] 



Polymerase Kinases 



mfx moles TTP^^ incorporated . /x/x moles ester formed 

 into DNA in 2 hr. per mg. in i • 5 hr. per mg. 

 protein protein 



TMP TDP TTP 



Ascites enzymes 4-7 430 830 2130 

 Ascites enzymes 



+ liver extract 04 14 12 o 



activity of whole extracts of liver suggests that they might contain factors 

 which interfere with polymerase or kinase action and this has indeed 

 been found to be the case [12, 14]. Addition of extract of normal rat 

 liver to partly purified polymerase and kinases for the thymidine system 

 prepared from extracts of Ehrlich ascites cells causes a very pronounced 

 drop in activity (Table III). Similarly the addition of extract of normal 

 liver to extract of regenerating liver greatly reduces the polymerase and 

 kinase activity in the latter. The activity of these enzymes in bone marrow 

 is likewise reduced by extracts of normal liver. 



The presence of factors interfering with polymerase and kinase activity 

 is not confined to liver tissue. Factors inhibiting polymerase are pro- 

 nounced in liver, kidney and serum but not in brain or muscle. The 

 phosphorylation of thymidine is strongly inhibited by liver extracts but 

 not by extracts of other tissues so far tested while the formation of TDP 

 and TTP is inhibited by liver and kidney extracts, by serum and to a 



