ENZYMIC FORMATION OF DEOXYRIBONL'CLEIC ACID 



107 



inhibition was located in the reductive step, as shown by the results of 

 Fig. 5. Our interpretation of these results is the following: the addition 

 of ATP + Mg ^ ^ to this crude enzyme system resulted in the phosphoryla- 

 tion of CAIP to CDF and CTP. As found by paper chromatography an 



Fig. 4. Inhibition of DXA formation from CMP by an ATP-regenerating 

 system. Conditions as in Fig. i. Where indicated 4-5 /tmoles of creatine phosphate 

 + o-i mg. of creatine kinase were added. 



equilibrium between mono-, di- and triphosphates was rapidly established 

 and CDP was the predominating nucleotide. However, when the ATP- 

 regenerating system was added, this resulted in a very efficient phosphoryla- 

 tion of CMP to CTP, and little CDP was left in the system. Since CDP is 



Fig. 5. Inhibition of CMP-reduction by an ATP-regenerating system. 

 Conditions as in Figs, i and 4. The formation of dCMP + deoxycytidine was 

 measured as described earlier [4]. 



the substrate for the reduction, the addition of the ATP-regenerating 

 system greatly decreased ribotide reduction. According to this interpreta- 

 tion our experiments indicate that in vitro the synthesis of DNA from 

 ribonucleotides is dependent on the maintenance of a critical level of 



