ENZYMIC FORMATION OF DEOXYRIBONUCLEIC ACID 



109 



(Fig. 6 lower curve). Furthermore, it was found (Fig. 7) that each single 

 deoxvnucleoside triphosphate inhibited DXA formation from CMP to 

 about the same extent. 



It seemed obvious that here again the reductive step was the point of 

 inhibition, and we therefore studied the influence of the diflFerent deoxy- 

 nucleoside triphosphates on the formation of dCMP from CMP, A strong 

 inhibition bv dATP and dGTP was observed, TTP inhibited much less, 

 while dCTP showed almost no effect (Fig. 8). Addition of the purine 

 deoxyribonucleotides thus resulted in about a 50^0 inhibition at an initial 

 concentration of io~^ M. The initial concentrations of ATP and CMP 

 in these experiments were 2-5 x lO"'^ M and 0-4 x 10^ m, respectively. 



Similar experiments were also carried out with guanine nucleotides. 

 It was found that DNA svnthesis from dGMP was stimulated more than 



MxlO^ 



Fig. 8. Effects of deoxvnucleoside triphosphates on the reduction of CMP. 



twofold by an equimolar mixture of dCTP, dATP and TTP. The effects 

 of such a mixture on the incorporation of isotope from GMP into DXA 

 were quite small and inconsistent. When each trip)hosphate was added 

 alone, different types of effects were observed (Fig. 9). Thus DNA 

 formation from GMP was strongly inhibited by the addition of dATP, 

 but was stimulated by the two pyrimidine deoxynucleoside triphosphates. 

 Similar divergent effects were observed when the formation of dGMP 

 from G^MP was investigated (Fig. 10). In this case the effect of dGTP 

 could also be investigated and it was found that this deoxyribonucleotide 

 acted as a strong inhibitor. Thus the results for the GMP^-dGMP trans- 

 formation showed that purine deoxyribonucleotides were inhibitors while 

 pyrimidine deoxyribonucleotides, stimulated the reaction. 



We believe that our results show that in an in vitro system with soluble 

 enzvmes the reduction of both a purine and a pyrimidine ribonucleotide 

 was regulated to a large extent by the levels of deoxynucleotides present 



