. Biological structure and function; proceedings. Biochemistry; Cytology. 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
. Biological structure and function; proceedings. Biochemistry; Cytology. 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 purin
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