Labelling of the thymidine and deoxycytidine bases of DNA by [2-14C]deoxycytidine in cultured L1210 cells.

Exposure of cultured L1210 cells to [2-14C]deoxycytidine and analysis or radioactivity incorporated into DNA-pyrimidines revealed that 2.7--5.5-fold more radioactivity is incorporated into DNA-thymine than into cytosine bases. Thus the pathway involving deamination of deoxycytidylate to deoxyuridylate and methylation to thymidylate is highly favored over successive phosphorylation to dCTP. Several modified and endogenous pyrimidines altered the labelling of DNA-thymine and DNA-cytosine with [2-14C]deoxycytidine. 3-Deazauridine at 0.1 mM caused a 56% increase in the labelling of DNA-thymine. Both thymidine and 3-deazauridine (greater than or equal to 10 microM) increased the specific activity of DNA-cytosine by 4-fold. Cytosine arabinoside (ara-C) (greater than or equal to 10 microM) reduced the labelling of both DNA-cytosine and DNA-thymine. Excess cytidine (0.1 mM) reduced the labelling of DNA-cytosine by 40%. Tetrahydrouridine at concentrations up to 1 mM had no effect.