Letter: On the source of "minor thymine" in DNA from a Novik off rat hepatoma cell line

To determine if a fraction of DNA-thymine might derive from deamination of DNA-5-methylcytosine in polymeric DNA, the occurrence of tritiated DNA-thymine residues was measured in DNA isolated from Novikoff rat hepatoma cells incubated with varying levels of unlabeled thymidine and with G-³H-labeled deoxycytidine, unlabeled deoxyguanosine, deoxyadenosine, adenosine, serine, glycine and amethopterin. At 0 and 10^?5 M-thymidine, significant incorporation of label was recovered as DNA-thymine. At higher levels of thymidine (5× 10^?5m to 10^?3m) label in DNA-thymine progressively decreased to levels that could be accounted for by deamination of DNA-thymine by the two different two-dimensional Chromatographic systems used to separate free DNA bases. “Minor thymine” in DNA of Novikoff hepatoma cells is thus generated by incomplete amethopterin blockade, insufficient dilution by exogenous thymidine and methodological deamination.