De Novo and Repair Replication of DMA in Liver of Carcinogen-treated Animals

The effect of certain carcinogens on DNA replication in the intact animal was studied for the determination of whether repair replication was induced and whether there was a change in the rate of de novo replication. Carcino gens were selected to include those known to react with DNA and those for which there was apparently no pre vious evidence for reaction with DNA in vivo. The method used depends on the increase in nuclear size that occurs in replicating cells. Replicating and nonreplicating diploid and tetraploid nuclei were fractionated in a sucrose gra dient in a zonal rotor. Incorporation of [mef/iy/-3H]thymidine into replicating nuclei measured de novo replication of DNA, and hydroxyurea-resistant incorporation into nonreplicating nuclei measured repair replication. Diethylnitrosamine, ethyl methanesulfonate, aflatoxin, and retrorsine were shown to induce DNA repair replica tion in vivo but not to alter de novo synthesis two hr after injection. Carbon tetrachloride and ethionine also induced repair replication but only after a delay period. This suggests that the repair was that of damage caused by an indirect mechanism, such as by deoxyribonuclease activ ity resulting from lysosomal damage (carbon tetrachlo ride) or from nonenzymic reaction of DNA with a metabo lite of the carcinogen that is slow to accumulate in liver (S-adenosylethionine after ethionine). Thioacetamide did not cause detectable repair replication, a result that correlates with the lack of evidence for a reaction between thioacetamide and DNA.