Suppression of nuclear ADP-ribosyltransferase activity in Ehrlich ascites tumor cells by 5-azacytidine. Modification of DNA as a cause of suppression.

Exposure of Ehrlich ascites tumor cells to 5-azacytidine for 5 h resulted in a partial loss of ability of DNA to stimulate ADP-ribosyltransferase activity, as assessed in a reconstituted in vitro enzyme system consisting of purified calf thymus enzyme, calf thymus whole histone and DNA isolated from the cells. The degree of suppression in vitro varied depending on the amount of histone and DNA added and it reached a maximum with a value of 83% and 62% of control for DNAs from cells exposed to 10 microM and 30 microM 5-azacytidine, respectively, at a histone/DNA mass ratio of 0.4. In the absence of histone (conditions of auto-ADP-ribosylation of the enzyme), no suppression was detectable.