[Participation of the NAD-poly(ADP ribose) system in the degradation of chromatin in irradiated thymocytes].

The diminution of NAD level in mouse thymus lymphocytes precedes their death under the effect of various genotoxic agents and manifests itself by the time of the onset of chromatin degradation. At the same time, in vitro, NAD does not influence the activity of micrococcus nuclease of Ca2+,Mg2+-dependent endonuclease from human spleen. Stimulation of protein poly(ADP-ribosylation) by exogenous NAD does not change the sensitivity of chromatin to micrococcus nuclease. In contrast to hepatocytes, in the thymus, no inhibition of Ca2+,Mg2+-endonuclease, resulting from ADP-ribosylation, occurs which may be due to low activity of ADP-ribosyl transferase in thymocytes. Incubation of thymus lymphocytes with benzamide prior to irradiation does not inhibit chromatin degradation. It is suggested that the decrease in the NAD level is one of the indications of the injury to thymocytes which is not related to the induction of their death. In contrast to thymocytes, the pretreatment of Ehrlich ascites tumor cells with benzamide produces a radiosensitizing effect.