Inhibition of Poly(ADP-ribose) Polymerase as a Possible Reason for Activation of Ca²⁺/Mg²⁺-dependent Endonuclease in Thymocytes of Irradiated Rats

Summary

The molecular mechanism of activation of Ca²⁺/Mg²⁺-dependent endonuclease in thymocytes of irradiated rats was studied. Thymocyte nuclei of control and irradiated rats were pre-incubated with NAD under conditions favourable for poly ADP-ribosylation. Pre-incubation results in a decrease in the rate of autolytic DNA digestion by Ca²⁺/Mg²⁺-dependent endonuclease of 6–7- and 2–3-fold for control and irradiated animals, respectively. The activity of Ca²⁺/Mg²⁺-nuclease extracted from the nuclei pre-incubated with NAD is also considerably decreased. The presence of nicotinamide and thymidine in the preincubation medium prevents the suppression of Ca²⁺/Mg²⁺-nuclease activity. In the experiments performed with isolated nuclei and permeabilized thymocytes the synthesis of poly(ADP-ribose) does not significantly change within 1 h after irradiation at a dose of 10 Gy, whereas 2 and 3 h after the exposure it decreases by 35–40 and 45–55 per cent, respectively. The activity of poly(ADP-ribose) glycohydrolase in this period is similar to that in the controls. The average size of the de novo synthesized chains of poly(ADP-ribose) increases from 11 to 17 ADP-ribose units by the second hour after irradiation. Inhibition of poly(ADP-ribose) polymerase in the postirradiation period preceded the internucleosomal fragmentation of chromatin. The results suggest that activation of Ca²⁺/Mg²⁺-nuclease in irradiated thymocytes is accounted for by the disturbance of its poly ADP-ribosylation