title: Inhibition of poly(ADP-ribose) polymerase as a possible reason for activation of Ca2+/Mg2+-dependent endonuclease in thymocytes of irradiated rats.
abstract: 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.
mesh_terms: D000818:Animals; D002467:Cell Nucleus / Q000201:enzymology; D004249:DNA Damage; D004706:Endodeoxyribonucleases / Q000378:metabolism*; D004789:Enzyme Activation; D008297:Male; D009243:NAD / Q000378:metabolism; D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D051381:Rats; D011919:Rats, Inbred Strains; D013950:Thymus Gland / Q000201:enzymology / Q000528:radiation effects*
chemical_list: D000067856:Poly(ADP-ribose) Polymerase Inhibitors; D009243:NAD; D004706:Endodeoxyribonucleases; C029367:calcium magnesium dependent endodeoxyribonuclease