PROPERTIES OF SOME NUCLEAR NUCLEASES OF RAT THYMOCYTES AND THEIR CHANGES IN RADIATION-INDUCED APOPTOSIS

Three nuclease activites have been found and characterized in rat thymocyte nuclear extracts. A Mn2+-dependent nuclease is loosely bound to nuclear components and can be extracted with 0.35 M NaCl. The enzyme is activated by Mn2+ but not by Mg2+, Ca2+, or both. Its molecular mass is 36-40 kDa when measured by gel filtration and 37 kDa by SDS/PAGE. An acidic nuclease is independent of divalent ions, produces DNA strand breaks with 5'-OH ends, its molecular mass is about 37 kDa. Two fractions of Ca2+/Mn2+-dependent nuclease, differing in binding to CM-Sepharose but identical in other respects, are active in the presence of Mn2+ but can be additionally activated by Ca2+. They are inactive in the presence of Mg2+ or Ca2+ but cleave DNA in Ca2+/Mg2+-containing medium. The molecular mass of the enzyme is 22 kDa as determined by both gel filtration and electrophoresis. The dependence of nuclease activities on pH, ions, and sulfhydryl reagents is described. Cycloheximide injection to both control and irradiated animals strongly inhibits the activities of Ca2+/Mn2+-dependent nuclease from thymocyte nuclei separated by chromatography on CM-Sepharose and does not change the activities of Mn2+-dependent and acidic nucleases. Nuclease activity in thymocyte nuclei from irradiated rats is increased in Ca2+/Mg2+-containing and Ca2+/Mn2+-containing media whereas there is no change in the activity of acidic nuclease. Ca2+/Mn2+-dependent nuclease is extracted from thymocyte nuclei of irradiated rats with 0.35 M NaCl but from control nuclei only with 0.5 M NaCl. Possible reasons of labilization of Ca2+/Mn2+-dependent-nuclease binding to the nuclear structures in dying thymocytes are discussed.