Differential effects of Ca2+ and Mg2+ on endonuclease activation in isolated promyelocytic HL-60 cell nuclei

In autodigestion assays, endonuclease activity in non apoptotic HL 60 promyelocytic leukemia cell nuclei cleaved the chromatin of the autologous cells to an oligonucleosomal length pattern. Both EGTA and EDTA inhibited the activation of endonuclease activity in isolated HL 60 cell nuclei. The inhibition by EDTA could be reversed by exogenous Ca 2+ , but not by exogenous Mg 2+ . In Ca 2+ /Mg 2+ free nuclei digestion buffer, addition of Ca 2+ (1\_10 mmol/L) induced endonuclease activity in the isolated nuclei, while addition of Mg 2+ had no effect. In the presence of Ca 2+ (0.1 mmol/L), endonuclease activity was enhanced by exogenous Mg 2+ (0.1\_10 mmol/L). These results suggest that the endonuclease responsible for internucleosomal DNA fragmentation in HL 60 cells during apoptosis is activated by Ca 2+ and further modulated by Mg 2+ in the presence of Ca 2+ .