Differential effects of ca(2+) and Mg (2+) on endonuclease activation in isolated promyelocytic HL-60 cell nuclei

In autodigestion assays, endonucleaw activity in non-apoptotic HL-60 promydocytic leukemia cell nuclei cleaved the chromatin of he 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 digation 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+) (0.1-10mmol/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+).