Endonuclease Activities Associated with High Molecular Weight and Internucleosomal DNA Fragmentation in Apoptosis

We have used pulsed-field gel electrophoresis and electron microscopy to correlate stages of DNA fragmentation with alterations of nuclear structure during apoptosis. DNA fragmentation occurs in two stages. The first is initiated by a previously undescribed endonucleolytic activity that cleaves DNA into 50- to 300-kb fragments. Electron microscopy showed that this degree of cleavage was sufficient to cause the chromatin to undergo condensation. The second stage of fragmentation is catalyzed by the previously described calcium-magnesium endonuclease. The enzyme activity responsible for the initial fragmentation of DNA was found to be distinct from that causing subsequent internucleosomal DNA cleavage based upon its cation requirements, independence of proteolysis and lack of inhibition by zinc. Both activities were found to preexist in nuclei from thymocytes, liver, HL60, and IL2-dependent CTLL cells. Thus, in apoptosis DNA degradation involves two distinct endonucleolytic activities, with only the first activity being essential for cell death.