A plasma nucleosome releasing factor (NRF) with serine protease activity is instrumental in removal of nucleosomes from secondary necrotic cells

We observed that interaction of secondary necrotic (sn) cells with human serum or plasma leads to loss of DNA staining. The decrease turned out to be a result of nucleosome release and was specific for apoptotic cells as necrotic cells did not show this phenomenon. We named this activity in plasma nucleosome releasing factor (NRF). NRF activity was completely inhibited by trypsin inhibitors suggesting that a serine protease is involved. Upon testing a number of plasma candidate serine proteases we found that plasmin did have NRF activity. However, plasminogen-deficient plasma still had NRF activity indicating that NRF is not plasmin. We conclude that a yet unidentified plasma serine protease is involved in removal of nucleosomes from sn cells.