Histones efficiently activate FSAP in serum and are subsequently degraded. (A) Nucleosomes (100 µg/mL) were preincubated with or without benzonase nuclease (5000 U/mL) at 37°C for 1 hour to digest nucleosome DNA before incubation with serum (50%) from 5 healthy donors at 37°C for 1 hour. As a readout for activation of FSAP in serum, complexes of FSAP with AP were determined by ELISA. (B) Nucleosomes that had been preincubated with or without benzonase were incubated with increasing concentrations (0% to 40%) of healthy donor serum, FSAP-deficient serum, FSAP-depleted serum, or mock-depleted serum at 37°C for 1 hour. Total histones and nucleosomes present in the samples were precipitated using Poros HS resin and separated by SDS-PAGE, and histone H3 proteolysis was assessed using anti–histone H3 antibody on immunoblot. Data are expressed as mean ± standard of the mean of 3 independent experiments. Representative blots of 3 independent experiments are shown. Arrows indicate intact histone H3 and a cleavage product of histone H3. **P < .01 (calculated using a paired 2-tailed Student t test).