Histones enhance thrombin generation in the absence of FV and phospholipids. (A-B) Prothrombin cleavage by FXa in the presence of FVa, H3, or H4 with (A) or without (B) phospholipids (40% phosphatidylcholine, 20% phosphatidylserine, and 40% phosphatidylethanolamine) after 60-minute incubation with calcium (5 mmol/L) at 37°C. (C) Densitometric quantification of prothrombin bands from gels (A and B), presented as mean ± SD from 3 independent experiments. Student t test shows a significant decrease in prothrombin cleavage in the absence of phospholipids (*P < .05). (D) Thrombin generation from FXa activation of prothrombin in the presence of FVa with (+) or without (−) phospholipids (PL). All reactions were performed for 90 seconds in the presence of calcium (5 mmol/L) and terminated by the addition of EDTA (10 mmol/L) (mean ± SD from 3 independent experiments). Student t test shows significant decrease in prothrombin generation in the absence of phospholipids (*P < .05). (E) Thrombin generation during 90 seconds activation of prothrombin by FXa with FVa, H3, or H4 in the presence of calcium (5 mmol/L) and absence of phospholipids (mean ± SD from 3 independent experiments). Student t test shows a significant increase in thrombin generation compared with FVa (*P < .05).