Platelet polyphosphate exists in 2 pools and displays nanoparticle-like behavior. (A) Gel electrophoresis of soluble platelet polyphosphate (phenol/chloroform [Phen/Chl]) or total platelet polyphosphate (ion exchange [Ion exe]). Polyphosphate (1 nmol/lane; expressed as monophosphate units) was separated on 10% polyacrylamide TBE-urea (7 M) gels and visualized by DAPI-negative staining. PSP (10 U/mL) indicates phosphatase treatment prior to separation. Synthetic polyphosphate preparations with polymer sizes of 39, 97, and 383 phosphate units served as molecular size standard. A representative image for 3 separate experiments is shown. (B) Total platelet polyphosphate (Ion exc; 1 nmol/lane, expressed as monophosphate units) from 3 individual donors. (C) Kallikrein-like activity, triggered by platelet polyphosphate (PolyP), from 2 different sources (20 μM; expressed as monophosphate units). ΔFXII plasma indicates congenital FXII-deficient plasma. (D) Fractionation of SYTOX-supplemented platelet lysate by ultracentrifugation on sucrose density gradients. Data show normalized means ± standard deviation of 3 independent fractionation studies. (E) Microscopic images (acquired at ×1000 magnification) of representative samples that were taken from fractions, indicated in panel D by roman numerals I-III. Polyphosphate was detected with DAPI (blue) and SYTOX (orange). DIC, differential interference contrast microscopy; OD₄₀₅, optical density at 405 nm.