Polyphosphate (
polyP) is a pro-inflammatory agent and a potent modulator of the human blood-clotting system. The presence of
polyP of 60
phosphate units was identified in rat basophilic
leukemia (RBL-2H3) mast cells using specific enzymatic assays,
urea-
polyacrylamide gel electrophoresis of
cell extracts, and staining of cells with
4,6-diamidino-2-phenylindole (
DAPI), and the
polyP-binding domain of
Escherichia coli exopolyphosphatase.
PolyP co-localizes with
serotonin- but not with
histamine-containing granules.
PolyP levels greatly decreased in mast cells stimulated to degranulate by
IgE. Mast cell granules were isolated and found to be acidic and decrease their
polyP content upon alkalinization. In agreement with these results, when RBL-2H3 mast cells were loaded with the fluorescent
calcium indicator fura-2 acetoxymethyl
ester to measure their intracellular Ca(2+) concentration ([Ca(2+)](i)), they were shown to possess a significant amount of Ca(2+) stored in an acidic compartment different from lysosomes.
PolyP derived from RBL-2H3 mast cells stimulated
bradykinin formation, and it was also detected in human basophils. All of these characteristics of mast cell granules, together with their known elemental composition, and high density, are similar to those of acidocalcisomes. The results suggest that mast cells
polyP could be an important mediator of their pro-inflammatory and pro-
coagulant activities.