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Previous studies have suggested that factor V in platelets is derived from plasma. In order to test this hypothesis, we studied the subcellular localization, release by aggregating agents, molecular size, and stability of factor V in human platelets. When platelet homoge-nates were fractionated on sucrose density gradient the factor V activity distributed primarily into the granules (0.076 units factor V activity/mg protein) with less amounts in the membrane fraction (0.014 units/mg protein) and virtually none in the dense granules or cytosol. Collagen released 79-3% of total platelet factor V clotting activity while ADP and epinephrine failed to liberate factor V activity, further supporting significant localization in the lysozomal granules rather than the dense granules. Platelet factor V can be solubilized by 0.1% Triton and has an apparent molecular weight of 470,000 as determined by 4% agarose gel filtration. In contrast, plasma factor V in the presence or absence of Triton has a molecular weight of 300,000. Factor V activity from the platelet homogenate exhibited biphasic decay at 37°C, suggesting more than one form, whereas plasma factor V shows a first order decay curve. These data suggest that platelet factor V is predominantly localized in the granules and is an intrinsic constituent of the platelet with different properties from plasma V.

Subcellular Localization and Secretion of Factor V by Human Platelets