Summary
The formation of microvesicles from platelets was induced either by activation of
the complement system by a monoclonal antibody to CD9, or by incubation of platelets
with the calcium ionophore A23187. A filter technique to isolate the microvesicles
without plasma contamination is described. The microvesicles contained FXIIIa2 from the platelet cytoplasm which shows that these particles contain significant
amounts of intracellular material. This was shown by the use of crossed immunoelectrophoresis
with rabbit antibodies to total human platelet proteins in the second dimension gel
and polyclonal antibodies against the a- and b-subunit of FXIII in the intermediate
gel. The FXIIIa2 in the microvesicle was found to be functional as an enzyme. To prove this, it was
shown that FXIII in its immunoprecipitate arc could catalyze the incorporation of
monodansylcadaverine into casein as identified by fluorescence of this arc in ultraviolet
light. The observation that the plasma form of FXIII (FXIIIa2b2) was absent from the microvesicles collected by the filtration technique, whereas
it was present in platelet fragments obtained by mechanical disruption by ultrasonication,
indicates that the activation-dependent microvesicles are formed by a true budding
process with the inclusion of intracellular, but not extracellular material.
