IN COMPARISON TO PROGENITOR PLATELETS, MICROPARTICLES ARE DEFICIENT IN GPIB, GPIB-DERIVED CARBOHYDRATES, GLYCEROPHOSPHOLIPIDS, GLYCOSPHINGOLIPIDS, AND CERAMIDES

Activated blood platelets shed microparticles with procoagulant activi ty that probably participate in normal hemostasis. We have isolated sp ontaneously formed microparticles from human blood and analysed them f or ultrastructure, antigenic profile, and biochemical composition. In transmission electron microscopy microparticles appeared as regular ve sicles with a mean diameter of 300 nn (50-600 nm). In flow cytometry a lmost all microparticles reacted with fluorescein isothiocyanate (FITC ) labeled antibody to platelet glycoprotein complex IIb-IIIa (GpIIb-II Ia) and with FITC annexin V but only 40-50% of microparticles reacted with FITC-antibody to platelet glycoprotein Ib (Gpib). The latter resu lt was confirmed by double labeling of microparticles with FITC-antibo dy to GpIIb-IIIa and phycoerythrin (PE) labeled antibody to GpIb. Larg e microparticles reacted better with anti-GpIb than the small ones. A decreased level of GpIb was also demonstrated by SDS/polyacrylamide ge l electrophoresis of microparticles. Compositional studies indicated, that in terms of cholesterol and protein contents, microparticles rese mbled platelets rather than platelet membranes as previously thought. They are, however, deficient in certain components. Thus, in compariso n to platelets, microparticles had reduced contents of sialic acid (by 56.4%), galactosamine (by 48.2%), glucosamine (by 22.4%), galactose b y (11.8%) and fucose (by 21.6%). Mannose content was increased by 11.8 %. Total phospholipids in microplatelets mere lower by 17.8%. Glycerop hospholipids only were affected with phosphatidylserine being decrease d as much as by 43.2%. Neutral glycosphingolipids, gangliosides and ce ramides in microparticles were reduced by half. We conclude that the b iochemical composition of microparticles probably reflects previous ac tivation of progenitor platelets.