Human platelets secrete the plasma type of platelet-activating factor acetylhydrolase primarily associated with microparticles.
OBJECTIVE
Platelet-activating factor acetylhydrolase (PAF-AH) expresses a Ca2+-independent phospholipase A2 activity and hydrolyzes platelet-activating factor as well as oxidized phospholipids. Two major types of PAF-AH have been described: the plasma type, which is associated with lipoproteins, and the intracellular type II PAF-AH.


RESULTS
We investigated the type(s) of PAF-AH expressed in human platelets as well as the mechanism and the enzyme type secreted from platelets during activation. The majority of the enzyme activity (75.1+/-14.3% of total) is found in the cytosol, whereas 24.9+/-7.3% is associated with the membranes. Immunofluorescence microscopy studies and Western blotting analysis showed that platelets contain the plasma type as well as the intracellular type II PAF-AH. Furthermore, platelets contain high levels of the mRNA of plasma PAF-AH, whereas only a small quantity of the type II PAF-AH mRNA was detected. On activation, platelets secrete the plasma type of PAF-AH mainly associated with platelet-derived microparticles (PMPs). The enzyme activity was also detected on circulating PMPs in plasma from normolipidemic healthy subjects.


CONCLUSIONS
This is the first indication that in addition to lipoproteins, PAF-AH in human plasma is carried by PMPs, suggesting that the PMP-associated PAF-AH may play a role in the dissemination of biological activities mediated by these particles.
Mitsios
John V
JV
Laboratory of Biochemistry, Department of Chemistry, University of Ioannina, Ioannina, Greece, 45110.
Vini
Maria P
MP
Stengel
Dominique
D
Ninio
Ewa
E
Tselepis
Alexandros D
AD
D043203:1-Alkyl-2-acetylglycerophosphocholine Esterase / Q000097:blood* / Q000235:genetics; D001792:Blood Platelets / Q000201:enzymology* / Q000648:ultrastructure*; D003600:Cytosol / Q000201:enzymology; D006801:Humans; D007425:Intracellular Membranes / Q000201:enzymology* / Q000648:ultrastructure*; D007527:Isoenzymes / Q000097:blood / Q000235:genetics; D010316:Particle Size; D054467:Phospholipases A2; D015539:Platelet Activation; D012333:RNA, Messenger / Q000097:blood
D007527:Isoenzymes; D012333:RNA, Messenger; D054467:Phospholipases A2; D043203:1-Alkyl-2-acetylglycerophosphocholine Esterase