Endogenous factor V synthesis in megakaryocytes contributes negligibly to the platelet factor V pool.
OBJECTIVE
Coagulation factor V (FV) is distributed between two pools: 80% circulates in plasma and 20% is stored in platelets. The aim of the study was to determine the origin of platelet FV.


METHODS
We investigated a FV Leiden heterozygous patient who had received an allogeneic bone marrow transplant from a normal donor. The patient had been referred to our laboratory for his marked activated protein C (APC) resistance in the apparent absence of FV Leiden. Analysis of the DNA from a buccal swab showed that the patient was indeed a heterozygous carrier of FV Leiden. The difference in FV genotype between the hepatocytes (heterozygous FV Leiden) and the blood cells (homozygous normal) of the patient provided a good model to investigate the origin of platelet FV. Platelets were isolated from the patient and the bone marrow donor and activated with thrombin and ionomycin to release and activate FV. APC was then added and the inactivation of platelet FVa was followed over time with a highly sensitive prothrombinase-based assay.


RESULTS
While the donor's platelet FVa showed a normal inactivation time course, the patient's platelet FVa was considerably resistant to APC. The kinetic pattern of APC-catalyzed inactivation of the patient's platelet FVa was indistinguishable from that of plasma FVa from a FV Leiden heterozygote.


CONCLUSIONS
These data indicate that platelet FV is derived from plasma and that endogenous FV synthesis by megakaryocytes contributes negligibly to the platelet FV pool.
Thomassen
M Christella  L G D
MC
Department of Biochemistry, Cardiovascular Research Institute Maastricht, Maastricht University, Maastricht, The Netherlands.
Castoldi
Elisabetta
E
Tans
Guido
G
Magdeleyns
Elke J P
EJ
Delaunoit
Christian
C
Debusscher
Louisette
L
Van Assche
Kris J A J
KJ
Rosing
Jan
J
D020016:Activated Protein C Resistance / Q000097:blood; D000328:Adult; D001779:Blood Coagulation Factors / Q000378:metabolism* / Q000502:physiology; D001792:Blood Platelets / Q000737:chemistry / Q000378:metabolism / Q000473:pathology; D001798:Blood Proteins / Q000096:biosynthesis; D004252:DNA Mutational Analysis / Q000379:methods; D004273:DNA, Neoplasm / Q000235:genetics; D004789:Enzyme Activation / Q000502:physiology; D005165:Factor V / Q000096:biosynthesis* / Q000235:genetics / Q000502:physiology; D006580:Genetic Carrier Screening; D006801:Humans; D008297:Male; D008533:Megakaryocytes / Q000378:metabolism* / Q000473:pathology; D017354:Point Mutation / Q000235:genetics / Q000502:physiology; D054198:Precursor Cell Lymphoblastic Leukemia-Lymphoma / Q000097:blood / Q000473:pathology; D011486:Protein C / Q000502:physiology
D001779:Blood Coagulation Factors; D001798:Blood Proteins; D004273:DNA, Neoplasm; D011486:Protein C; C095381:factor V Leiden; C037685:platelet factor V; D005165:Factor V