Summary
It has recently been proved that, in vitro, red blood cells (RBCs) from patients with
homozygous β-thalassemia behave as procoagulant cells. The procoagulant activity of
β-thalassemia RBCs might be the result of an increased exposure of procoagulant phospholipids
(i. e. phosphatidylserine) in the outer leaflet of the membrane. In order to test
this hypothesis, we compared the catalytic properties of RBCs of patients with β-thalassemia
and homozygous sickle cell disease (SS-RBCs) with that of controls. The catalytic
parameters (Km, kcat) of prothrombin activation by factor Xa were determined both
in the absence and in the presence of RBCs. The turn-over number (kcat) of the reaction
was not modified by normal, SS- or (3-thalassemia RBCs. The Km was lower in the presence
of normal RBCs (mean value: 9.1 µM) than in the absence of cells (26 µM). The Km measured
in the presence of either SS-RBCs (mean value: 1.6 µM) or β-thalassemia RBCs (mean
value: 1.5 pM) was significantly lower compared to normal RBCs (p <0.001). No significant
difference was observed between SS-RBCs and p-thalassemia RBCs. Annexin V, a protein
with high affinity and specificity for anionic phospholipids, inhibited the procoagulant
activity of both SS-RBCs and (3-thalassemia RBCs, in a dose-dependent manner. More
than 95% inhibition was achieved at nanomolar concentrations of annexin V. These results
indicate that the procoagulant activity of both β-thalassemia RBCs and SS-RBCs may
be fully ascribed to an abnormal exposure of phosphatidylserine at the outer surface
of the red cells.
