Platelet factor 3 (Pf3) together with factor V accelerates the conversion of prothrombin
to thrombin by factor X a. Our method to determine platelet factor 3 activity is based
on the measurement of thrombin formation with the substrate H-D-Phe-Pip-Arg-pNA in
a system containing normal platelet poor plasma and Russells’ viper venom. There is
a linear relationship between the rate of thrombin formation and the concentration
of Pf3. The method is very sensitive. Thus 10 μl of frozen platelet rich plasma(PRP)
or a lipid emulsion prepared from brain give absor-bances > 2.0. When PRP is aggregated
with ADP or collagen in concentrations which give maximal aggregation, no Pf3 activity
whatsoever can be demonstrated. Treatment with kaolin or simply prolonged shaking
of the PRP on the other hand increase the Pf3 activity extensively. Our conclusion
is that aggregation of platelets per se do not make Pf3 available.
