Platelets are known to process human and bovine factor V during secretion and/or membrane
binding. We therefore studied the functional and structural changes produced in human
factor V and Va by purified platelet calpain. A maximum increase in factor V coagulant
activity of 2.5-fold over control incubations was observed for calpain (0.6 u/ml)
at 25°C in comparison with a 10-fold increment for a thrombin (1 u/ml). Thrombin addition
to reactions initiated by calpain resulted in further activation comparable to that
of thrombin alone, while subsequent addition of calpain had no effect on the extent
or pattern of the activation of factor V by thrombin. The cleavage pattern of factor
V produced by these two enzymes are distinctly different. Calpain yields initial components
of 210 kDa and 160 kDa within 1 min. Further digestion of the 210 kDa species give
rise to polypeptides of 150, 140 and 120 kDa by 2 min with and increase in coagulant
activity. The degradation of the 160 kDa polypeptide gives rise to smaller fragments
of 130, 100, 90, and 87 kDa. Immunob lot ting of these fragments with the monoclonal
antibody B10 directed to factor V and the thrombin generated Cl fragments yields results
demonstrating an immunological relationship to the calpain generated components of
210, 160, 140 and 120 kDa. Thus platelet calpain generates a complex cleavage pattern
different from thrombin which may explain the partial activation observed.
