title: Factor V activity of platelets: evidence for an activated factor V molecule and for a platelet activator.
abstract: This study was prompted by the observation that fresh platelet suspensions--prepared by gel filtration or albumin density gradient centrifugation--possessed only minimal factor V activity, whereas frozen-and-thawed platelet suspensions possessed striking factor V activity. Results of experiments with fresh suspensions suggested that unaltered platelets did not bind plasma factor V. The factor V activity of frozen-and-thawed platelet suspensions was markedly diminished after exposure to a factor V antibody, was not activated by thrombin, and was not associated with an increase in factor V antigen over that found in fresh platelet suspensions. Consequently, disruption by freezing and thawing must have resulted in the appearance of small amounts of an activated factor V molecule in platelet suspensions. Disrupted platelets were shown to activate native factor V, but an interaction between a platelet activator and traces of native factor V in fresh suspensions could not be demonstrated to account for the full activity of frozen-and-thawed suspensions. Apparently, therefore, platelets also contained an activated factor V molecule. Adding collagen, but not adenosine 5'-diphosphate to fresh platelet suspensions increased their factor V activity. Release of an activated platelet factor V molecule after exposure to collagen could represent a physiologically significant early step in hemostasis.
mesh_terms: D000906:Antibodies; D000937:Antigen-Antibody Reactions; D001792:Blood Platelets / Q000378:metabolism*; D002850:Chromatography, Gel; D005165:Factor V / Q000276:immunology / Q000378:metabolism*; D005169:Factor VIII / Q000378:metabolism; D005615:Freezing; D006801:Humans; D011517:Prothrombin Time; D013048:Specimen Handling
chemical_list: D000906:Antibodies; D005165:Factor V; D005169:Factor VIII