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The activation of factor IX represents a transition step between the “contact phase” and the “phospholipid-dependent phase” of intrinsic blood coagulation. A study was made to see if any of the constituents of either of these phases would influence this activation. Trypsin-activated factor XIa and factor IX were preincubated with various combinations of kaolin, high molecular weight kininogen, factor XII and human brain cephalin before being added to factor IX deficient plasma to determine the clotting time. Of these, only cephalin in the preincubation mixture apparently increased factor IXa activity. Measurements of the rate of activity generation with and without cephalin and with early and late addition of cephalin indicated that cephalin caused a five-fold increase in the maximum obtainable factor IXa activity. Late addition started at lower activity and gradually increased to the maximum. Factor IXa, isolated from factor XIa by means of an immobilized factor Xl-antibody column, responded to cephalin addition by becoming more active and to factor XIa addition by becoming less active. Centrifugation of cephalin-protein mixtures showed that neither factor XIa nor factor IX but only factor IXa was bound to the phospholipid. These data suggest that: 1) the activation of factor IX by factor XIa does not occur on a phospholipid surface; 2) once cleaved, the factor IXa that is produced remains bound to the active site of factor XIa, preventing further factor IX activation until a phospholipid surface is available for the reaction sequence to continue; 3) phospholipid competes with factor XIa for the bound factor IXa and these, in a time-dependent interaction, then form an active complex. This mechanism would prevent the production and dissemination of large amounts of factor IXa until required for further clotting.

Control Of Factor IXa Production By The Formation Of A Factor XIa-Factor IXa Complex