Human blood coagulation factor XI. Purification, properties, and mechanism of activation by activated factor XII.

Human blood coagulation Factor XI was purified in 20% yield from plasma by ion exchange chromatography. The specific clotting activity of purified Factor XI was 250 & 20 units/mg implying that its concentration in normal titrated human plasma is 4 pg/ml. The purified Factor XI in its precursor form gave a single protein band on polyacrylamide gels in the presence of sodium dodecyl sulfate with an apparent molecular weight of 160,000 in the absence of the reduction of disulfide bonds, whereas in the presence of mercaptoethanol a single protein band at M, = 83,000 was observed. Proteolytic activation of ‘2”I-Factor XI by a mixture of purified Factor XII, prekallikrein, high molecular weight kininogen and kaolin was demonstrated. The appearance of activated Factor XI activity was directly correlated with the extent of cleavage of lzLI-Factor XI. The cleaved activated ?-Factor XI retained its apparent molecular weight of 160,000 in the absence of reducing agents on sodium dodecyl sulfate polyacrylamide gels, whereas reduction of activated 1Z51-Factor XI yielded M, = 50,000 and 33,000 fragments. When normal human plasma containing ‘V-Factor XI was subjected to contact activation by kaolin, the Y-Factor XI gave similar molecular weight profiles on sodium dodecyl sulfate gels in the absence and presence of reducing agents. These results show that the activation of human Factor XI both by purified activated Factor XII and by the contact activation system in plasma involves limited proteolytic cleavage of Factor XI yielding polypeptide chains of IV, = 50,000 and 33,000 which are held together by disulfide bonds.