Human Factor XI is a plasma glycoprotein which circulates in a non-covalent complex with high molecular weight kininogen (1). The mature molecule is synthesized in the liver. It is a two-chain homodimer with a molecular weight of ~160kD (2,3). It is estimated that 5% of the total mass is attributable to carbohydrate (2). The two identical monomers have molecular weights of 80kD. They are joined together by disulfide bonds. By SDS-PAGE analysis, factor XI appears as a single band both non-reduced (Mr 160kD) and reduced (Mr 80kD).
Factor XI circulates as a zymogen. It requires proteolytic activation to acquire serine protease activity. The conversion of factor XI to factor XIa is catalyzed by factor XIIa. This results in cleavage of the Arg369-Ile370 bond in each monomer (3). Factor XIa consists of two NH2-terminal derived heavy chains and two COOH-terminal derived light chains. All of which are held together by disulfide bonds. Factor XIa participates within the intrinsic pathway of coagulation by catalyzing the conversion of factor IX to factor IXa. A bleeding disorder called plasma thromboplastin antecedent deficiency results from a lack of factor XI procoagulant activity (4,5).
The variable bleeding tendencies observed in factor XI deficient patients do not correlate with either factor XI activity or antigen levels. This latter observation may be related to the ability of the tissue factor/factor VIIa complex to also activate factor IX to IXa.
Storage and Stability:May be stored at 4°C for short-term only. For long-term storage, store at -20°C. Aliquots are stable for at least 6 months at -20°C. For maximum recovery of product, centrifuge the original vial after thawing and prior to removing the cap. Further dilutions can be made in assay buffer.
