Tenase

Tenase is a term used in hemostasis to describe enzyme complexes that activate factor X to factor Xa, a pivotal step in the coagulation cascade. There are two canonical forms: intrinsic tenase and extrinsic tenase.

The intrinsic tenase complex consists of activated factor IX (IXa) and activated factor VIII (VIIIa) as a calcium-dependent assembly on a phospholipid surface, typically provided by activated platelets. This IXa–VIIIa complex cleaves and activates factor X to Xa with high efficiency. The extrinsic tenase complex is the tissue factor–bound factor VIIa complex (TF–VIIa) on cell surfaces, also requiring calcium and phospholipids; TF–VIIa activates factor X to Xa, providing a rapid initiation of coagulation following vascular injury. In both cases, the generated factor Xa then combines with factor Va on a phospholipid surface to form the prothrombinase complex, which converts prothrombin into thrombin and leads to fibrin formation and clot stabilization.

Clinical relevance stems from the need for functional tenase activity. Deficiencies of factors VIII or IX impair intrinsic tenase and contribute to hemophilia A and B, respectively, while deficiencies in factor VII affect extrinsic tenase and the initiation of coagulation. Laboratory assays such as the activated partial thromboplastin time (aPTT) and prothrombin time (PT) reflect the status of these pathways and their respective tenases.