[21] Limulus Clotting Factor B

This chapter describes the limulus clotting factor B. Horseshoe crab hemocytes are highly sensitive to gram-negative bacterial endotoxins, which are lipopolysaccharides (LPS); exposure to LPS results in activation of intracellular coagulation systems. In the Japanese horseshoe crab, Tachypleus tridentatus, the coagulation system consists of three intracellular serine protease zymogens—namely, proclotting enzyme, factor B, and factor C, in addition to the fibrinogen-like substance, coagulogen. In the presence of LPS, a LPS-sensitive serine protease zymogen, factor C, is autocatalytically activated. The resulting clotting enzyme converts soluble coagulogen to an insoluble coagulin gel. Like the mammalian coagulation and complement systems, this cascade reaction is propagated by limited proteolysis. These zymogens and coagulogen constitute the cascade pathway responsible for the LPS-mediated activation.