A new (1 → 3)‐β‐D‐glucan‐mediated coagulation pathway found in limulus amebocytes

The amebocytes of horseshoe crab (Limulus) hemolymph contain a coagulation system highly sensitive to bacterial endotoxins (lipopolysaccharides) [ 1,2]. This system seems to participate not only in haemostasis but also in defence against invading microorganisms [3]. In the amebocyte lysate, endotoxin has been reported to mediate directly the activation of a proclotting enzyme resulting in the transformation of coagulation to coagulin gel [4]. However, our recent studies have indicated that both amebocyte lysates from Limulus polyphemus and Tachypleus tridentatus contain at least two components beside coagulogen, all of which are associated with the coagulation system [5 1. One of them is a new component sensitive to endotoxin, tentatively named factor B, and the other is a component corresponding to the known proclotting enzyme but insensitive to endotoxin. During these studies, we were informed that, in addition to endotoxin, a water-soluble antitumor carboxymethylated (1 + 3)Q-D-glucan (CMPS) activated the Limulus coagulation system and induced the clot formation [ 161. This result prompted us to examine which component associated with the coagulation system is activated by CMPS. We report here that neither factor B nor proclotting enzyme previously identified is sensitive to CMPS but that there exists a third component (tentatively named factor G) sensitive to CMPS, which mediates the activation of the proclotting enzyme to its active form. The results also suggest that the Limulus amebocytes contain two independent coagulation pathways, endotoxin-mediated and (1 + 3)O-D-glucan-mediated pathways, both of which result in the transformation of coagulogen to coagulin. The latter pathway seems to correspond