A sensitive substrate for the clotting enzyme in horseshoe crab hemocytes.

An endotoxin-activated hemocyte lysate from horseshoe crab (Tachypleus and Limulus) was found to hydrolyze specifically BZ-Ile-Glu-Gly-Arg-p-nitroanilide, which was recently introduced as the substrate for assay of the blood coagulation factor, Factor Xa. Further, this amidase activity increased by increasing the concentration of bacterial endotoxin (Salmonella minnesota R595) added to the lysate. Thus, the measurement of the amidase activity in the hemocyte lysate can be very useful to detect and determine the endotoxin.