A clotting enzyme associated with the hemolymph coagulation system of horseshoe crab (Tachypleus tridentatus): its purification and characterization.

A clotting enzyme associated with the hemolymph coagulation system of Japanese horseshoe crab (Tachypleus tridentatus) was highly purified from the amebocyte lysate. The method for purification consisted of gel-filtration of the lysate on a pyrogen-free Sepharose CL-6B column and affinity chromatography of the endotoxin-treated clotting enzyme on a benzamidine-Sepharose 4B column. Through these procedures, about 3 mg of the purified enzyme was obtained from 70 ml of the lysate and about 390-fold purification was achieved. The purified preparation was found to give a single major band, respectively, on polyacrylamide-gel disc electrophoresis at pH 3.2 in the presence of 6 M urea and on sodium dodecyl sulfate (SDS)-gel electrophoresis in the presence and absence of 2-mercaptoethanol. It also gave a single symmetrical peak on QAE-Sephadex A-25 column chromatography. The molecular weight of the clotting enzyme was estimated to be approximately 42,000 for the unreduced sample by SDS-gel electrophoresis. For the reduced sample, it was 30,000, suggesting that the protein consists of plural polypeptide chains bridged by disulfide(s). The Tachypleus clotting enzyme was a glycoprotein, as shown by the positive periodic acid-Schiff reaction for the protein band on SDS-gel and the amino acid analysis. The purified clotting enzyme transformed Tachypleus coagulogen to coagulin gel and hydrolyzed a chromogenic peptide substrate, Tos-Ile-Glu-Gly-Arg-p-nitroanilide for Factor Xa, liberating p-nitroaniline. The enzyme was sensitive to DFP and benzamidine. It was also inhibited partially by PCMB. Antithrombin III and alpha 2-plasmin inhibitor (alpha 2-antiplasmin) were very effective inhibitors of this enzyme among ten kinds of naturally occurring proteinase inhibitors tested. The clotting enzyme had a restricted specificity towards protein substrates and activated only prothrombin among plasma zymogens including Factor IX, Factor X, fibrinogen, plasminogen and prekallikrein. The cleavage sites of bovine prothrombin for this enzyme were the same Arg-Thr and Arg-Ile linkages as those for Factor Xa, resulting in the formation of alpha-thrombin. These results indicate that the horseshoe crab clotting enzyme is a Factor Xa-like serine proteinase rather than alpha-thrombin. It seems likely that the Tachypleus clotting enzyme is a prototype of mammalian serine proteinases participating in blood coagulation.