On the occurrence of thrombin-like enzymes in mosquitoes

A thrombin-like enzyme has been identified in mosquitoes and partially purified. The enzyme preparation displays on SDS gel electrophoresis a major protein band of about 22 kDa and one minor of about 28 kDa. The enzyme preparation cleaves a synthetic substrate (S-2238) for thrombin with Km 113 mu mol/L (as compared to 3 mu mol/L for thrombin). The enzyme clots fibrinogen and plasma. During activation of fibrinogen, fibrinopeptide A (but scarcely fibrinopeptide B) is released. The enzyme is not inhibited by hirudin and activates (if at all) factor XIII differently from thrombin. Predominantly gamma-dimers an formed in the cross linking process. As compared to thrombin a larger extent of activation is required to induce gelation (clotting) by the mosquito enzyme. At a given clotting time the enzyme produces tighter gel structures than thrombin. In its action the enzyme resembles the snake venom enzyme, batroxobin.