Triapsin, an unusual activatable serine protease from the saliva of the hematophagous vector of Chagas' disease Triatoma infestans (Hemiptera: Reduviidae).

Salivary anticoagulant activities are widely distributed among hematophagous arthropods. Most of them are inhibitors of the serine proteases of the coagulation cascade. Here we show that the saliva of the exclusively hematophagous insect Triatoma infestans, an important vector in the transmission of Chagas' disease, contains an uncommon trypsin-like activity, triapsin. This novel enzyme was purified and characterized. It is a serine protease that is stored as a zymogen in the luminal content of the salivary glands D2. Triapsin is activated by trypsin treatment, or when the saliva is ejected during the insect bite. The enzyme was purified 300-fold from the released saliva by anion exchange chromatography in a HiTrap Q column, followed by chromatography in Phenyl-Superose, and Superdex HR75. The purified triapsin shows an apparent molecular mass of around 40 kDa in non-reduced SDS gels and in sieving chromatography, and 33 kDa in reduced SDS-gels. Its activity is lost after incubation with dithiothreitol indicating that cysteine bridges are essential for activity. Triapsin cleaves gelatin and synthetic substrates showing preference for arginine at P1 residues. The best p-nitroanilide substrate is isoleucyl-prolyl-arginine. It does not cleave bradykinin, angiotensin and other lysine containing substrates. The triapsin amidolytic activity against chromogenic substrates is similar to plasminogen activators, such as urokinase and tissue plasminogen activator. However, it does not activate plasminogen. The fact that triapsin is released at the bite in its active form suggests that it has a role in blood feeding.