Human thrombin receptors are insensitive to thrombin-like snake venom enzymes

Thrombin-like snake venoms enzymes, flavoxobin, and okinaxobin I isolated from Trimeresurus flavoviridis and Trimeresurus okinavensis, respectively, were examined in SH-EP cells and evaluated whether or not they can activate human thrombin receptors. Flavoxobin was almost completely inactive in both assays for phosphoinositide turnover and DNA synthesis. In contrast, okinaxobin I stimulated phosphoinositide turnover in a dose dependent manner, but considerably weakly. The EC₅₀ value was about 100 nM, which was 4,000 times larger than that of α-thrombin. This stimulation was not inhibited by hirudin, an effective inhibitor of α-thrombin. Okinaxobin I also induced a very weak stimulation of DNA synthesis. These results suggest that thrombin-like snake venom enzymes interact with human thrombin receptors in inefficient ways. Weak interactions of the enzymes with thrombin receptor and inhibitor were ascribed to the incomplete formation of a lysine-cation cluster necessary for electrostatic molecular recognition.