A simple activity assay for thrombin and hirudin.

Cloning of the thrombin cDNA has made it possible to study thrombin function by site-directed mutagenesis. Quantitative results from studies of thrombin mutants are often hindered by difficulties in assaying the enzyme activity. The high enzyme concentrations required for activity determination by standard methods limit their usefulness to thrombin mutants that cannot be readily produced in large quantities. We have developed a novel method using the synthetic substrate S-2238 and hirudin, a tight-binding inhibitor of thrombin, that allows for the active-site titration of thrombin at concentrations as low as 20 pM, with an error of < or = 5%. In addition, hirudin activity can be determined by this method to concentrations as low as 40 pM, with an error of < or = 5%.