A fluorogenic active-site titrant for serine proteases

The synthesis of 3'-(4-guanidinobenzoyloxy)-spiro(isobenzofuran-1(3H),9H-[9H]xan then]-3-one or 3HFGB, an active-site titrant for serine proteases, is described. 3HFGB has two advantages over previously described titrants. It is very sensitive (it can measure serine proteases at the 10(-11)-10(-12) M level), and it can be used at stoichiometric ratios of titrant to protease of unity or less. Moreover, the fluorogenic leaving group, 3'-hydroxyspiro[isobenzofuran-1(3H),9'-[9H]xanthen]-3- one, is suitable for use in a range of esteropeptidase substrates. The use of 3HFGB in the active-site titration of trypsin is described.