pBenzyl p-guanidinothiobenzoate hydrochloride was synthesized and demonstrated to be useful for active-site titration of bovine trypsin, bovine thrombin, human lung tryptase, bovine activated protein C, human Factor XIIa fragment and bovine Factor Xa beta. The titration is based on rapid formation of a stable acyl-enzyme with a stoichiometric release of benzyl thiol. Thiol production is measured quantitatively by including 4,4′-dithiodipyridine in the reaction mixture and measuring the increase in absorbance at 324 nm. Ellman9s reagent has also been successfully employed, allowing measurement at 410 nm. Unlike p-nitrophenyl p9-guanidinobenzoate, the thioester titrant reacts slowly with chymotrypsin A alpha thus eliminating interference by this enzyme in most titrations. Advantages of this reagent as a titrant include: flexibility in detection of the released thiol, selectivity between trypsin and chymotrypsin-like enzymes, minimal pH-dependence of the epsilon of the absorbing species, relative stability of the reagent under titration conditions, and high epsilon at pH 7.2 with either 4,4′-dithiodipyridine or Ellman9s reagent. The reagent should prove useful as an alternative to p-nitrophenyl p9-guanidinobenzoate hydrochloride for the determination of active-site concentrations of the enzymes employed, as well as of other related enzymes./p
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