Active-site titration of horse urinary kallikrein.

Horse urinary kallikrein was titrated with the reagent 4-nitrophenyl 4-guanidino-benzoate. The titration was shown to be dependent upon the concentration of the titrant. This finding, which distinguishes horse urinary kallikrein from other enzymes, is explained by the relatively small ratio between its rate of acylation and deacylation (k2/k3 = 16.8) and by a low affinity of the reagent (Km = 1.16 microM). By an appropriate kinetic treatment, it was possible to establish the relationship between the hydrolysis of 4-nitrophenyl 4-guanidinobenzoate and the actual concentration of the active enzyme.