Peptide esters and nitroanilides as substrates for the assay of human urinary kallikrein

Ac-Phe-ArgOMe is hydrolyzed much faster than are Bz-ArgOEt, Z-ArgOMe, or Ac-Gly-ArgOMe by the kallikrein from human urine. The synthesis of Ac-Phe-ArgOEt is described. Hydrolysis of this substrate can be conveniently monitored by a coupled spectrophotometric procedure. Increase in absorbance is linear with time and proportional to the amount of kallikrein up to a deltaA366 of at least 0.22/10 min. This assay for human urinary kallikrein is 46-fold more sensitive than that based on Bz-ArgOEt and 38-fold more sensitive than that with D-Val-Leu-Arg-p-nitroanilide. A number of other arginine p-nitroanilides are hydrolyzed by this enzyme at still lower rates. The assay of human urinary kallikrein with D-Val-Leu-ArgOEt is about a factor of two less sensitive than the assay with Ac-Phe-ArgOEt. This also holds for Z-TyrONp, which displays a rapid spontaneous hydrolysis. Furthermore, the rate of the enzymic reaction with Z-TyrONp drops off rapidly.