Hippuryl-His-Leu-OH (N-Benzoyl-Gly-His-Leu) hydrate is a substrate for detecting the activity of angiotensin I converting enzyme. The His-Leu released by Hippuryl-His-Leu-OH hydrate can react with o-phtbaldialdehyde or Fluorescamine (HY-D0715) for fluorescence detection[1][2][3].