Hippurl-1-histidyl-l-leucine (HHL),ACE (EC 3.4.15.1, from rabbit
lung), Azocasein, L-Lysine p-nitroanilide dihydrobromide (Lys–
pNA) and N-benzoyl-L-tyrosine-p-nitroanilide (NBTpNA) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All other
chemicals were of the highest grade available commercially.