Gamma Glutamyl Transpeptidase of Sprouted Onion

SUMMARY –An enzyme capable of liberating p‐nitroaniline from γ‐ l ‐glutamyl p‐nitroanilide has been purified 800‐fold from sprouted onions. The enzyme acts optimally at pH 9.0, is activated by amino acids and inhibited by borate and competitively by γ‐glutamyl derivatives. With the p‐nitroanilide as substrate it exhibits a K m of 14.3 mM whereas the K i values with glutathione and γ‐glutamyl‐S‐methyl‐ l ‐cysteine as inhibitors are 0.20 and 2.14 mM respectively. From chromatography of the reaction products and from kinetic considerations it is concluded that the purified enzyme is a transpeptidase (E.C.2.3.2.1) whereas crude onion extract may, in addition, possess a true γ‐glutamyl hydrolase. It is suggested that such enzymes may play a role in the disappearance of peptides in post‐dormant onions and may be useful evoking the full flavor potential of onion.