CHARACTERIZATION OF PURIFIED GAMMA-GLUTAMYL-TRANSPEPTIDASE IN ONIONS - EVIDENCE FOR IN-VIVO ROLE AS A PEPTIDASE

gamma-Glutamyl transpeptidase, from sprouting onion bulbs, was purifie d to homogeneity and characterized as a glycoprotein of M(r) 56 700. S tudies to determine the mode of action of purified enzyme were carried out using the synthetic substrate gamma-glutamyl p-nitroanilide. At p H 6.0, equal amounts of two products, glutamic acid and p-nitroaniline , were formed, demonstrating hydrolysis of the gamma-glutamyl substrat e. Between pH 6.0 and 9.0, the ratio of p-nitroaniline to glutamic aci d increased from 1:1 to 17:1 indicating autotranspeptidation of gamma- glutamyl p-nitroanilide. The enzyme showed a wide range of substrate s pecificity for intermediates in the biosynthetic pathway to flavour pr ecursors. gamma-Glutamyl transpeptidase was detected in leaves, roots and bulbs of the growing plant, but not in dormant bulbs. In vivo the enzyme acts as a hydrolase of gamma-glutamyl peptides during the biosy nthesis of flavour precursors.