Characterization of purified γ-glutamyl transpeptidase in onions: Evidence for in vivo role as a peptidase

Abstract

γ-Glutamyl transpeptidase, from sprouting onion bulbs, was purified to homogeneity and characterized as a glycoprotein of M r , 56 700. Studies to determine the mode of action of purified enzyme were carried out using the synthetic substrate γ-glutamyl p-nitroanilide. At pH 6.0, equal amounts of two products, glutamic acid and p- nitroaniline, were formed, demonstrating hydrolysis of the γ-glutamyl substrate. Between pH 6.0 and 9.0, the ratio of p- nitroaniline to glutamic acid increased from 1:1 to 17:1 indicating autotranspeptidation of γ-glutamyl p-nitroanilide. The enzyme showed a wide range of substrate specificity for intermediates in the biosynthetic pathway to flavour precursors. γ-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 γ-glutamyl peptides during the biosynthesis of flavour precursors.