Metabolism of γ-glutamyl peptides during development, storage and sprouting of onion bulbs

Abstract

Quantitative analysis of γ-glutamyl peptides in onion bulbs showed that γ-glutamyl trans-(+)- S-(1-propenyl)- l-cysteine sulphoxide (γ-glutamyl propenyl CSO) and S-2-carboxypropyl glutathione (2CPGTH) were absent prior to bulbing, and then at bulbing accumulated to levels of 2.1 and 0.4 mg g −1 fr. wt respectively. These levels were maintained throughout storage. The hydrolytic enzyme γ-glutamyl transpeptidase [EC 2.3.2.1] was of minimal activity in a stored bulb. During sprouting, activity increased ca five-fold. Levels of γ-glutamyl propenyl CSO and 2CPGTH decreased by 50% to 0.94 and 0.14 mg g −1 fr. wt respectively. These data support the hypothesis that the two major γ-glutamyl peptides function as storage compounds during the overwintering of onion bulbs.