The onion (Allium cepa L.) bulb has a high level of glutathione S-transferase (GST) activity, and it is a rich source of sulfur compounds as well as flavonoids. To investigate interactions between onion bulb GSTs and metabolites, we separated onion bulb GSTs (GSTa and GSTb as minor GSTs and GSTc, GSTd and GSTe as dominant GSTs) by DEAE-cellulose chromatography. In Western blot analysis with anti-CmGSTF1 antiserum, GSTc and GSTd fractions showed a thick band. A cDNA (AcGSTF1) corresponding to GSTc was immunoscreened with the same antiserum from an onion bulb cDNA library and its bacterial expression product was also subjected to investigation. Among the sulfur compounds, nonphysiological compounds, S-hexyl glutathione (GSH) and S-butyl GSH, showed strong inhibitory effects on 1-chloro-2,4-dinitrobenezene (CDNB)-conjugating activities of GSTa, GSTb and GSTe. However, physiological sulfur compounds, S-methyl GSH, S-propyl GSH, S-lactoyl GSH and S-ethyl-L-cysteine sulfoxide, had small or almost no inhibitory effects. Therefore, onion sulfur compounds might have the least possibility to be substantial inhibitors of onion GSTs. On the other hand, the activities of GSTc, GSTd and AcGSTF1 were strongly inhibited by flavonoids, quercetin, luteolin, apigenin and kaempferol. Ethylacetate (EtOAc) extract of onion bulb contained quercetin-40-glucoside as a major inhibitory substance. The strong inhibitory effects of quercetin-4'-glucoside on GSTc and GSTd as well as on AcGSTF1 (50% inhibitory concentration (IC₅₀): 9.5, 7.5 and 11.2 µM, respectively) along with its high concentration (226 µM) in the onion bulb indicates that quercetin-4'-glucoside is a physiological inhibitor of dominant GSTs in the onion bulb.
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