Levels of a dominant Glutathione S-transferase in onion bulbs have a seasonal relationship with physiological inhibitors

Isoforms of onion bulb GSTs (designated as GSTa, GSTb, as minor, and GSTc, GSTd and GSTe as dominant GSTs) were separated by DEAE-cellulose chromatography, and variation in the activity of dominant GSTs was studied in winter and spring. Among the dominant GSTs, GSTc and GSTd were found to have similar activity levels in both seasons. In case of GSTe, distinct activity levels were observed in winter and spring, where, a decreased activity was found in spring. An expressional relation was observed between GSTe and physiological inhibitor, quercetin-4 ́-glucoside in onion calli. However, GSTe was not responsive to quercetin-3,4 ́-diglucoside. The concentrations of the two inhibitors were also compared in onion bulb tissues collected from winter and spring seasons. Quercetin-4 ́-glucoside and quercetin-3,4 ́-diglucoside containing fractions, ethylacetate (EtOAc) and water fraction, were isolated from onion bulb extract, and IC50s of the fractions were measured on GSTc, a sensitive GST to the inhibitors. The IC50s of EtOAcand water fraction on GSTc (in winter 84 and 283 mg ml -1 fresh tissue, respectively, and in spring 102 and 214 mg ml -1 fresh tissues, respectively) and HPLC profile suggested that the concentration of quercetin-4 ́-glucoside decreased over time of storage in onion bulb, while the concentration of quercetin3,4 ́-diglucoside increased in spring. Therefore, lower activity of GSTe in onion bulb in spring is due to lower content of quercetin-4 ́-glucoside in onion bulb tissues, and hence, quercetin-4 ́-glucoside might have important role in expressing the activity of this GST in onion bulb.