Effects of redox agents on the activity of V-ATPase and H+-pyrophosphatase in red beet root

Plant vacuolar membrane contains two functionally and physically distinct phosphohydrolases of the vacuolar type: H+-ATPase (EC 3.6.1.34) and H+-pyrophosphatase (EC 3.6.1.1), which catalyze electrogenic H+-translocation. The ability of glutathione and sulfite to modulate the activity of V-ATPase has been recently investigated in a variety of fungal and animal cells. Redox agents can also regulate the activity of vacuolar proton pumps in higher plants. Here we show that antioxidants, such as glutathione in the reduced form (GSH) and sulfite, induce an increase in the H+-ATPase activity in red beet roots (Beta vulgaris L.), while an oxidizing agent, glutathione in the oxidized form (GSSG), decreases the enzyme activity. Sulfite opposed an inhibitory action of nitrate on the H+-ATPase activity. It is proposed that nitrate may act as an oxidizing agent and may inhibit the H+-ATPase activity by promoting the formation of disulfide bonds. However, GSH did not prevent NO3-induced inhibition of V-ATPase. Both reducing agents studied, GSH and sulfite, decreased the activity of H+-pyrophosphatase. Analysis of lipid peroxidation indicated that GSSG promoted the formation of malondialdehyde, while in the presence of GSH, sulfite, or nitrate, the malondialdehyde concentration was notably reduced. These results support the hypothesis that a redox condition of a cell is essential for tuning the activity of the vacuolar proton pumps.