Effects of orthovanadate on H+ secretion, K+ uptake, electric potential difference and membrane ATPase activity of higher plant tissues

Orthovanadate at concentrations between 5 and 20 μM inhibited the K+-stimulated, DCCD-sensitive ATPase activity of membrane preparations (fraction sedimenting between 13 000 and 80 000 g) from radish seedlings while it had no effect on either the oligomycin-sensitive ATPase activity of mitochondrial preparations, or the phosphorylating activity of isolated mitochondria from radish seedlings or from pea internode segments. In whole germinating radish seeds orthovanadate inhibited the increase in fresh weight, net K+ uptake and H+ extrusion without inducing any detectable change of respiration nor of ATP and ADP levels, and it induced a rapid, partial depolarisation of the transmembrane electric potential difference. These effects in vivo were interpreted as indicating that orthovanadate inhibits an ATP-driven electrogenic proton pump corresponding to a plasmalemma ATPase.