Inhibition of Malate Synthesis by Vanadate: Implications for the Cytosolic Alkalinization Induced by Vanadate Concentrations Partially Inhibiting the Plasmalemma H+Pump

Abstract

The effects of Na-orthovanadate, at concentrations only partially inhibiting net H+ extrusion, were determined on vacuolar and cytosolic pH by the weak base and weak acid distribution at equilibrium. Treatment with vanadate induces in Elodea densa leaves and in Arabidopsis thaliana seedlings a moderate acidification of both cell sap and vacuole. Conversely, it induces an alkalinization of cytosol, this effect being in apparent contrast with a condition of reduced activity of the H+-transporting plasmalemma ATPase, which should be associated with a cytosolic acidification. In Arabidopsis seedlings treated with vanadate, the increase in pH of both cytosol and external medium is associated with a decrease in cell sap buffer capacity, more evident for higher vanadate concentrations, and particularly marked in the pH range between 3·5 and 5·5. In these conditions, the malate content is strongly reduced, its decrease almost completely accounting for the decrease in cell sap buffer capacity. An in vitro analysis of the vanadate effect on phosphoenolpyruvate carboxylase indicates that the decrease in malate content seems substantially due to an inhibiting effect of vanadate on this enzyme. These results stress that the in vivo use of vanadate as an inhibitor of the plasmalemma H+-ATPase must be taken with caution; in particular, for studying the correlations between changes in net H+ extrusion and changes in cytosolic pH and related processes.