Effect of magnesium and ATP on ATPase of sugarcane vacuoles

Kinetic analysis of the Mg(2+)-dependence of tonoplast ATPase from suspension-cultured cells of sugarcane showed that the enzyme activity increased with increasing magnesium concentrations till 1-3 mM and then decreased consideably for higher concentrations. This kinetic could be explained by the assumption that MgATP(2-) is the substrate of ATPase: MgATP(2-) concentration increases with increasing concentration of magnesium till, at high concentrations of magnesium, Mg2ATP is formed. No evidence for a direct role of Mg(2+) as activator or inhibitor was found. These data corroborate previous findings that MgATP(2-) is the sole substrate of the vacuolar ATPase of sugarcane (Thom and Komor 1984). High concentrations of ATP seemed to inhibit the ATPase. This result, however, could be traced back to interference of ATP with the Fiske-Subbarow method of phosphate determination. After adjustment of the test conditions, inhibition by ATP was no longer found. Reported data for ATPases of other plant materials, showing inhibition of enzyme activity with high magnesium or ATP concentrations, might be explicable in a similar way.