Mg(2+)-Dependent, cation-stimulated inorganic pyrophosphatase associated with vacuoles isolated from storage roots of red beet (Beta vulgaris L.)

Abstract

Vacuoles isolated from storage roots of red beet (Beta vulgaris L.) posess a Mg(2+)-dependent, alkaline pyrophosphatase (PPase) activity which is further stimulated by salts of monovalent cations. The requirement for Mg(2+) is specific. Mn(2+) and Zn(2+) permitted only 20% and 12%, respectively, of the PPase activity obtained in the presence of Mg(2+) while Ca(2+), Co(2+) and Cu(2+) were ineffective. Stimulation of Mg(2+)-PPase activity by salts of certain monovalent cations was due to the cation and the order of effectiveness of the cations tested was K(+)=Rb(+)=NH 4 (+) >Cs(+). Salts of Li(+) and Na(+) inhibited Mg(2+)-PPase activity by 44% and 24%, respectively. KCl-stimulation of Mg(2+)-PPase activity was maximal with 60-100 mM KCl. There was a sigmoidal relationship between PPase activity and Mg(2+) concentrations which resulted in markedly non-linear Lineweaver-Burk plots. At pH 8.0, the optimal [Mg(2+)]:[PPi] ratio for both Mg(2+)-PPase and (Mg(2+)+KCl)-PPase activities was approximately 1:1, which probably indicates MgP2O7 (2-) is the true substrate.