Mg2+-Dependent, cation-stimulated inorganic pyrophosphatase associated with vacuoles isolated from storage roots of red beet (Beta vulgaris L.)

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Abstract
Vacuoles isolated from storage roots of red beet (Beta vulgaris L.) posess a Mg2+-dependent, alkaline pyrophosphatase (PPase) activity which is further stimulated by salts of monovalent cations. The requirement for Mg2+ is specific. Mn2+ and Zn2+ permitted only 20% and 12%, respectively, of the PPase activity obtained in the presence of Mg2+ while Ca2+, Co2+ and Cu2+ were ineffective. Stimulation of Mg2+-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 Mg2+-PPase activity by 44% and 24%, respectively. KCl-stimulation of Mg2+-PPase activity was maximal with 60–100 mM KCl. There was a sigmoidal relationship between PPase activity and Mg2+ concentrations which resulted in markedly non-linear Lineweaver-Burk plots. At pH 8.0, the optimal [Mg2+]:[PPi] ratio for both Mg2+-PPase and (Mg2++KCl)-PPase activities was approximately 1:1, which probably indicates MgP2O7 2- is the true substrate.

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