REGULATION OF DIVALENT-CATIONS OF THE MEMBRANE-BOUND PYROPHOSPHATASE OF RHODOSPIRILLUM-RUBRUM, AS SHOWN BY THE HYDROLYSIS OF TRIPOSITIVE-PYROPHOSPHATE COMPLEXES

Tripositive-pyrophosphate [M(III)-PPi] complexes were used to investig ate the role of tree divalent cations on the membrane-bound pyrophosph atase. Divalent cations remain free and the M(III)-PPi complexes were employed as substrates. Formation of a La-PPi complex was studied by f luorescence, and the fact that Zn2+ and Mg 2+ remain free in the solut ion was validated. Hydrolysis of La-PPi is stimulated by the presence of fixed concentrations of free Mg2+ or Zn2+ and this stimulation depe nds on the concentration of the cations when the La-PPi complex is fix ed. The divalent cation stimulation order is Zn2+ > Co2+ > Mg2+ > Mn2 > Ca2+ (at 0.5 mm of free cation). With different M(III)-PPi complexe s, Zn2+ produces the same K(m) for all the complexes and Mg2+ stimulat es with a different K(m). The results suggest that both Mg2+ and Zn2activate the membrane-bound pyrophosphatase but through different mech anisms.