Orthophosphate – pyrophosphate exchange catalyzed by soluble and membrane‐bound inorganic pyrophosphatases

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Abstract
A comparative study of the orthophosphate—pyrophosphate exchange reaction catalyzed by the soluble pyrophosphatase from baker's yeast and by the membrane‐bound pyrophosphatase of Rhodospirillum rubrum chromatophores was performed.In both systems the rate of exchange increased when the pH of the medium was raised from 6.0 to 7.8 and when the MgCl2 concentration was raised from 0.1 mM to 20 mM. For the yeast pyrophosphatase the exchange rates measured at different pH values and in the presence of 6.7 to 8.8 mM free Mg2+ superimposed as a single curve when plotted as a function of the concentrations of either HPO42− or MgHPO4. This was not observed with the use of R. rubrum chromatophores. With yeast pyrophosphatase, the Km for Pi was higher than 10 mM and could not be measured when the free Mg2+ concentration in the medium was lower than 0.5 mM. There was a decrease in the Km for Pi when the free Mg2+ concentration was raised to 6.7–8.8 mM or when, in the presence of low free Mg2+, the organic solvents dimethylsulfoxide (20% v/v) or ethyleneglycol (40% v/v) were inclded in the assay medium. In the presence of 6.7–8.8 mM free Mg2+ the Km for total Pi was 7 mM at pH 7.0 and 12 mM at pH 7.8. For the ionic species HPO42− and MgHPO4, the Km values were 5.8 mM and 4.2 mM respectively. In the presence of 0.24–0.42 mM free Mg2+ and either 20% (v/v) dimethylsulfoxide or 40% (v/v) ethyleneglycol the Km values for total, Pi, HPO42− and MgHPO4 were 7.6, 3.5 and 0.5 mM respectively. With R. rubrum chromatophores, the Km for Pi in the presence of 5.5–7.5 mM free Mg2+ was very high and could not be measured.In the presence of 0.24–0.45 mM free Mg2+ the ratio between the velocities of hydrolysis and synthesis of pyrophosphate mesured at pH 7.8 with yeast pyrophosphatase and chromatophores of R. rubrum were practically the same. When the free Mg2+ concentration was raised to 5.5–8.8 mM this ratio decreased from 1028 to 540 when the yeast pyrophosphatase was used and from 754 to 46 when chromatophores were used.