Two-stage mechanism of the fluoride inhibition of inorganic pyrophosphatase using the fluoride ion

Some kinetic and spectral approaches have been used to study the interactions in the enzyme-Mg2+-F--pyrophosphate (or imidodiphosphate, a non-hydrolyzeable pyrophosphate analog) system underlying the mechanism of yeast inorganic pyrophosphatase inhibition by fluoride. The continuous curves of the enzymatic reaction were obtained with an automatic phosphate analyzer operating on the time scale of seconds. Increasing concentrations of NaF caused an increase in the inactivation rate constant to a constant level of 5.3 min-1 for PPi (pH 6.2-7.2) and 3.9 min-1 for imidodiphosphate, (pH 7.2). At a saturating fluoride concentration, the initial rate of PPi hydrolysis dropped to 10%. NaF and imidodiphosphate changed the protein spectrum at 270-310 nm and strengthened the binding of each other to the protein. The binding of F- required a Mg2+-binding site with Kd = 0.15 mM being filled in. The free enzyme and its Ca2+ complex did not bind F-. The experimental results indicate that pyrophosphatase inhibition by fluoride occurs in two steps. The inhibitor adds first to the Mg2+ ion on the enzyme in a readily reversible reaction causing a 90% decrease of the catalytic activity. Thereafter, a slow isomerization of the enzymesubstrate complex takes place, resulting in a complete loss of activity.     

Catalytic properties of the inorganic pyrophosphatase in rat liver mitochondria.

Intact rat liver mitochondria have very low hydrolytic activity, if any, toward exogenous pyrophosphate. The activity can be unmasked by making mitochondria permeable to PPi by toluene treatment or disrupting them with detergents or ultrasound, indicating that the active site of pyrophosphatase is located in the matrix. Initial rates of PPi hydrolysis by toluene-permeabilized mitochondria and purified pyrophosphatase were found to depend in a similar manner on PPi and Mg2+ concentrations. The simplest model consistent with the data in both cases implies that the reaction proceeds through two pathways and requires MgPPi as the substrate and, at least, one Mg2+ ion as the activator. In the presence of 0.4 mM Mg2+ (physiological concentration), the inhibition constant for Ca2+ is 12 microM and the enzyme activity is, at least, 50% maximal. The results suggest that the activity of pyrophosphatase in mitochondria is high enough to keep free PPi concentration at a level close to that at equilibrium.     

Catalytic properties of inorganic pyrophosphatase in rat liver mitochondria]

Intact rat liver mitochondria possess a very low hydrolytic activity, if any, towards exogenous pyrophosphate. This activity can be unmasked by making mitochondria permeable to PPi by toluene treatment or by disrupting them with detergents or ultrasound, thus indicating that the active site of pyrophosphatase is localized in the matrix. The initial rates of PPi hydrolysis of toluene-permeabilized mitochondria and purified pyrophosphatase were found to depend, in a similar manner, on PPi and Mg2+ concentrations. The simplest model consistent with these data in both cases implies that the reaction proceeds via two pathways and requires MgPPi as substrate and at least one Mg2+ ion as activator. In the presence of 0.4 mM Mg2+ (physiological concentration) the inhibition constant for Ca2+ is 12 microM and the enzyme activity is no less than 50% of the maximal one. The data obtained suggest that the activity of pyrophosphatase in mitochondria is high enough to keep free PPi concentration at a level close to the equilibrium one.     

Comparative kinetic studies on the two interconvertible forms of Streptococcus faecalis inorganic pyrophosphatase.

In this work the two interconvertible forms of inorganic pyrophosphatase (EC 3.6.1.1) of Streptococcus faecalis were shown to differ in kinetics. The highly active form of the enzyme was more sensitive to the changes in the Mg2+ concentration, and thus also more sensitive to the inhibition caused by ATP, which competes with PPi for the chelation of Mg2+ ions. We have previously described a kinetic model for the less-active form of S. faecalis inorganic pyrophosphatase [Lahti & Jokinen (1985) Biochemistry 24, 3526-3530]. The kinetic model of the highly active enzyme form is proposed to be a modification of the model of the less-active form in which enzyme activation by free Mg2+ is necessary for the reaction to occur. In this model the enzyme exists in two states, referred to as R- and T-states. In the absence of ligands the enzyme is in the T-state. R-state, i.e. the catalytically active state, exists only in the presence of free Mg2+. Mg1PPi2- is the primary substrate, and free pyrophosphate is a weak inhibitor that cannot serve as a substrate for the highly active form of S. faecalis inorganic pyrophosphatase. This model closely resembles that previously presented for yeast inorganic pyrophosphatase.     

Reversible inactivation of rat liver inorganic pyrophosphatase by substrate and its analogs.

Dissociation of Mg2+ from one of the two metal-binding sites whose occupancy is absolutely required for catalysis by rat liver inorganic pyrophosphatase is a slow reaction (tau 1/2 = 3 h). Polycarboxylic Mg2+ complexons markedly accelerate this process due to their binding with Mg2+ on the enzyme. PPi, ATP and a number of diphosphonate analogs of PPi also bind with Mg2+ on the enzyme with concomitant decrease in enzyme activity by 75% but do not release the bound Mg2+. The resulting ternary complex rapidly (tau 1/2 of several seconds) dissociates upon dilution into substrate-free medium. PPi and imidodiphosphate, which are substrates for pyrophosphatase, decrease the rate of reactivation by at least two orders of magnitude. The results can be explained by existence of two interconvertible forms of the enzyme, of which one is inactive and is stabilized by substrate or its analogs.     

Inhibition of mouse spleen inorganic pyrophosphatase by methylene diphosphonic acid]

Some properties of mouse spleen cytosol inorgainc pyrophosphatase (PPi-ase) (E. C. 3.6.1.1) as well as the effect of methylene diphosphonic acid (PCP) on the PPi-ase activity were studied. Specific staining for the enzyme PAAG disc-electrophoresis was developed; it was shown that the PPi-ase formed only one band in 7.5% PAAG. The enzyme pH optimum being 8.0, the optimal [Mg++]/[PPi] ratio was about 2; Km =7.7x10(-4) M, Vmax=0.77 mkM. min-1. mg protein-1. PCP was shown to competitively inhibit the pyrophosphatase reaction, Ki=2.5x10(-4) M +/- 0.2x10(-4) M.     

Kinetic characterization and partial purification of the membrane-bound inorganic pyrophosphatase from Rhodopseudomonas palustris

A membrane-bound inorganic pyrophosphatase from Rhodopseudomonas palustris has been studied by kinetic analysis. The enzymatic activity was stimulated by Mg2+, and the (Mg-PPi) complex is regarded to be the functional substrate. Free Mg2+ revealed a significant influence on the membrane-bound PPiase activity. Kinetic data were determined at various fixed concentrations of free Mg2+. Mg2+ is proposed to act as an activator in two ways. It may interact with the enzyme directly, and may combine with PPi to yield the functional substrate Mg-PPi. Ca2+ revealed a non-competitive type of inhibition on the Mg2+-activated enzyme. The membrane-bound PPiase activity was firmly attached to the chromatophore membrane. To achieve an almost entire solubilization, both, Triton X-100 and high concentrations of Mg2+, had to be applied. An enrichment method along with stepwise lowering the concentrations of Triton X-100 and Mg2+ after the solubilization has been established. The solubilized and partially purified enzyme was stimulated by phospholipids while the influence of free Mg2+ was lost. Three different energies of activation as a function of temperature were derived from Arrhenius plots for the membrane-bound as well as for the solubilized PPiase activity.     

Leucine aminopeptidase (bovine lens). Effect of pH on the relative binding of Zn2+ and Mg2+ to and on activation of the enzyme.

Incubation of leucine aminopeptidase (bovine lens) (EC 3.4.1.1) with various concentrations of Mg2+ at various pH values in 1 M KCl and 0.155 M trimethylamine-HCl at 37 degrees confirms that Mg2+ competes with Zn2+ for binding only 1 site per 54,000-dalton subunit. The ratio of the apparent association constants (1KZn:1KMg = 1KZn/Mg) at this site (site 1) was estimated to be 20,720 at pH 8.16, 10,570 at pH 8.44, 3,590 at pH 8.78, and 660 AT PH 9.14. The decrease in values of 1KZn/Mg with increasing pH in the activation of leucine aminopeptidase by Mg2+ is attributed to the lowering of the free Zn2+ concentration relative to that of free Mg2+ caused by the formation of ZnOH+ and Zn(OH)2 complexes with increasing OH- concentration. When corrections are made for the binding of Zn2+ by OH- ions, the pH-independent ratio of association constants (1KZn:1KMg = 1KZn/Mg) for the relative binding of Zn2+ and Mg2+ at site 1 of leucine aminopeptidase in 29,800. From the effect of pH on the relative binding constant, a value (beta2) for the product of the two stepwise association constants for the formation of Zn(OH)2 from Zn2+ and OH- (Zn2+ + OH- in equilibrium ZnOH+; ZnOH+ + OH- in equilibrium Zn(OH)2) was estimated to be 4.42 X 10(10) M-2 at 37 degrees. Values of Km at pH 7.5 AND 30 degrees with L-leucine p-nitroanilide as substrate in the presence of 0.01 M NaHCO3 are 4.13 and 2.01 mM for the zinc-zinc and magnesium-zinc enzymes, respectively. Values for Vmax are 0.2 and 2.49 mumol/min/mg, respectively.     

Purification and characterization of a novel 4-methyleneglutamine synthetase from germinated peanut cotyledons (Arachis hypogaea)

A newly detected amide synthetase, designated 4-methyleneglutamine synthetase, has been partially purified from extracts of 5- to 7-day germinated peanut cotyledons (Arachis hypogaea). Purification steps include fractionation with protamine sulfate and ammonium sulfate followed by column chromatography on Bio-Gel and DEAE-cellulose; synthetase purified over 300-fold is obtained. The enzyme has a molecular weight estimated to be approximately 250,000 and a broad pH optimum with maximal activity at approximately pH 7.5. Maximal rates of activity are obtained with NH+4 (Km = 3.7 mM) as the amide donor and the enzyme is highly specific for 4-methylene-L-glutamic acid (Km = 2.7 mM) as the amide acceptor. Product identification and stoichiometric studies establish the reaction catalyzed to be: 4-methyleneglutamic acid + NH4+ + ATP Mg2+----4-methyleneglutamine + AMP + PPi. PPi accumulates only when F- is added to inhibit pyrophosphatase activity present in synthetase preparations. This enzymatic activity is completely insensitive to the glutamine synthetase inhibitors, tabtoxinine-beta-lactam and F-, and is only partially inhibited by methionine sulfoximine. It is, however, inhibited by added pyrophosphate in the presence of F- as well as by certain divalent metal ions (other than Mg2+) including Hg2+, Ni2+, Mn2+, and Ca2+. All data obtained indicate that this newly detected synthetase is distinct from the well-known glutamine and asparagine synthetases.     

Two pathways for phosphate/water oxygen exchange by yeast inorganic pyrophosphatase

A scheme of Mg2+ and Pi binding to yeast inorganic pyrophosphatase has been deduced from the concentration dependencies of the rate of oxygen exchange between Pi and water. The exchange reaction requires the binding of MgPi and free Pi (pathway I) or two MgPi (pathway II) in addition to two Mg2+ ions bound in the absence of Pi. Pathway II predominates above 0.16 mM Mg2+. The rate of formation of bound PPi from bound Pi for pathway II is three times as high as that for pathway I. The results suggest that the binding of the fourth Mg2+ ion to pyrophosphatase stimulates its synthetic vs its hydrolytic capability.     

Effects of spermine and spermidine on the inorganic pyrophosphatase of Streptococcus faecalis. Interactions between polyamines and inorganic pyrophosphate.

We have shown a dual role for Mg2+ in the hydrolysis of PPi catalysed by inorganic pyrophosphatase (PPase; EC 3.6.1.1) of Streptococcus faecalis; Mg2+ is necessary for the formation of the substrates, Mg1PPi2- and Mg2PPi0, and it also acts as an allosteric activator [Lahti + Jokinen (1985) Biochemistry 24, 3526-3530]. No activity can be observed with S. faecalis PPase in the absence of bivalent cations, which indicates that free PPi cannot serve as a substrate for this enzyme. However, significant activities were observed in the presence of spermine and spermidine, even though no bivalent cations were present. It was shown by particle-induced gamma-ray emission and particle-induced X-ray-emission analysis that the polyamines used were not contaminated with Mg2+ or any other bivalent cations that could support PPase activity. Hence it is obvious that polyamines are able to form a complex with PPi that serves as a substrate for PPase. The apparent stability constants for the 1:1 adducts of spermine and spermidine were estimated by a resin competition method. The values obtained at pH 7.5 were 2.7 X 10(3) M-1 and 6.4 X 10(2) M-1 respectively. Kinetic results further suggested that polyamines can also substitute for Mg2+ as an activator in vitro. The physiological significance of these polyamine effects were discussed.     

Influence of free Mg2+ on the kinetics of human erythrocyte phosphofructokinase

The influence of Mg2+ on the reaction catalyzed by human erythrocyte phosphofructokinase has been investigated using kinetic methods. The catalytic activity of PFK is dependent upon the presence of Mg2+ which constitutes with ATP the true Mg-ATP2- substrate. Free Mg2+ has no influence on the affinity of the enzyme for Mg-ATP2- substrate. Erythrocyte PFK is more inhibited by ATP4- and uncomplexed citrate than it is by Mg-ATP2- and Mg-citrate. Free Mg2+ relieves the MgATP2- and Mg-citrate inhibition under conditions where free ATP4-is negligible. We can assume that uncomplexed Mg2+ acts as positive effector by direct binding to the enzyme. These results emphasize the role of Mg2+ in the regulation of PFK activity in the erythrocyte.     

ATP sulfurylase-dependent assays for inorganic pyrophosphate: applications to determining the equilibrium constant and reverse direction kinetics of the pyrophosphatase reaction, magnesium binding to orthophosphate, and unknown concentrations of pyrophosphate

A continuous, coupled, spectrophotometric assay is described in which the enzyme ATP sulfurylase is employed to measure the concentration of inorganic pyrophosphate (PPi) at equilibrium with known concentrations of inorganic orthophosphate (Pi) in the presence of excess inorganic pyrophosphatase (PPitase). In agreement with previous reports, the apparent equilibrium constant (Keq,app) of the PPi hydrolysis reaction was shown to decrease as the concentration of Mg2+ is increased. At pH 7.3, 30 degrees C, in the presence of 150 mM NaCl and 1 mM free Mg2+, Keq,app (calculated as [Pi]t2/[PPi]t) was 1950. Measurements of Keq,app at different total concentrations of Mg2+ and Pi permitted the determination of K0, the dissociation constant of the Mg-Pi complex. In 0.05 M Tris-Cl, pH 8.0, at 30 degrees C, K0 was 3.6 mM. In the presence of excess ATP sulfurylase, yeast PPitase catalyzed PPi formation from Pi with a specific activity (Vmax) of 9 units X mg protein-1 at pH 8.0, 30 degrees C, and 1 mM free Mg2+. Half-maximum reverse reaction velocity was observed at a total Pi concentration of 18 mM. (Under the same conditions, Vmax of the PPi hydrolysis reaction was 530 units X mg protein-1.) A radiochemical end point ("reaction-to-completion") assay for measuring unknown concentrations of PPi was devised. In the presence of excess 35S-adenosine-5'-phosphosulfate ([35S]APS) as the cosubstrate, 35SO2-4 formation was stoichiometric with added PPi. (The 35SO2-4 and [35S]APS are separated by adsorption of the latter onto charcoal.) The sensitivity of the assay can be adjusted by varying the specific radioactivity of the [35S]APS. In the absence of interfering substances, as little as 2 pmol of PPi per 1.0 ml assay volume can be measured. The sensitivity of the assay is reduced in the presence of ATP plus perchlorate (which synergistically inhibit the enzyme). However, if the bulk of the ATP is removed from perchloric acid extracts of tissues with glucose and hexokinase, initial intracellular levels as low as 1 microM can be measured. The possibility that most of the cellular PPi extracted with perchloric acid was originally enzyme bound is discussed.     

Thermodynamics, kinetics, and mechanism in yeast inorganic pyrophosphatase catalysis of inorganic pyrophosphate: inorganic phosphate equilibration

We have developed two methods for quantitatively measuring inorganic pyrophosphate (PPi) in the presence of 10(3)--10(4) molar excesses of inorganic phosphate (Pi) and used them to measure the extent of enzyme-bound pyrophosphate (EPPi) formation in solutions of yeast inorganic pyrophosphatase and Pi. We have also measured the rate of enzyme-catalyzed H2O--phosphate oxygen exchange. We find both processes to have essentially identical dependence on Mg2+ and Pi concentrations, thus providing important confirmation for the recent proposal by Janson et al. (1979) that oxygen exchange proceeds via EPPi formation. Our results are consistent with a model in which three Mg2+ per active site are required for EPPi formation but inconsistent with a model requiring only two Mg2+ per active site and permit the formulation of an overall scheme for inorganic pyrophosphatase catalysis of PPi--Pi equilibration as well as the evaluation of equilibrium and rate constants in this scheme. The major results and conclusions of our work are the following: (a) the equilibrium constant for PPi (enzyme-bound) in equilibrium with 2Pi (enzyme-bound) is 4.8; (b) following PPi hydrolysis, the first released Pi contains an oxygen from solvent water; (c) the steps for PPi hydrolysis on the enzyme and for release of both product Pi's are all partially rate determining in overall enzyme-catalyzed PPi hydrolysis; (d) PPi formation on the enzyme is rate determining for H2O--Pi oxygen exchange; (e) PPi dissociation from the enzyme is very slow and is the rate-determining step in Pi--PPi exchange (Cohn, 1958; Janson et al., 1979). This also accounts for the observation that the calculated dissociation constant for MgPPi complex binding to enzyme is considerably lower than the measured Km for enzyme-catalyzed MgPPi hydrolysis.     

Two pathways of pyrophosphate hydrolysis and synthesis by yeast inorganic pyrophosphatase.

Initial rates of pyrophosphate hydrolysis and synthesis by baker's yeast inorganic pyrophosphatase and equilibrium amounts of enzyme-bound and free pyrophosphate were measured over wide ranges of Mg2+ and respective substrate concentrations. Computer analysis of these data, in conjunction with those on phosphate/water oxygen exchange [Kasho, V. N. & Baykov, A. A. (1989) Biochem. Biophys. Res. Comm. 161, 475-480], yielded values of the equilibrium constants for Mg2+ binding to free enzyme and central complexes and values of the forward and reverse rate constants for the four reaction steps, namely, PPi binding/release, PPi hydrolysis/synthesis and two Pi binding/release steps. All catalytic steps were found to proceed through two parallel pathways, involving 3 or 4 Mg2+/PPi or 2 Pi bound. Product release is the slowest catalytic event in both hydrolysis and synthesis of pyrophosphate, at least, for the four-metal pathway. In the hydrolytic reaction, magnesium pyrophosphate binding is faster for the four-metal pathway, dissociation of the second Pi is faster for the three-metal pathway, while PPi hydrolysis and the release of the first Pi may proceed with similar rates. Release of pyrophosphate formed on the enzyme is faster for the three-metal pathway. Both pathways are expected to operate in vivo, and their relative contributions will vary with changes in the Mg2+ concentration, thus providing a means for pyrophosphatase-activity regulation.     

Steady-state properties of calcium binding to parvalbumins from bullfrog skeletal muscle: effects of Mg2+, pH, ionic strength, and temperature

To improve our understanding of the physiological roles of parvalbumins, PA-1 (pI 4.78) and PA-2 (pI 4.97) parvalbumins were prepared from bullfrog skeletal muscle and their calcium binding properties were examined in a medium of constant ionic strength (I = 0.106, pH 6.80, at 20 degrees C) containing various concentrations of Mg2+ by using a metallo-indicator, tetramethylmurexide. Apparent binding constants for Ca2+ in the presence of Mg2+ changed in the manner expected if Ca2+ and Mg2+ compete for two independent homogeneous binding sites. The following values were obtained: for PA-1, KCa = 1 X 10(7) M-1, KMg = 900 M-1; for PA-2, KCa = 6 X 10(6) M-1, KMg = 830 M-1 (I = 0.106, pH 6.80, at 20 degrees C). The apparent binding constants are strongly dependent on temperature: at 10 degrees C for PA-1, KCa = 2 X 10(8) M-1, KMg = 10(4) M-1; for PA-2, KCa = 5 X 10(7) M-1, KMg = 5 X 10(3) M-1 (I = 0.106, pH 6.80). The dependence of the affinities for Ca2+ on ionic strength is similar to or less than that of GEDTA (EGTA). The affinities for Ca2+ and Mg2+ of parvalbumins are unchanged between pH 6.5 and 7.2.     

Inhibition of mitochondrial-matrix inorganic pyrophosphatase by physiological [Ca2+], and its role in the hormonal regulation of mitochondrial matrix volume.

1. The pyrophosphatase activity in cytosolic and mitochondrial fractions of rat liver was 1.7 and 0.26 units/mg of protein respectively when assayed at 37 degrees C in the presence of physiological [Mg2+] (0.3 mM). 2. Approx. 80% of the mitochondrial pyrophosphatase was inaccessible to extramitochondrial PPi, of which 40% represented soluble matrix enzyme (0.38 unit/mg of matrix protein). 3. Ca2+ inhibited the soluble matrix enzyme; the effective K0.5 for inhibition increased as [Mg2+], an essential cofactor of the enzyme, increased. Measured values were 0.39, 1.15, 3.7, 8.3 and 12.5 microM at 0.04 mM-, 0.1 mM-, 0.3 mM-, 0.6 mM- and 1 mM-Mg2+ respectively. 4. The data were analysed by a kinetic model similar to that for yeast pyrophosphatase, which assumes the substrate to be MgPPi (Km 5 microM) with Mg2+ also activating at an additional site (K0.5 23 microM). Ca2+ inhibits through the formation of CaPPi, a strong competitive inhibitor (Ki 0.067 microM). 5. Heart mitochondria also contain a soluble matrix pyrophosphatase of similar activity to that of liver mitochondria and with the same sensitivity to [Ca2+]. 6. The data provide an explanation for the increase in mitochondrial PPi, mediated by Ca2+, which is responsible for the increase in matrix volume induced by gluconeogenic hormones [Davidson & Halestrap (1988) Biochem. J. 254, 379-384].     

Isolation and primary characterization of two forms of inorganic pyrophosphatase from ox heart muscle mitochondria

A method allowing to obtain two highly purified forms (membrane-bound and soluble ones) of inorganic pyrophosphatase from bovine heart mitochondria is described. Both forms have the isoelectric point of 5.8, pH optimum with Mg2+ at 7--9, are maximally stable at pH 5.8 and absolutely specific to PPi and require Mg2+ or Co2+ for their activity. The soluble pyrophosphatase is also activated by Zn2+. Besides, the two forms differ in their electrophoretic mobilities, affinity for DEAE cellulose and stability in acidic (pH less than 5.8) and alkaline media.     

Kinetic models for the action of cytosolic and mitochondrial inorganic pyrophosphatases of rat liver

Initial rates of PPi hydrolysis by cytosolic and mitochondrial inorganic pyrophosphatases of rat liver have been measured in the presence of 0.2-100 microM MgPPi and 0.01-50 mM Mg2+ at pH 7.2 to 9.3. The apparently simplest model consistent with the data for both enzymes implies that they bind substrate, in the form of MgPPi, and three Mg2+ ions, of which two are absolutely required for activity. The third metal ion facilitates substrate binding but decreases maximal velocity for the cytosolic enzyme, while substrate binding is only modulated for the mitochondrial enzyme. The model is also applicable to bovine heart mitochondrial pyrophosphatases. The active form of the substrate for the cytosolic pyrophosphatase is MgP2O7(-2); the catalytic and metal-binding steps require a protonated group with pKa = 9.2 and an unprotonated group with pKa = 8.8, respectively. The results indicate that the mitochondrial pyrophosphatase is more sensitive to variations of Mg2+ concentration in rat liver cells than is the cytosolic one.     

Spectral and kinetic studies of phosphate and magnesium ion binding to yeast inorganic pyrophosphatase

Inorganic pyrophosphatase must bind two phosphate molecules in order to catalyze pyrophosphate synthesis. In this report it is shown that Pi causes marked effect on the absorption spectrum of baker's yeast inorganic pyrophosphatase and this effect can be used to analyze Pi binding to this enzyme. A series of absorbance versus Pi concentration curves in the presence of 0.5-20 mM free Mg2+ were obtained at pH 7.2 and computer-fitted to 19 models. The dissociation constant of magnesium phosphate (8.5 +/- 0.4 mM) used in this analysis was measured with a Mg2+-sensitive electrode. The best model implies successive binding of two magnesium phosphate molecules or random-order binding of magnesium phosphate and free phosphate molecules. The first route predominates at physiological concentrations of Mg2+. The Pi-inhibition pattern of pyrophosphate hydrolysis confirmed that Pi adds to the active site and provided further evidence for the existence of an activating Pi-binding site. The possibility is raised that the pathways of pyrophosphate synthesis and hydrolysis by inorganic pyrophosphatase may differ in the sense that the binding of the fourth metal ion/subunit may facilitate the synthesis and inhibit the hydrolysis.