Kinetic studies on the interactions of two forms of inorganic pyrophosphatase of heart mitochondria with physiological ligands

A scheme of interactions of Mg2+ ions and their 1:1 complex with PPi (PPiMg') with two forms of inorganic pyrophosphatase isolated from beef heart mitochondria has been deduced from the analysis of enzyme kinetics at pH varying from 5.6 to 8.5. The scheme implies the existence of two catalytically important metal-binding sites on the enzyme. The two enzyme forms differ in maximal velocity and affinity for the metal activator. The pH dependence of kinetic parameters suggests that the active form of the substrate is MgP2O2-7. Ca2+ ions strongly inhibit pyrophosphatase activity and the corresponding Hill coefficient is 1.5. Phosphate and ATP are weak inhibitors of pyrophosphatase of the competitive and noncompetitive type respectively. The results show that these forms of mitochondrial pyrophosphatase are similar to pyrophosphatases isolated from other sources.     

Orthophosphate-pyrophosphate exchange catalyzed by soluble and membrane-bound inorganic pyrophosphatases. Role of H+ gradient

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 HPO4(2-) 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 included 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 HPO4(2-) 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, HPO4(2-) 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 measured 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.     

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 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].     

Studies on an activator of the (Ca2+ plus Mg2+)-ATPase of human erythrocyte membranes.

1. An activator of the (Ca2+ plus Mg2+)-stimulated ATPase present in the human erythrocytes (membrane) has been isolated in soluble form from hemolysates of these cells. Partial purification has been achieved through use of carboxymethyl-Sephadex chromatography. The resulting activator fraction contained no hemoglobin and only 0.3% of the total adenylate kinase activity of the cell. 2. Whereas the activator was released from erythrocytes subjected to hemolysis in 20 miosM buffer at pH 7.6 or at pH 5.8, only the membranes prepared at pH 7.6 were affected by it. 2. Whereas the activator was released from erythrocytes subjected to hemolysis in 20 miosM buffer at pH 7.6 or at pH 5.8, only the membranes prepared at pH 7.6 were affected by it. 3. When (Ca2+ plus Mg2+)-ATPase activity was measured by 32Pi release from (gamma-32P)ATP, freeze-thawed erythrocytes, as well as membranes prepared at pH 5.8 and at pH 7.6, expressed lower values than noted by assay for total Pi release. When ADP instead of ATP was used as substrate, significant amount of Pi were released by these erythrocyte preparations. Further study revealed (a) production of ATP and AMP from ADP with membranes and hemolysate alone, and (b) exchange of the gamma-and B-position phosphate on (gama-32P)ATP in the presence of membranes plus hemolysates. These observations established the presence of adenylate kinase activity in the (membrane-free) hemolysates and in membranes. It further supports the conclusion that Pi release from ADP by human erythrocytes (freeze-thawed) and by their isolated membranes is due to formation of ATP by adenylate kinase and hydrolysis of this generated ATP by (Ca2+ plus Mg2+)-ATPase. 4. The following points were also established: (a) absence of an ADPase in human erythrocytes; (b) the (Ca2+ plus Mg2+)-ATPase activator enhanced cleavage only of the gama-position of ATP and (c) the (Ca2+ plus Mg2+)-ATPase activator is neither adenylate kinase nor hemoglobin.     

Inhibition of inorganic pyrophosphatase of animal mitochondria by calcium

Calcium ion is an uncompetitive inhibitor of the inorganic pyrophosphatases of bovine heart and rat liver mitochondria with respect to substrate MgPPi at pH 8.5 and a non-competitive inhibitor of the former enzyme at pH 7.2. The concentration of Ca2+ required to decrease the maximal velocities for both enzymes at pH 8.5, 0.4 mM Mg2+ was about 10 microM. The inhibition results from the binding of two Ca2+ ions to both free enzymes and their complexes with the substrate. The results suggest that Ca2+ regulates pyrophosphatase activity and hence PPi level in mammalian mitochondria.     

Purification and kinetic properties of human erythrocyte Mg2+-dependent inorganic pyrophosphatase

Inorganic pyrophosphatase (pyrophosphate phosphohydrolase, EC 3.6.1.1) from human erythrocyte hemolysates has been purified up to 10 000-fold. The purified enzyme is homogenous and has a specific activity of 79.75 mumol PPi hydrolysed.min-1.mg-1 at pH 8 and 37 degrees C. It was confirmed that it is a dimer with a molecular weight of 42 000, composed of two identical protomers. From kinetic studies, it is proposed that human erythrocyte inorganic pyrophosphatase activity depends on free Mg2+ concentration in different ways. This ion constitutes part of the substrate (the Mg.PPi complex; Km = 1.4.10(-4) M) and probably acts as an allosteric activator (kinetic activation constant: KMg2+a = 7.5.10(-4) M). Equilibrium binding studies performed in the absence of PPi showed 4 binding sites for Mg2+, all having the same high affinity (dissociation constant: KMg2+d = 4.10(-6) M). Since the concentration of free Mg2+ in red blood cells is very low and may vary with the oxygenation state, it is likely that in vivo erythrocyte pyrophosphatase activity is regulated.     

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.     

Purification and characterization of an inorganic pyrophosphatase from the extreme thermophile Thermus aquaticus.

An inorganic pyrophosphatase was purified over 600-fold to homogeneity as judged by polyacrylamide gel electrophoresis. The enzyme is a tetramer of Mr = 84,000, has a sedimentation coefficient of 5.8S, a Stokes radius of 3.5 nm, and an isoelectric point of 5.7. Like the enzyme of Escherichia coli, the pyrophosphatase appears to be made constitutively. The pH and temperature optima are 8.3 and 80 degrees C, respectively. The Km for PPi is 0.6 mM. A divalent cation is essential, with Mg2+ preferred. The enzyme uses only PPi as a substrate.     

H(+)-translocating inorganic pyrophosphatase of plant vacuoles. Inhibition by Ca2+, stabilization by Mg2+ and immunological comparison with other inorganic pyrophosphatases.

The effects of divalent cations, especially Ca2+ and Mg2+, on the proton-translocating inorganic pyrophosphatase purified from mung bean vacuoles were investigated to compare the enzyme with other pyrophosphatases. The pyrophosphatase was irreversibly inactivated by incubation in the absence of Mg2+. The removal of Mg2+ from the enzyme increased susceptibility to proteolysis by trypsin. Vacuolar pyrophosphatase required free Mg2+ as an essential cofactor (K0.5 = 42 microM). Binding of Mg2+ stabilizes and activates the enzyme. The formation of MgPPi is also an important role of magnesium ion. Apparent Km of the enzyme for MgPPi was about 130 microM. CaCl2 decreased the enzyme activity to less than 60% at 40 microM, and the inhibition was reversed by EGTA. Pyrophosphatase activity was measured under different conditions of Mg2+ and Ca2+ concentrations at pH 7.2. The rate of inhibition depended on the concentration of CaPPi, and the approximate Ki for CaPPi was 17 microM. A high concentration of free Ca2+ did not inhibit the enzyme at a low concentration of CaPPi. It appears that for Ca2+, at least, the inhibitory form is the Ca2(+)-PPi complex. Cd2+, Co2+ and Cu2+ also inhibited the enzyme. The antibody against the vacuolar pyrophosphatase did not react with rat liver mitochondrial or yeast cytosolic pyrophosphatases. Also, the antibody to the yeast enzyme did not react with the vacuolar enzyme. Thus, the catalytic properties of the vacuolar pyrophosphatase, such as Mg2+ requirement and sensitivity to Ca2+, are common to the other pyrophosphatases, but the vacuolar enzyme differs from them in subunit mass and immunoreactivity.     

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.     

The formation of enzyme-bound and medium pyrophosphate and the molecular basis of the oxygen exchange reaction of yeast inorganic pyrophosphatase.

Yeast inorganic pyrophosphatase, with 10 mM 32Pi and 10 mM Mg2+ present at pH 7.3 TO 7.6, rapidly forms enzyme-bound pyrophosphate equivalent to about 5% of the total catalytic sties on the two enzyme subunits. The enzyme thus appears to bind PPi so as to favor thermodynamically its formation from Pi. The enzyme catalyzes a measurable equilibrium formation of free PPi at a much slower rate. Under similar conditions, the enzyme catalyzes a rapid exchange of oxygen atoms between Pi and water with the relative activation by metals being Mg2+ greater than Zn2+ greater than Co2+ greater than Mn2+. Millisecond mixing and quenching experiments demonstrate that the rate of formation and cleavage of the enzyme-bound PPi is rapid enough to explain most or all of the oxygen exchange reaction.     

Purification and characterization of membrane-bound inositolpolyphosphate 5-phosphatase

Membrane-bound inositolpolyphosphate 5-phosphatase was solubilized and highly purified from a microsomal fraction of rat liver. Its physiochemical and enzymological properties were compared with those of highly purified preparations of two types of soluble enzyme (soluble Type I and Type II) from rat brain. The molecular masses of the membrane-bound and soluble Type I enzymes were 32 kDa, while that of soluble Type II enzyme was 69 kDa, as determined by molecular sieve chromatography. The membrane-bound and soluble Type I enzymes showed similar broad peaks on isoelectric focusing (pI 5.8-6.4), while soluble Type II enzyme showed multiple peaks in the region between pI 4.0-5.8. All three enzymes required divalent cation for activity. Mg2+ was the most effective for both the membrane-bound and soluble Type I enzymes, while Co2+ enhanced soluble Type II enzyme activity about 1.5-fold relative to Mg2+ at 1 mM. The optimal pH of both the membrane-bound and soluble Type I enzymes was 7.8, while that of soluble Type II was 6.8. The Km values for inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] of all three enzymes were similar (5-8 microM), but those for inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] were quite different, the Km values of membrane-bound and soluble Type I enzymes being 0.8 microM, while that of soluble Type II was 130 microM. These similarities between the membrane-bound and soluble Type I enzymes suggest that these two molecules may be the same protein, and that concentrations of Ins(1,4,5)P3 and Ins(1,3,4,5)P4, both of which are considered to play critical roles in the regulation of intracellular Ca2+-concentration, may be differently regulated by two functionally distinct enzymes.     

The role of nucleoside triphosphate pyrophosphohydrolase in in vitro nucleoside triphosphate-dependent matrix vesicle calcification

Nucleoside triphosphate pyrophosphohydrolase (EC 3.6.1.8) activity is associated with matrix vesicles purified from collagenase digests of fetal calf epiphyseal cartilage. This enzyme hydrolyzes nucleoside triphosphates to nucleotides and PPi, the latter inducing precipitation in the presence of Ca2+ and Pi. An assay for matrix vesicle nucleoside triphosphate pyrophosphohydrolase is developed using beta, gamma-methylene ATP as substrate. The assay is effective in the presence of matrix vesicle-associated ATPase, pyrophosphatase, and alkaline phosphatase activities. A soluble nucleoside triphosphate pyrophosphohydrolase is obtained from matrix vesicles by treatment with 5 mM sodium deoxycholate. The solubilized enzyme induced the precipitation of calcium phosphate in the presence of ATP, Ca2+, and Pi. Extraction of deoxycholate-solubilized enzymes from matrix vesicles with 1-butanol destroys nucleoside triphosphate pyrophosphohydrolase activity while enhancing the specific activities of ATPase, pyrophosphatase, and alkaline phosphatase. In solutions devoid of ATP and matrix vesicles, concentrations of PPi between 10 and 100 microM induce calcification in mixtures containing initial Ca2+ X P ion products of 3.5 to 7.9 mM2. This finding plus the discovery of nucleoside triphosphate pyrophosphohydrolase in matrix vesicles supports the view that these extracellular organelles induce calcium precipitation by the enzymatic production of PPi. Nucleoside triphosphate pyrophosphohydrolase is more active against pyrimidine nucleoside triphosphates than the corresponding purine derivatives. The pH optimum is 10.0 and the enzyme is neither activated nor inhibited by Mg2+ or Ca2+ ions or mixtures of the two. Vmax at pH 7.5 for beta, gamma-methylene ATP is 0.012 mumol of substrate hydrolyzed per min per mg of protein and Km is below 10 microM. The enzyme is irreversibly destroyed at pH 4 and is stable at pH 10.5.     

A soluble pyrophosphatase, a key enzyme for polyphosphate metabolism in Leishmania

We report the functional characterization in Leishmania amazonensis of a soluble pyrophosphatase (LaVSP1) that localizes in acidocalcisomes, a vesicular acidic compartment. LaVSP1 is preferentially expressed in metacyclic forms. Experiments with dominant negative mutants show the requirement of LaVSP1 functional expression for metacyclogenesis and virulence in mice. Depending on the pH and the cofactors Mg2+ or Zn2+, both present in acidocalcisomes, LaVSP1 hydrolyzes either inorganic pyrophosphate (Km = 92 microM, kcat = 125 s(-1)), tripolyphosphate (Km = 1153 microM, kcat = 131 s(-1)), or polyphosphate of 28 residues (Km = 123 microM, kcat = 8 s(-1)). Predicted structural analysis suggests that the structural orientation of the residue Lys78 in LaVSP1 accounts for the observed increase in Km compared with the yeast pyrophosphatase and for the ability of trypanosomatid VSP1 enzymes to hydrolyze polyphosphate. These results make the VSP1 enzyme an attractive drug target against trypanosomatid parasites.     

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.     

Phosphomonoesterase hydrolysis of polyphosphoinositides in rat kidney: Properties and subcellular localization of the enzyme system.

Tthe properties of diphosphoinositide and triphosphoinositide phosphatases from rat kidney homogenate were studied in an assay system in which non-specific phosphatase activity was eliminated. The enzymes were not completely metal-ion dependent and were activated by Mg2+. The detergent sodium deoxycholate, Triton X-100 and Cutscum inhibited the reaction; cetyltrimethylammonium bromide only activated when added with the subtrates and in the presence Mg2+. Both enzymes had a pH optimum of 7.5. Ca2+ and Li+ both activated triphosphoinositide phosphatase, but Ca2+ inhibited and L+ had little effect on diphosphoinositide phosphatase. Cyclic AMP had no effect on either enzyme. The enzymes were three times more active in kidney cortex than in the medulla. On subcellular fractionation of kidney-cortex homogenates by differential and density-gradient centrifugation, the distribution of the enzymes resembled that of thiamin pyrophosphatase (assayed in the absence of ATP), suggesting localization in the Golgi complex. However, the distribution differed from that of the liver Golgimarker galactosyltransferase. Activities of both diphosphoinositide and triphosphoinositide phosphatases and thiamin pyrophosphatase were low in purified brush-border fragments. Further experiments indicate that at least part of the phosphatase activity is soluble.     

Identification of cytosolic Mg2+-dependent soluble inorganic pyrophosphatases in potato and phylogenetic analysis

Using polyclonal antibodies raised against a previously cloned potato Mg2+-dependent soluble inorganic pyrophosphatase (ppa1 gene) [8], a second gene, called ppa2, could be isolated. A single locus homologous to ppa2 was mapped on potato chromosomes, unlinked to the two loci identified for ppa1. From a phylogenetic and structural point of view, the PPA1 and PPA2 polypeptides are more closely related to prokaryotic than to eukaryotic Mg2+-dependent soluble inorganic pyrophosphatases (soluble PPases). Subcellular localization by immunogold electron microscopy, using sections from leaf parenchyma cells, showed that PPA1 and PPA2 are localized to the cytosol. Based on these observations, the likely phylogenetic origin and the physiological significance of the cytosolic soluble pyrophosphatases are discussed.     

Subcellular Localization of Inorganic Pyrophosphatases in Rabbit Skeletal Muscle and Some Properties of a Microsomal Acid Pyrophosphatase

Subcellular localization of muscle inorganic pyrophosphatase was examined using rabbit skeletal muscle homogenates. The pyrophosphatases were found to be contained in the microsomal, mitochondrial, and cytosol fractions. The microsomal and mitochondrial pyrophosphatases were most likely bound to the respective subcellular fractions. The pyrophosphatases associated with microsome and mitochondria showed their optimal activities at about pH 5.5 and 7, respectively. They were not dissociated from the particles by washing with salt solution or by ten times freezing-thawing. The activity of microsomal acid pyrophosphatase was not affected by Mg,²⁺ Ca,²⁺ or EDTA, but that of the mitochondrial neutral pyrophosphatase was enhanced by the addition of Mg.²⁺ The microsomal acid pyrophosphatase was stable between pH values of 5.5 and 8.5 during storage at 4°. The activity was inhibited by p-chloromercuribenzoate. The activity was irreversibly inhibited by sodium dodecyl sulfate, but reversibly inhibited by neutral salts and membrane solubilizing detergents such as Triton X-100, octaethylene glycol mono-n-dodecylether, and sodium cholate.     

A novel phosphodiesterase I from the soluble fraction of erythrocytes

A previously unrecognized erythrocyte phosphodiesterase I with activity against thymidine-5'-monophospho-p-nitrophenyl ester is described. The enzyme is present in the soluble fraction of the erythrocyte, and was purified about 500-fold by chromatography using DEAE-cellulose, followed by gel chromatography with Sephadex G-200. Erythrocyte phosphodiesterase I has a molecular weight of about 70 000, when fully active as a monomer. Its pI is 5.4 and the pH optimum is 8.5. The Km value for thymidine-5'-monophospho-p-nitrophenyl ester is rather high, about 4 mmol/l. The enzyme has a barely detectable nucleotide pyrophosphatase activity. It is extremely sensitive to SH-inhibitors such as N-ethyl-maleimide, p-chloromercuribenzoate and disulphides (a reversible 50% inhibition was obtained by cystamine, 0.01 mmol/l). It is a metalloenzyme with loosely bound metal, and is stimulated by Mg2+. This activation by Mg2+ is counteracted by Zn2+. Gel chromatography revealed that the enzyme is a monomer in the presence of Mg2+. When inhibited by Zn2+, it forms polymers that can be reconverted to the monomer by thiols. All of the above properties of the erythrocyte enzyme support the conclusion that it is different from plasma membrane phosphodiesterase I (oligonucleate 5'-nucleotidohydrolase, EC 3.1.4.1).