Characterization of intermediate-molecular-weight acid phosphatase from bovine kidney cortex

The distribution of acid phosphatases of intermediate molecular weight was determined in various mammalian tissues. The intermediate-molecular-weight acid phosphatases (designated P-II-1 and 2) comprised about 25% of the p-nitrophenyl phosphatase activity in the supernatant of bovine kidney cortex homogenate. The P-II-1 and 2 purified 2,000 fold showed the pI values of 5.9 and 5.7, respectively, on isoelectric focusing. Apparent molecular weights of both P-II-1 and 2 were estimated to be 42,000 by Sephadex G-100 gel filtration and 44,000 by SDS-polyacrylamide disc gel electrophoresis. Both the enzymes catalyzed the hydrolysis of a wide variety of natural phosphomonoesters, except for the phosphoproteins phosphoserine and o-phosphocholine. The enzymes showed a high activity on pyridoxal phosphate, beta-glycerophosphate, and 2'-AMP. The optimum activity pH was near 5 with p-nitrophenyl phosphate, but was shifted to the neutral range when pyridoxal phosphate was the substrate. The cations Hg2+ and Ag+ had a marked inhibitory effect. Neither enzyme was inhibited significantly by L-(+)-tartrate or pCMB. The two other types of acid phosphatases, the high-molecular-weight (designated P-I) and low-molecular-weight (designated P-III), were also purified to homogeneity from bovine kidney cortex, and were compared with P-II from several aspects including substrate specificity and susceptibility to various compounds.     

Acid and alkaline phosphatases activities in vascular smooth muscle: species differences and subcellular distribution

1. Acid and alkaline phosphatase activities were studied in rat and dog aortic muscle using p-nitrophenyl phosphate (p-NPP) as the substrate. Alkaline phosphatase activity was quite comparable to acid phosphatase activity in rat aortic microsomes as well as further purified plasma membranes, but considerably lower than acid phosphatase activity in dog aortic membranes. 2. Subcellular distribution of acid and alkaline phosphatase activities in these vascular muscles indicated that alkaline phosphatases and a large portion of acid phosphatase activities were primarily associated with plasma membranes and the distribution of acid phosphatase showed little resemblance to that of N-acetyl-beta-glucosaminidase, a lysosomal marker enzyme. 3. The rat aortic plasmalemmal acid and alkaline phosphatase activities responded very differently to magnesium, fluoride, vanadate and EDTA. The alkaline phosphatase was more susceptible to heat inactivation than acid phosphatase. 4. These results suggest that these two phosphatases are likely to be two different enzymes in the smooth muscle plasma membranes. The implication of the present findings is discussed in relation to the alteration of these phosphatases in hypertensive vascular diseases.     

The cell surface associated phosphatase activity of Mycobacterium bovis BCG is not regulated by environmental inorganic phosphate

Non-specific phosphomonoesterase activities (alkaline phosphatase (EC 3.1.3.1) and acid phosphatase (EC 3.1.3.2)) were examined at the cell surface of Mycobacterium bovis BCG. Using p-nitrophenylphosphate as the substrate, peaks of phosphatase activity were detected at pH 6.0, pH 10.0 and pH 12.0, suggesting the presence of one acid phosphatase and two alkaline phosphatases with distinct optimum pH values. Contrary to the situation observed in several other microorganisms, the expression of these enzymes is not regulated by the environmental inorganic phosphate concentration.     

Rat liver lowM r phosphotyrosine protein phosphatase isoenzymes: Purification and amino acid sequences

Two lowM _r phosphotyrosine protein phosphatases have been isolated from rat liver. The enzymes were previously known as lowM _r acid phosphatases, but several recent studies have demonstrated that this family of enzymes possesses specific phosphotyrosine protein phosphatase activity. We determined the complete amino acid sequences of the two isoenzymes and named them AcP1 and AcP2. Both consist of 157 amino acid residues, are acetylated at the NH₂-terminus, and have His as the COOH-terminus. The molecular weights calculated from the sequences are 18,062 for AcP1 and 17,848 for AcP2. They are homologous except in the 40–73 zone, where about 50% of residues are different. This fact suggests that the two isoenzymes are produced by an alternative splicing mechanism. There is no homology between these two isoenzymes and the receptor-like phosphotyrosine protein phosphatases LAR, CD45, human placenta PTPase 1B, and rat brain PTPase-1. AcP1 and AcP2 are also distinct from rat liver PTPase-1 and PTPase-2, since these last enzymes have higher molecular weights. AcP1 differs from AcP2 with respect to (1) substrate affinity and (2) its sensitivity to activators and inhibitors, thus suggesting a their different physiological function.     

Characterization of Leishmania donovani acid phosphatases

A crude membrane fraction from promastigotes of Leishmania donovani grown in a liquid culture medium containing 20% fetal calf serum was prepared by freeze-thawing, centrifugation (200,000 X g, 30 min), and extraction with 2% (w/v) sodium cholate. After removal of the bile salt by chromatography on a Sephadex G-75 column, the solubilized membrane protein fraction, rich in acid phosphatase activity, was chromatographed on columns containing concanavalin A-Sepharose, QAE-Sephadex, and Sephadex G-150 and G-100. Three distinct acid phosphatases were resolved: the major phosphatase activity (70% of the total) was L-(+)-tartrate-resistant (designated ACP-P1) and corresponds to the acid phosphatase localized to the outer surface of the parasite's plasma membrane; the other two phosphatases (ACP-P2 and ACP-P3) account for the remaining 30% of the particulate acid phosphatase activity, and both of these enzymes are L-(+)-tartrate-sensitive. Using a combination of sucrose density gradient centrifugation, gel filtration chromatography, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, it was determined that ACP-P1 is a 128,000-dalton protein composed of two subunits of 65,000-68,000 daltons. ACP-P1 has an isoelectric point of 4.1, a pH optimum of 5.5, hydrolyzes fructose 1,6-diphosphate, but no other sugar phosphates and dephosphorylates phosphotyrosine, yeast mannan, and the phosphorylated form of rat liver pyruvate kinase. ACP-P2 (pI, 5.4) and ACP-P3 (pI, 7.1) with molecular masses of 132,000 and 108,000 daltons, respectively, are both tartrate-sensitive and are distinguished from each other on the basis of their sensitivity to inhibition by polyanionic molybdenum complexes. These two phosphatases also have their pH optima in the pH 5.0-6.0 range, but have a considerably broader substrate specificity than ACP-P1.     

Rat liver lowMr phosphotyrosine protein phosphatase isoenzymes: Purification and amino acid sequences

Two lowM _r phosphotyrosine protein phosphatases have been isolated from rat liver. The enzymes were previously known as lowM _r acid phosphatases, but several recent studies have demonstrated that this family of enzymes possesses specific phosphotyrosine protein phosphatase activity. We determined the complete amino acid sequences of the two isoenzymes and named them AcP1 and AcP2. Both consist of 157 amino acid residues, are acetylated at the NH₂-terminus, and have His as the COOH-terminus. The molecular weights calculated from the sequences are 18,062 for AcP1 and 17,848 for AcP2. They are homologous except in the 40–73 zone, where about 50% of residues are different. This fact suggests that the two isoenzymes are produced by an alternative splicing mechanism. There is no homology between these two isoenzymes and the receptor-like phosphotyrosine protein phosphatases LAR, CD45, human placenta PTPase 1B, and rat brain PTPase-1. AcP1 and AcP2 are also distinct from rat liver PTPase-1 and PTPase-2, since these last enzymes have higher molecular weights. AcP1 differs from AcP2 with respect to (1) substrate affinity and (2) its sensitivity to activators and inhibitors, thus suggesting a their different physiological function.     

Comparison of the properties of the protein phosphatases from avian and mammalian smooth muscles: purification and characterization of rabbit uterine smooth muscle phosphatases

Three protein phosphatases were purified to near homogeneity from rabbit uterine muscle. These enzymes are termed rabbit uterine smooth muscle phosphatase (RU SMP)-I, -II, and -IV. RU SMP-I is composed of three subunits (Mr 60,000, 55,000, and 38,000) which comigrated with the subunits of turkey gizzard smooth muscle phosphatase (TG SMP)-I. Ethanol treatment of RU SMP-I dissociated the subunits and led to the purification of its catalytic subunit (Mr 38,000), RU SMP-Ic. Structural homology between the turkey gizzard and rabbit uterine SMP-I is indicated by the cross-reactivity of RU SMP-I with the polyclonal antibodies against TG SMP-I and -Ic. Like TG SMP-II, RU SMP-II is inactive in the absence of divalent cations and can be activated by Mg2+ and Mn2+. However, their electrophoretic profiles on sodium dodecyl sulfate-polyacrylamide gel are different. RU SMP-II shows two bands (Mr 42,000 and 44,000) while TG SMP-II is monomeric (Mr 43,000). Western blot analysis revealed that the 42,000 and 44,000-Da proteins cross-react with anti-TG SMP-II antibodies, suggesting that these proteins share common structural properties. The anti-TG SMP-I and Ic antibodies do not cross-react with RU SMP-II and -IV. Likewise, the anti-TG SMP-II antibodies do not cross-react with RU SMP-I and -IV, implying that these enzymes are distinct. RU SMP-IV is composed of a catalytic subunit (Mr 40,000) and a subunit with a molecular weight of 60,000 or 58,000. All three rabbit uterine smooth muscle phosphatases dephosphorylate the isolated myosin light chains but only RU SMP-IV dephosphorylates heavy meromyosin. However, when the catalytic subunit of RU SMP-I is dissociated from the regulatory subunits, it is active toward heavy meromyosin and exhibits higher activity toward myosin light chains and phosphorylase a than its holoenzyme. The substrate specificity of these enzymes and the effects of ATP, NaF, pyrophosphate, okadaic acid, Mg2+, Mn2+, and Ca2+ on their activities are very similar to those of the turkey gizzard smooth muscle phosphatases.     

Molecular basis for substrate specificity of protein kinases and phosphatases

Regulation of various metabolic processes occurs by the phosphorylation/dephosphorylation of enzymes. Both the protein kinases that catalyze the phosphorylations and the protein phosphatases that catalyze the dephosphorylations display relatively broad specificity, reacting with a number of distinct sites in target enzymes. In this way changes in the activity of a particular kinase or phosphatase can cause coordinated and pleiotropic responses. However, the kinases and phosphatases do not exhibit a one-to-one correspondence in their reactions. Residues at different positions may be phosphorylated by a single kinase, yet dephosphorylated by different individual phosphatases. Conversely, sites which are substrates for different individual kinases may be dephosphorylated by a single phosphatase. In exploring the molecular basis for these differences this article shows that whereas kinases react with specific primary structures that often times appear as beta bends, the phosphatases recognize higher order structure, less strictly ruled by amino acid sequence surrounding the phosphorylated site. The differences, seen in the ability of these enzymes to utilize synthetic peptide substrates, might be rationalized in terms of function. Kinases need protruding segments of structure that can be enwrapped to exclude water, thereby minimizing ATP hydrolysis and enhancing phosphotransferase activity. On the other hand phosphatases are hydrolytic enzymes that may operate especially well on protein interfaces. Hydrolytic action often measured with p-nitrophenylphosphate is not necessarily indicative of a protein phosphatase and consideration of the mechanism reveals why this substrate can be misleading.(ABSTRACT TRUNCATED AT 250 WORDS)     

The MgATP-dependent protein phosphatase and protein phosphatase 1 have identical substrate specificities

The MgATP-dependent phosphorylase phosphatase was found to have a broad substrate specificity. Its activity against all phosphoproteins tested was dependent upon preincubation with the activating factor FA and MgATP. The enzyme dephosphorylated and inactivated phosphorylase kinase and inhibitor 1, and dephosphorylated and activated glycogen synthase and acetyl-CoA carboxylase. Glycogen synthase was dephosphorylated at similar rates whether it had been phosphorylated by cyclic-AMP-dependent protein kinase, phosphorylase kinase or glycogen synthase kinase 3. The enzyme also catalysed the dephosphorylation of ATP citrate lyase, initiation factor eIF-2, and troponin I. The properties of the MgATP-dependent protein phosphatase from either dog liver or rabbit skeletal muscle showed a remarkable similarity to highly purified preparations of protein phosphatase 1 from rabbit skeletal muscle. The relative activities of the two enzymes against all phosphoproteins tested was very similar. Both enzymes dephosphorylated the beta-subunit of phosphorylase kinase 40-fold faster than the alpha-subunit, and both enzymes were inhibited by identical concentrations of the two proteins termed inhibitor 1 and inhibitor 2, which inhibit protein phosphatase 1 specifically. These results demonstrate that the MgATP-dependent protein phosphatase is a type-1 protein phosphatase, and is distinct from type-2 protein phosphatases which dephosphorylate the alpha-subunit of phosphorylase kinase and are unaffected by inhibitor 1 and inhibitor 2. The possibility that the MgATP-dependent protein phosphatase is an inactive form of protein phosphatase 1 and that both proteins share the same catalytic subunit is discussed.     

The protein phosphatases involved in cellular regulation. 1. Classification and substrate specificities

The protein phosphatase activities involved in regulating the major pathways of intermediary metabolism can be explained by only four enzymes which can be conveniently divided into two classes, type-1 and type-2. Type-1 protein phosphatases dephosphorylate the beta-subunit of phosphorylase kinase and are potently inhibited by two thermostable proteins termed inhibitor-1 and inhibitor-2, whereas type-2 protein phosphatases preferentially dephosphorylate the alpha-subunit of phosphorylase kinase and are insensitive to inhibitor-1 and inhibitor-2. The substrate specificities of the four enzymes, namely protein phosphatase-1 (type-1) and protein phosphatases 2A, 2B and 2C (type-2) have been investigated. Eight different protein kinases were used to phosphorylate 13 different substrate proteins on a minimum of 20 different serine and threonine residues. These substrates include proteins involved in the regulation of glycogen metabolism, glycolysis, fatty acid synthesis, cholesterol synthesis, protein synthesis and muscle contraction. The studies demonstrate that protein phosphatase-1 and protein phosphatase 2A have very broad substrate specificities. The major differences, apart from the site specificity for phosphorylase kinase, are the much higher myosin light chain phosphatase and ATP-citrate lyase phosphatase activities of protein phosphatase-2A. Protein phosphatase-2C (an Mg2+-dependent enzyme) also has a broad specificity, but can be distinguished from protein phosphatase-2A by its extremely low phosphorylase phosphatase and histone H1 phosphatase activities, and its slow dephosphorylation of sites (3a + 3b + 3c) on glycogen synthase relative to site-2 of glycogen synthase. It has extremely high hydroxymethylglutaryl-CoA (HMG-CoA) reductase phosphatase and HMG-CoA reductase kinase phosphatase activity. Protein phosphatase-2B (a Ca2+-calmodulin-dependent enzyme) is the most specific phosphatase and only dephosphorylated three of the substrates (the alpha-subunit of phosphorylase kinase, inhibitor-1 and myosin light chains) at a significant rate. It is specifically inhibited by the phenathiazine drug, trifluoperazine. Examination of the amino acid sequences around each phosphorylation site does not support the idea that protein phosphatase specificity is determined by the primary structure in the immediate vicinity of the phosphorylation site.     

Turkey gizzard smooth muscle myosin phosphatase-III is a novel protein phosphatase.

Chromatography of turkey gizzard extract on Sephacryl S-300 has been shown to fractionate the various smooth muscle phosphatases. We have previously reported the purification and characterization of three of these enzymes, termed smooth muscle phosphatase (SMP)-I, -II, and -IV. Recently, we have purified SMP-III to near homogeneity. Although all of the smooth muscle phosphatases dephosphorylate the isolated myosin light chains, only SMP-III and -IV are active toward intact myosin and, therefore, are most likely to play a direct role in the muscle contraction-relaxation process. SMP-III has a higher molecular weight (390,000), as determined by gel filtration, than the other smooth muscle phosphatases and migrates as single band with a molecular weight of 40,000 in a sodium dodecyl sulfate-polyacrylamide gel. SMP-III is immunologically distinct from SMP-I and -II. It dephosphorylates heavy meromyosin and the isolated myosin light chains at a rapid rate but has low activity toward phosphorylase alpha. The activity of SMP-III is not affected by Ca2+ but is activated by Mn2+.Mg2+ stimulates the activity toward heavy meromyosin but inhibits the myosin light chain phosphatase activity. Attempts to classify SMP-III according to the scheme proposed by Ingebritsen and Cohen (Ingebritsen T. S., and Cohen, P. (1983) Science 221, 331-338) revealed that it is resistant to the heat stable inhibitor-2, suggesting that it is a Type 2 protein phosphatase. However, SMP-III is inhibited by concentrations of okadaic acid which are characteristic of Type 1 protein phosphatases and it binds to heparin-Sepharose like other Type 1 phosphatases. But most interestingly, SMP-III does not dephosphorylate the alpha- or beta-subunits of phosphorylase kinase, a property not reported for any Ser/Thr protein phosphatase.     

Purification, properties, and substrate specificities of phosphoprotein phosphatase(s) from rabbit liver.

The phosphoprotein phosphatase(s) acting on muscle phosphorylase a was purified from rabbit liver by acid precipitation, high speed centrifugation, chromatography on DEAE-Sephadex A-50, Sephadex G-75, and Sepharose-histone. Enzyme activity was recovered in the final step as two distinct peaks tentatively referred to as phosphoprotein phosphatases I and II. Each phosphatase showed a single broad band when examined by sodium dodecyl sulfate gel electrophoresis; the molecular weights derived by this method were approximately 30,500 for phosphoprotein phosphatase I and 34,000 for phosphoprotein phosphatase II. The s20, w value for each enzyme was 3.40. Using this value and values for the Stokes radii, the molecular weight for each enzyme was calculated to be 34,500. Both phosphatases, in addition to catalyzing the conversion of phosphorylase a to b, also catalyzed the dephosphorylation of glycogen synthase D, activated phosphorylase kinase, phosphorylated histone, phosphorylated casein, and the phosphorylated inhibitory component of troponin (TN-I). The relative activities of the phosphatases with respect to phosphorylase a, glycogen synthase D, histone, and casein remained essentially constant throughout the purification. The activities of both phosphatases with different substrates decreased in parallel when they were denatured by incubation at 55 degrees and 65 degrees. The Km values of phosphoprotein phosphatase I for phosphorylase a, histone, and casein were lower than the values obtained for phosphoprotein phosphatase II. With glycogen synthase D as substrate, each enzyme gave essentially the same Km value. Utilizing either enzyme, it was found that activity toward a given substrate was inhibited competitively by each of the alternative substrates. The results suggest that phosphoprotein phosphatases I and II are each active toward all of the substrates tested.     

Identification of two isoenzymes of protein phosphatase 2C in both rabbit skeletal muscle and liver

Protein phosphatase 2C was isolated from rabbit skeletal muscle by a procedure that involved chromatography on DEAE-cellulose, precipitation with ammonium sulphate, gel-filtration on Sephadex G-100, affinity chromatography on thiophosphorylated myosin-P-light-chain--Sepharose and chromatography on Mono Q. The enzyme was purified about 35,000-fold and 0.3-0.4 mg was isolated from 2500 g skeletal muscle within 5 days. The final step resolved the activity into two peaks, termed protein phosphatases 2C1 and 2C2, that possessed identical substrate specificities and enzymatic properties. About 2.5-fold more protein phosphatase 2C2 was isolated than protein phosphatase 2C1. Protein phosphatases 2C1 and 2C2 migrated as single bands on SDS/polyacrylamide gels yielding apparent molecular masses of 44 kDa and 42 kDa, respectively, and the native proteins were both monomeric at pH 7.5 as judged by their elution from Sephadex G-100 and Sephacryl S200. Peptide maps of protein phosphatases 2C1 and 2C2, obtained after separate digestions with four different proteinases, were different, indicating that they are isoenzymes. Protein phosphatases 2C1 and 2C2 were purified from rabbit liver by the same procedure, and 0.2 mg (2C1 + 2C2) was isolated from 120 g hepatic tissue. Hepatic protein phosphatases 2C1 and 2C2 were also isolated in a molar ratio of about 1:2.5, and their enzymatic properties and apparent molecular masses in the presence and absence of SDS were identical to the skeletal muscle enzymes. Protein phosphatases 2C1 from muscle and liver displayed identical peptide maps, as did protein phosphatases 2C2 from these two tissues. It is concluded that the same two isoenzymes of protein phosphatase 2C are present in skeletal muscle and liver.     

Separation and some properties of two acid phosphatases from suspension cultures ofIpomoea sp.

Two acid phosphatases isolated from culturedIpomoea (moring glory) cells were separated by column chromatography on DEAE-cellulose. The two acid phosphatases have different pH optima (pH 4.8–5.0 and 6.0) and do not require the presence of divalent ions. The enzymes possess high activity toward pyrophosphate,p-nitrophenylphosphate, nucleoside di- and triphosphates, and much less activity toward nucleoside monophosphates and sugar esters. The two phosphatases differ from each other in Michaelis constants, in the degree of inhibition by arsenate, fluoride and phosphate and have quantitative differences of substrate specificity. In addition, they also differ in their response to various ions. Issued as NRCC No. 20658     

Determination of Francisella tularensis AcpB acid phosphatase substrate preferences

The Francisella species encode 4 main acid phosphatases (Acp) that are potentially involved in pathogenesis through currently unknown mechanisms. Only 2 of these enzymes, AcpA and AcpC, have been biochemically characterized to date. In this work we describe the catalytic properties of Francisella tularensis AcpB utilizing an array of 120 phosphorylated substrates. In contrast to most acid phosphatases, the purified enzyme showed a narrow range of substrate preferences, with the highest affinity towards thiamine phosphate (Km = 150 μM). Francisella species do not possess a thiamine biosynthetic pathway even though vitamin B1 is indispensable in numerous cellular functions. Consequently, thiamine should be incorporated from the environment, in this case, from the host cell. Our results suggested that AcpB could provide the hydrolytic activity necessary to transform the nontransportable phosphorylated vitamin B1 present in tissues to a form that can be absorbed by the intracellular pathogen.     

Isolation and characterization of rabbit skeletal muscle protein phosphatases C-I and C-II

Previous studies have shown that phosphorylase phosphatase can be isolated from rabbit liver and bovine heart as a form of Mr approximately 35,000 after an ethanol treatment of tissue extracts. This enzyme form was designated as protein phosphatase C. In the present study, reproducible methods for the isolation of two forms of protein phosphatase C from rabbit skeletal muscle to apparent homogeneity are described. Protein phosphatase C-I was obtained in yields of up to 20%, with specific activities toward phosphorylase a of 8,000-16,000 units/mg of protein. This enzyme represents the major phosphorylase phosphatase activity present in the ethanol-treated muscle extracts. The second enzyme, protein phosphatase C-II, had a much lower specific activity toward phosphorylase a (250-900 units/mg). Phosphatase C-I and phosphatase C-II had Mr = 32,000 and 33,500, respectively, as determined by sodium dodecyl sulfate disc gel electrophoresis. The two enzymes displayed distinct enzymatic properties. Phosphatase C-II was associated with a more active alkaline phosphatase activity toward p-nitrophenyl phosphate than was phosphatase C-I. Phosphatase C-II activities were activated by Mn2+, whereas phosphatase C-I was inhibited. Phosphatase C-I was inhibited by rabbit skeletal muscle inhibitor 2 while phosphatase C-II was not inhibited. Both enzymes dephosphorylated glycogen synthase and phosphorylase kinase, but displayed different specificities toward the alpha- and beta-subunit phosphates of phosphorylase kinase (Ganapathi, M. K., Silberman, S. R., Paris, H., and Lee, E. Y. C. (1980) J. Biol. Chem. 246, 3213-3217). The amino acid compositions of the two proteins were similar. Peptide mapping of the two proteins showed that they are distinct proteins and do not have a precursor-proteolytic product relationship.     

A method for measuring activities of acid phosphatases separated by acrylamide gel electrophoresis

Methods for measuring the activities of acid phosphatases with the same substrate, alpha-naphthyl acid phosphate, both before and after acrylamide gel electrophoresis are described. The gel assay, which involves elution of the precipitated dye complex, can be used to measure the length of linear reaction rates of the enzymes separated in gels. It is possible with the use of these methods both to determine the effect of electrophoresis on the activity of acid phosphatases and to correlate the amount of precipitated dye with the area under the peak on a densitometric tracing of the same band.     

Mutagenesis of putative catalytic and regulatory residues of Streptomyces chromofuscus phospholipase D differentially modifies phosphatase and phosphodiesterase activities

Phospholipase D from Streptomyces chromofuscus (sc-PLD) is a member of the diverse family of metallo-phosphodiesterase/phosphatase enzymes that also includes purple acid phosphatases, protein phosphatases, and nucleotide phosphodiesterases. Whereas iron is an essential cofactor for scPLD activity, Mn2+ is also found in the enzyme. A third metal ion, Ca2+, has been shown to enhance scPLD catalytic activity although it is not an essential cofactor. Sequence alignment of scPLD with known phosphodiesterases and phosphatases requiring metal ions suggested that His-212, Glu-213, and Asp-389 could be involved in Mn2+ binding. H212A, E213A, and D389A were prepared to test this hypothesis. These three mutant enzymes and wild type scPLD show similar metal content but considerably different catalytic properties, suggesting different roles for each residue. His-212 appears involved in binding the phosphate group of substrates, whereas Glu-213 acts as a ligand for Ca2+. D389A showed a greatly reduced phosphodiesterase activity but almost unaltered ability to hydrolyze the phosphate group in p-nitrophenyl phosphate suggesting it had a critical role in aligning groups at the active site to control phosphodiesterase versus phosphatase activities. We propose a model for substrate and cofactor binding to the catalytic site of scPLD based on these results and on sequence alignment to purple acid phosphatases of known structure.     

Identification, purification, and characterization of phosphotyrosine-specific protein phosphatases from cultured chicken embryo fibroblasts.

Tyrosine phosphorylation catalyzed by a unique class of protein kinases is an important process in both normal cell proliferation and oncogenic transformation. In this study, phosphoprotein phosphatases specific for the dephosphorylation of phosphotyrosine residues were partially purified from secondary chicken embryo fibroblasts, using 32P-labeled immunoglobulin G phosphorylated by pp60src as substrate. Crude cell extracts contained ca. 70% of the activity in the soluble form and ca. 30% associated with a crude membrane fraction. The soluble activity was purified by using DEAE-cellulose and carboxymethyl cellulose column chromatography and gel filtration, and at least three enzyme species of apparent Mr 55,000 (pTPI), 50,000 (pTPII), and 95,000 (pTPIII)--comprising ca. 20, 45, and 35%, respectively, of the total activity--were resolved. All three enzymes possessed somewhat similar properties. They had a pH optimum of about 7.4, they were inhibited by Zn2+, vanadate, ATP, and ADP, and they were unaffected by divalent metal cations, EDTA, and F- under standard assay conditions employing a physiological ionic strength. These properties suggest that they represent a class of enzymes distinct from well-known phosphoseryl-phosphothreonyl-protein phosphatases and that dephosphorylation of phosphotyrosine-containing proteins may be carried out by a unique family of phosphoprotein phosphatases. Transformation by Rous sarcoma virus resulted in a small increase in phosphotyrosyl-protein phosphatase activity.