Purification and properties of polyphosphoinositide phosphomonoesterase from rat brain.

1. On subcellular fractionation of rat brain homogenate, polyphosphoinositide phosphomonoesterase activity was greater in the cytosol than the membranous fractions. 2. The enzyme was purified from the cytosol by column chromatography on DEAE-cellulose, calcium phosphate gel and Sephadex G-100. 3. The final preparation of the enzyme showed a 430-fold purification over the whole homogenate and appeared to be homogeneous since it gave a single band on sodium dodecyl sulphate-polyacrylamide gel electrophoresis and on isoelectric focusing. The enzyme has a relatively low molecular weight and an isoelectric point of 6.8. 4. The phosphatase showed a high affinity for triphosphoinositide. Without added Mg2+, the Km was 25 muM and V was 33 mumol Pi released/min/mg protein. 5. The enzyme hydrolysed diphosphoinositide at a slower rate than triphosphoinositide. In the presence of 10 mM Mg2+, the Km values for triphosphoinositide and diphosphoinositide were 5 muM and 25 muM respectively and V was the same for each substrate. 6. Both Mg2+ and Ca2+ activated the enzyme. While Ca2+ produced maximum activation at 100 muM, a much higher concentration of Mg2+ (10 mM) was required to elicit comparable activation. The enzyme did not show an absolute requirement for Mg2+ or Ca2+ as it exhibited low activity in the presence of 0.5 mM EDTA or EGTA. 7. The phosphatase showed maximum activity between 7.4 and 7.6. A drop in pH to 7.0 activated it almost completely, whereas an increase in pH to 8.0 halved the activity. 7.0 activated it almost completely, whereas an increase in pH to 8.0 halved the activity.     

Resolution and purification of taurine- and GABA-synthesizing decarboxylases from calf brain

The present work describes a procedure for the co-purification of cysteine sulfinate decarboxylase (CSAD) and glutamate decarboxylase (GAD) from calf brain. A crude enzyme preparation was first made from brain homogenate by acid precipitation and ammonium sulphate fractionation. Subsequent fractionation of the decarboxylase preparation by cation exchange chromatography on CM-Sepharose CL-6B revealed the existence of a specific CSAD enzyme, which has no GAD activity. The GAD activity peak was found to possess CSAD activity. Further fractionation by gel filtration on Sephacryl S-200 separated the specific CSAD activity into two enzyme forms, one of them having a molecular weight of 150,000 and the other of 71,000. GAD activity was eluted from the gel filtration column in a single peak (mol wt 330,000) and showed CSAD activity. The purification of the specific CSAD enzyme was 920-fold and that of GAD activity 850-fold as compared with the starting material, whole calf brain. SDS gel electrophoresis indicated that the purified CSAD and GAD enzymes consisted of two or more subunits. The crude decarboxylase preparation was analysed by isoelectric focusing in ultra-thin polyacrylamide gel in the pH range 3.5-10.0. The most active fraction of CSAD indicated an isoelectric point of 6.5 and that of GAD 6.8. The pH optimum for CSAD activity in the crude preparation was 7.2 and that for GAD activity 7.9.     

Purification of D-myo-inositol 1,4,5-trisphosphate 3-kinase from rat brain

The ATP-dependent, calmodulin-sensitive 3-kinase responsible for the conversion of D-myo-inositol 1,4,5-trisphosphate to D-myo-inositol 1,3,4,5-tetrakisphosphate has been purified 2,700-fold from rat brain to a specific activity of 2.3 mumol/min/mg protein. A method of purification is described involving chromatography on phosphocellulose, Orange A dye ligand, calmodulin agarose, and hydroxylapatite columns. Neither the highly purified enzyme nor enzyme eluting from the phosphocellulose column were activated by Ca2+. However, enzyme in the 100,000 x g supernatant from rat brain was activated by Ca2+ over the range from 10(-7) to 10(-6) M and Ca2+ sensitivity of the purified enzyme was restored by the addition of calmodulin. The enzyme has a catalytic subunit Mr of 53,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Size exclusion chromatography of the purified enzyme on a Superose 12 column gave a Mr value of 70,000, indicating that the purified enzyme was present as a monomer. In contrast, the 100,000 x g supernatant and the purified enzyme after addition of calmodulin and 10(-6) M Ca2+ chromatographed on size exclusion chromatography with a Mr of 150,000-160,000. These results imply that the native enzyme is a dimeric structure of two catalytic subunits plus calmodulin. The purified enzyme showed a Km of 0.21 +/- 0.08 microM for D-myo-inositol 1,4,5-trisphosphate and had a pH optimum of 8.5. Addition of calmodulin increased both the Km and the Vmax of the purified enzyme about 2-fold. The high affinity of the 3-kinase for D-myo-inositol 1,4,5-trisphosphate together with its activation by Ca2+/calmodulin suggests that this enzyme may exert an important regulatory role in inositol phosphate signaling by promoting the formation of additional inositol polyphosphate isomers.     

Acid phosphatase in rat liver lysosomal membranes: purification and characterization

Acid phosphatase associated with rat liver lysosomal membranes (M-APase) was purified about 4,200-fold over the homogenate with 10% recovery to apparent homogeneity, as determined from the pattern on polyacrylamide gel electrophoresis in the presence of SDS. The purification procedure included; preparation of lysosomal membranes, solubilization of the membranes with 1% Triton X-100, immunoaffinity chromatography, and gel filtration with FPLC equipped with a Sephacryl S-300HR column. The molecular weight, estimated by gel filtration through TSK SW 3000G, was approximately 320K and SDS gel electrophoresis showed that the enzyme is composed of four identical subunits with an apparent molecular weight of 67K. The enzyme contains about 24.3% carbohydrate consisting of mannose, galactose, fucose, N-acetylglucosamine, N-acetylgalactosamine, and N-acetylneuraminic acid in a molar ratio of 38:20:5:36:4:11, respectively. In addition, three soluble forms of acid phosphatase (C-APase I, II, and III) in lysosomal contents were separated from rat liver lysosomal contents with DEAE-Sephacel. These three enzymes were also purified using immunoaffinity chromatography followed by gel filtration. C-APase I, II, III, and M-APase have isoelectric points of 7.7-8.2, 6.6-7.0, 5.7-6.7, and 3.4-3.8, respectively. All four APases are sensitive to endo-beta-N-acetylglucosaminidase H. However, only C-APase III and M-APase are digestible with neuraminidase. Susceptibility of M-APase to neuraminidase in intact tritosomes was examined to study the topography of M-APase in tritosomal membranes. Neuraminidase susceptibility of M-APase was not observed in the intact tritosomes until the tritosomes had been disrupted by osmotic shock.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purification of a novel type of calcium-activated neutral protease from rat brain. Possible involvement in production of the neuropeptide kyotorphin from calpastatin fragments

We have found a novel type of Ca2(+)-activated neutral protease in rat brain cytosol which cleaves -Tyr-Arg-containing calpastatin fragments to release the neuropeptide kyotorphin. This enzyme was purified about 26,000-fold by column chromatography as follows: DE52 cellulose, Ultrogel AcA 44, thiopropyl-Sepharose 6B, second DE52 cellulose, Ultrogel AcA 34, and blue Sepharose CL-6B. The molecular mass of the enzyme was estimated to be 65-75 kDa by gel filtration. The purified enzyme gave a single band of 74 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Some properties of this enzyme were similar to those of the calpains, i.e. an absolute requirement for Ca2+, maximal activity at neutral pH, and inhibition by sulfhydryl reagents such as p-chloromercuriphenylsulfonic acid and N-ethylmaleimide. However, it differs from the calpains in that it possesses no caseinolytic activity, separates from the calpains on the first DE52 column, and is insensitive to leupeptin and E-64 (N-[N-(L-3-trans-carboxyoxrian-2-carbonyl)-L-leucyl]agmatine). Thus, the molecular mass, the substrate specificity, the chromatographic behavior, and the inhibitor spectrum all suggest that this enzyme is a novel type of Ca2(+)-activated neutral protease.     

Purification and some properties of membrane-bound phospholipase B from Torulaspora delbrueckii

Membrane-bound phospholipase B was purified to a homogeneous state from Torulaspora delbrueckii cell homogenate. Cell homogenate was extracted with Triton X-100, and the enzyme was precipitated with acetone. The acetone powder was washed repeatedly with Tris-HCl buffer (pH 8.0) until no phospholipae B activity was detected in the soluble fraction. The enzyme was extracted with Triton X-100 from the final residue and purified about 1,390-fold by sequential chromatofocusing, Sepharose 6B, and DEAE-Sephadex A-50 column chromatography. The final preparation showed a single broad protein band on SDS-polyacrylamide gel electrophoresis when stained with silver stain reagent and PAS-reagent. The molecular weight of phospholipase B was about 390,000 and 140,000-190,000 as estimated by gel filtration on Sepharose 6B and SDS-polyacrylamide gel electrophoresis, respectively, suggesting that phospholipase B is an oligomeric protein. The isoelectric point was at pH 4.5. Phospholipase B has two pH optima, one acidic (pH 2.5-3.0) and the other alkaline (pH 7.2-8.0). At acidic pH the phospholipase B activity was greatly increased in the presence of divalent metal ions, although metal ions are not a factor for enzyme activity. On the other hand, at alkaline pH the enzyme required Ca2+ or Mn2+ for activity. The pH- and thermal-stabilities at both pHs were similar. The phospholipase B hydrolyzed all diacylphospholipids tested at acidic pH, but hydrolyzed only phosphatidylcholine at alkaline pH. The hydrolysis rates of lysophospholipids were much higher (about 10-fold) than those of diacylphospholipids at both pHs.     

Adenosine deaminase from bovine brain: purification and partial characterization.

Bovine brain adenosine deaminase cytoplasmatic form was purified about 450 fold by salt fractionation, column chromatography on DEAE-cellulose, octyl-sepharose 4B and affinity chromatography on CH-sepharose 4B 9-(p-aminobenzyl)adenine. The purified enzyme was homogeneous on disc gel electrophoresis; the enzyme had a molecular mass of about 65 kDa with an isoelectric point at pH 4.87. The Km values for adenosine and 2'-deoxyadenosine were 4 x 10(-5) and 5.2 x 10(-5) M, respectively. The enzyme showed a great stability to temperature with a half life of 15 hours at 53 degrees C significantly different compared to that known for other mammalian forms of this enzyme. Aza and deaza analogs of adenosine and erythro-9-(2-hydroxy-3-nonyl) adenine were good inhibitors of the bovine brain enzyme with little difference with respect to those reported for the adenosine deaminases purified from other sources. Kinetic constants for the association and dissociation of coformycin and 2'-deoxycoformycin with the bovine brain adenosine deaminase are reported.     

Purification of and kinetic studies on a narrow specificity synaptosomal membrane pyroglutamate aminopeptidase from guinea-pig brain

A pyroglutamate aminopeptidase activity, distinct from that of cytoplasm, was released from a synaptosomal membrane preparation of guinea-pig brain by papain treatment. This activity was further purified 3560-fold relative to the homogenate with a yield of 17% by a procedure involving gel filtration chromatography, calcium phosphate cellulose chromatography and hydrophobic interaction chromatography on phenyl-Sepharose CL-4B. The purified synaptosomal pyroglutamate aminopeptidase hydrolysed only thyroliberin, acid-thyroliberin, the luliberin N-terminal tripeptide (Glp-His-Trp) and, only slightly, Glp-His-Gly. No hydrolysis was observed with dipeptides containing N-terminal pyroglutamic acid (Glp) or with pyroglutamyl peptides containing more than three amino acids. A Km value of 40 microM was recorded when thyroliberin was used as substrate; however, luliberin was found to inhibit the hydrolysis of thyroliberin competitively with a Ki value of 20 microM.     

Topological arrangement in microsomal membranes of hepatic haem oxygenase induced by cobalt chloride.

Arysulphatase A was purified from rabbit testis. The purification was accomplished by a four-step procedure involving (NH4)2SO4 fractionation, chromatography on DEAE-cellulose, SP(sulphopropyl)-Sephadex and affinity chromatography on concanavalin A-Sepharose. The specific activity of purified preparation was 135 mumol/min per mg of protein, which represented an increase of 900-fold above that of the crude homogenate. The purified enzyme (20-50 micrograms) was found to move electrophoretically as a single band on polyacrylamide gel at pH 7.2 and 8.4. The homogeneous enzyme was shown to be a glycoprotein with 0.8% (w/w) of N-acetylneuraminic acid and 20% neutral sugar. The treatment of purified enzyme with bacterial neuraminidase had no effect on enzyme activity or kinetic properties, but it changed the elution prolife of rabbit testis arylsulphatase A through DEAE-Sephadex. The purified enzyme was strongly inhibited by Cu2+, Fe3+ and Ag+. It hydrolysed several sulphate esters including cerebroside 3-sulphate, ascorbic acid 2-sulphate and steroid sulphates. Pure arysulphatase was effective in dispersing the cumulus cells of rabbit ova.     

Studies on phospholipase A in Trimeresurus flaoviridis venom. III. Purification and some properties of phospholipase A inhibitor in Habu serum.

Phospholipase A [EC 3.1.1.4] inhibitor was purified from Habu (Trimeresurus flavivurudls) serum by gel filtration on Sephadex G-200, chromatography on DE-23 cellulose and affinity chromatography on a Sepharose 4B-phospholipase A column. By these procedures, a 31-fold increase in specific activity was attained with a yield of 15%. The purified material was homogeneous as judged by cellulose acetate and polyacrylamide gel electrophoresis. It had an apparent molecular weight of 100,000 as measured by gel filtration on Sephadex G-200. The purified inhibitor was stable for 20 min at 80 degrees and was unstable below pH 6. It migrated before albumin in cellulose acetate electrophoresis and did not form any precipitin line with the crude venom or with purified phospholipase A in immunodiffusin tests. An 8-fold excess of the purified inhibitor by weight was required to inhibit completely both the egg yolk clearing action and the hemolytic action of phospholipase A.     

Purifcation and properties of multiple forms of brain acetylcholinesterase (EC 3.1.1.7)

—Approximately 70 per cent of the total AChE of bovine brain tissue was solubilized by repeated homogenization and centrifugation in 0.32 m sucrose containing EDTA. After ammonium sulphate fractionation, application of the enzyme preparation to an agarose affinity gel column effected a 700-fold purification. Subsequent molecular filtration separated three active forms of AChE with molecular weights of 130,000, 270,000 and 390,000 with an average specific activity of 575 mmol of acetylthiocholine hydrolysed/mg of protein/h. The complete procedure represented an approximate 23,000-fold purification of the enzyme from that in the original tissue homogenate. The three forms of AChE exhibited certain differences in properties, including apparent K_m values, pH optima and sensitivity to inhibitory agents. Ancillary studies on less purified enzyme preparations by use of polyacrylamide gel electrophoresis and isoelectric focusing techniques also suggested that brain AChE exists in multiple forms.     

Characteristics of solubilized human-somatotropin-binding protein from the liver of pregnant rabbits.

A specific binding site for somatotropin was solubilized by 1% (v/v) Triton X-100 from a crude particulate membrane fraction of pregnant rabbit liver, partially purified and characterized. The solubilized binding site retained many of the characteristics observed in the original particulate fraction, indicating that extraction of the binding site with Triton X-100 does not cause any major changes in its properties. The binding of human 125I-labelled-somatotropin to the solubilized binding site is a saturable and reversible process, depending on temperature, incubation time, pH and ionic environment. Analysis of the kinetic data revealed a finite number of binding sites with an affinity constant of 0.32 x 10(10)M-1. The binding activity for human 125I-labelled-somatotropin was adsorbed to a concanavalin-A-Sepharose column and was dissociated from the column with alpha-methyl-D-glucoside, suggesting that the binding protein may be a glycoprotein. Using affinity chromatography on concanavalin-A-Sepharose, ion-exchange chromatography on DEAE-cellulose and gel filtration on Sepharose 6B, the binding protein was purified 1000-4000-fold from the original liver homogenate. When the partially purified preparation was chromatographed on Sepharose 6B, the binding protein eluted as a molecule with an apparent molecular weight of 200000, with a Stokes' radius of 4.9 nm. Sucrose-density-gradient centrifugation of the preparation showed that the sedimentation coefficient of the binding protein was 7.2S. Isoelectric focusing experiments revealed that a major part of the protein has an acidic pI (4.2-4.5). Exposure of the protein to trypsin decreased the binding activity for human 125I-labelled-somatotropin or bovine 125I-labelled-somatotropin, whereas ribonuclease, deoxyribonuclease, phospholipase C or neuraminidase had little or no effect.     

Purification and characterization of thiamine triphosphatase from bovine brain.

Soluble thiamine triphosphatase (EC 3.6.1.28) of bovine brain has been purified 68,000-fold to an electrophoretically homogeneous state with an overall recovery of 5.5% by hydrophobic chromatography on Toyopearl HW-60, Sephadex G-75 gel filtration, DEAE-Toyopearl 650M chromatography and Blue Sepharose CL-4B chromatography. The enzyme has an absolute specificity among thiamine and nucleoside phosphate esters for thiamine triphosphate and shows no nonspecific phosphatase activities. Thiamine triphosphatase is composed of a single polypeptide chain with molecular mass of 33,900 kDa as estimated by Sephadex G-100 gel filtration and SDS-polyacrylamide gel electrophoresis. The enzyme has a pH optimum of 8.7 and is dependent on divalent metal ions. Mg2+ has been found to be the most effective among cations tested. A study of the reaction kinetics over a wide range of thiamine triphosphate concentrations has revealed a biphasic saturation curve being described by higher-degree rational polynomials.     

Purification of Phosphatidylinositol Synthetase from Rat Brain by CDP-Diacylglycerol Affinity Chromatography and Properties of the Purified Enzyme

A purification procedure for rat brain phosphatidylinositol synthetase (PI synthetase; CDP-1,2-diacyl-sn-glycerol:myo-inositol 3-phosphatidyltransferase; EC 2.7.8.11) is described. The enzyme was purified 200-250-fold from the homogenate by solubilization with Triton X-100 from microsomal membranes and affinity chromatography on CDP-diacylglycerol-Sepharose. Elution of enzyme activity required the presence of Triton X-100, CDP-diacylglycerol, and either phosphatidylcholine or asolectin. The product that was obtained in 5-10% yield from whole brain and in 70% yield from the microsomal fraction contained three protein bands as determined by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The final preparation contained levels of CDP-diacylglycerol hydrolase and CDP-diacylglycerol: sn-glycero-3-phosphate 3-phosphatidyltransferase activities that were less than 1% of PI synthetase activity. The purified enzyme displayed a pH optimum of 8.5-9.0, required either Mg2+ or Mn2+ and exhibited a Km of 4.6 mM for myo-inositol.     

A rapid method for the purification of S-adenosylmethionine: Protein-carboxyl O-methyltransferase by affinity chromatography

A simple method to purify S-adenosylmethionine: protein-carboxyl O-methyltransferase (protein methylase II, EC 2.1.1.24) from calf brain has been developed using affinity chromatography. The product of the reaction, S-adenosyl-l-homocysteine, which is a competitive inhibitor of the enzyme, was covalently linked to Sepharose beads. This gel proved to be an effective binder for protein methylase II at pH 6.2 and allowed for specific removal of the enzyme by the addition of the methyl donor substrate, S-adenosyl-l-methionine to the elution buffer. One step using this affinity chromatography column resulted in 377-fold purification of the enzyme and 71% recovery of the activity. Subsequent Sephadex G-100 chromatography enabled the enzyme to be purified 3000-fold from the calf brain whole homogenate. The purified enzyme showed a number of protein methylase II activity peaks following preparative gel electrophoresis with one major enzyme peak.     

Partial purification of γ-glutamyltransferase from human brain microvessels

Two forms of gamma-glutamyltransferase from human brain cortex microvessels were partially purified by gel permeation and ion-exchange and group-affinity chromatography. The specific activity of the purified preparations was 320-fold (detergent form) and 830-fold (proteolytic form) higher than that of the enzyme in the brain cortex homogenate. The relative molecular mass of the proteolytic form of the enzyme was about 90,000 as determined by gel permeation chromatography. The major part of the enzyme (about 80%) was absorbed on Con A-Sepharose 4B. The pH optima for transfer reactions with -glutamyl-4-nitroanilide as donor and glycylglycine andl-cystine as acceptors were in the range of 8.2 to 9.0. The studied enzyme was inhibited by a mixture ofl-serine and borate and by bromcresol green.     

Purification and characterization of membrane-bound phosphatidylinositol kinase from rat brain

A membrane-bound phosphatidylinositol (PI) kinase was purified from rat brain. The enzyme was solubilized with Triton X-100 from salt-washed membrane and purified 11,183-fold, with a final specific activity of 150 nmol/min/mg of protein. Purification steps included several chromatography using Q-Sepharose Fast Flow, cellulose phosphate, Toyopearl HW 55 and Affi-Gel Blue. The purified PI kinase had an estimated molecular weight of 80,000 by gel filtration and 76,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The purified kinase phosphorylated only PI and did not phosphorylate phosphatidylinositol 4-phosphate or diacylglycerol. Km values for PI and ATP were found to be 115 and 150 microM, respectively. The enzyme required Mg2+ (5-20 mM) or Mn2+ (1-2 mM) for activity, was stimulated by 0.1-1.0% (w/v) Triton X-100, and completely inhibited by 0.05% sodium dodecyl sulfate. The enzyme activity showed a broad pH optimum at around 7.4. The enzyme utilized ATP and not GTP as phosphate donor. Nucleoside triphosphates other than ATP and diphosphates significantly inhibited the kinase activity. However, inhibitory effects of adenosine, cAMP, and quercetin were weak.     

Properties of a Zn2+-glycerophosphocholine cholinephosphodiesterase from bovine brain membranes

A Zn²⁺-glycerophosphocholine cholinephosphodiesterase was purified with a specific activity of 4.6 μmole/min·mg protein from bovine brain membranes by procedures involving PI-PLC solubilization, concanavalin A affinity chromatography, CM-sephadex chromatography and Sephadex G-150 chromatography. Based on molecular weight determination gel chromatography and SDS polyacrylamide gel electrophoresis, the phosphodiesterase activity appears to be a dimeric protein (110 kDa) composed of two subunits with a molecular weight of approximately 54 kDa. The Km value for p-nitrophenylphosphocholine and the optimum pH were found to be 16 μM and pH 10.5, respectively. The phosphodiesterase was inhibited by Cu²⁺, but not the other divalent metal ions. The activity of the apoenzyme was remarkably activated by Co²⁺ or Zn²⁺, but not Mn²⁺ or Mg²⁺. In addition, the inactivation of the enzyme in glycine buffer was prevented by Mn²⁺ or Zn²⁺, but not Co²⁺ or Mg². In a separate experiment, comparing properties of the purified and membrane-bound phosphodiesterases, the forms of two enzymes were quite similar except in stability. Both enzymes were more stable at pH 7.4 than pH 5 or 10. However, the membrane-bound enzyme was more stable than the soluble enzyme at all three pHs. These data suggest that the activity of the phosphodiesterase may be stabilized in-vivo.     

The minor anionic form of arylsulphatase B (arylsulphatase Bm) of monkey brain. Purification and phosphoprotein nature

The anionic form of arylsulphatase B (arylsulphatase Bm) was purified to apparent homogeneity from monkey brain through steps involving chromatography on diethylaminoethyl-cellulose, Blue-Sepharose, Biogel HTP and finally Biogel P-300 gel filtration. The molecular weight of the purified enzyme as deduced by gel filtration on Biogel P-300 and by sodium dodecylsulphate gel electrophoresis was ∼ 30,000.Escherichia coli alkaline phosphatase treatment of arylsulphatase Bm resulted in the conversion of upto 84% of the enzyme into a less charged form of enzyme, that could not bind to diethylaminoethyl cellulose. Potassium phosphate an inhibitor of alkaline phosphatase prevented this conversion. Upon acid hydrolysis the purified enzyme yielded approximately 7.0 mol of inorganic phosphate per mol of protein.Vibrio cholerae neuraminidase treatment did not alter the charge on arylsulphatase Bm.     

Calcium-dependent cyclic nucleotide phosphodiesterase from brain: comparison of adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate as substrates.

A Ca²⁺-dependent cyclic nucleotide phosphodiesterase has been partially purified from extracts of porcine brain by column chromatography on Sepharose 6 B containing covalently linked protamine residues, ammonium sulfate salt fractionation, and ECTEOLA-cellulose column chromatography. The resultant preparation contained a single form of cyclic nucleotide phosphodiesterase activity by the criteria of isoelectric focusing, gel filtration chromatography on Sephadex G-200, and electrophoretic migration on polyacrylamide gels. When fully activated by the addition of Ca²⁺ and microgram quantities of a purified Ca²⁺-binding protein (CDR), the phosphodiesterase hydrolyzed both adenosine 3′,5′-monophosphate (cyclic AMP) and guanosine 3′,5′-monophosphate (cyclic GMP), with apparent K_m values of 180 and 8 μm, respectively. Approximately 15% of the total enzymic activity was present in the absence of added CDR and Ca²⁺. This activity exhibited apparent K_m values for the two nucleotides identical to those observed for the maximally activated enzyme. Competitive substrate kinetics and heat destabilization studies demonstrated that both cyclic nucleotides were hydrolyzed by the same phosphodiesterase. The purified enzyme was identical to a Ca²⁺-dependent phosphodiesterase present in crude extract by the criteria of gel filtration chromatography, polyacrylamide-gel electrophoresis, and kinetic behavior.Apparent K_m values of the Ca²⁺-dependent phosphodiesterase for cyclic AMP and cyclic GMP were lowered more than 20-fold as CDR quantities in the assay were increased to microgram amounts, whereas the respective maximal velocities remained constant. The apparent K_m for Mg²⁺ was lowered more than 50-fold as CDR was increased to microgram amounts. Half-maximal activation of the phosphodiesterase occurred with lower amounts of CDR as a function of either increasing degrees of substrate saturation or increasing concentrations of Mg²⁺. At low cyclic nucleotide substrate concentrations i.e., 2.5 μm, cyclic GMP was hydrolyzed at a fourfold greater velocity than cyclic AMP. At high substrate concentrations (millimolar range) cyclic AMP was hydrolyzed at a threefold greater rate than cyclic GMP.