Characterization of a Ca2+-calmodulin-stimulated cyclic GMP phosphodiesterase from bovine brain

A calmodulin-stimulated form of cyclic nucleotide phosphodiesterase from bovine brain has been extensively purified (1000-fold). Its specific activity is approximately 4 mumol min-1 (mg of protein)-1 when 1 microM cGMP is used as the substrate. This form of calmodulin-sensitive phosphodiesterase activity differs from those purified previously by showing a very low maximum hydrolytic rate for cAMP vs. cGMP. The purification procedure utilizing ammonium sulfate precipitation, ion-exchange chromatography on DEAE-cellulose, gel filtration on Sephacryl S-300, isoelectric focusing, and affinity chromatography on calmodulin-Sepharose and Cibacron blue-agarose results in a protein with greater than 80% purity with 1% yield. Kinetics of cGMP and cAMP hydrolysis are linear with Km values of 5 and 15 microM, respectively. Addition of calcium and calmodulin reduces the apparent Km for cGMP to 2-3 microM and increases the Vmax by 10-fold. cAMP hydrolysis shows a similar increase in Vmax with an apparent doubling of Km. Both substrates show competitive inhibition with Ki's close to their relative Km values. Highly purified preparations of the enzyme contain a major protein band of Mr 74 000 that best correlates with enzyme activity. Proteins of Mr 59 000 and Mr 46 000 contaminate some preparations to varying degrees. An apparent molecular weight of 150 000 by gel filtration suggests that the enzyme exists as a dimer of Mr 74 000 subunits. Phosphorylation of the enzyme preparation by cAMP-dependent protein kinase did not alter the kinetic or calmodulin binding properties of the enzyme. Western immunoblot analysis indicated no cross-reactivity between the bovine brain calmodulin-stimulated gGMP phosphodiesterase and the Mr 60 000 high-affinity cAMP phosphodiesterase present in most mammalian tissues.     

Purification and characterization of cyclic GMP-stimulated cyclic nucleotide phosphodiesterase from calf liver. Effects of divalent cations on activity

Cyclic GMP-stimulated cyclic nucleotide phosphodiesterase purified greater than 13,000-fold to apparent homogeneity from calf liver exhibited a single protein band (Mr approximately 102,000) on polyacrylamide gel electrophoresis under denaturing conditions. Enzyme activity comigrated with the single protein peak on analytical polyacrylamide gel electrophoresis, sucrose density gradient centrifugation, and gel filtration. From the sedimentation coefficient of 6.9 S and Stokes radius of 67 A, an Mr of 201,000 and frictional ratio (f/fo) of 1.7 were calculated, suggesting that the native enzyme is a nonspherical dimer of similar, if not identical, peptides. The effectiveness of Mg2+, Mn2+, and Co2+ in supporting catalytic activity depended on the concentration of cGMP and cAMP present as substrate or effector. Over a wide range of substrate concentrations, optimal concentrations for Mg2+, Mn2+, and Co2+ were about 10, 1, and 0.2 mM, respectively. At concentrations higher than optimal, Mg2+ inhibited activity somewhat; inhibition by Co2+ (and in some instances by Mn2+) was virtually complete. At low substrate concentrations, activity with optimal Mn2+ was equal to or greater than that with Co2+ and always greater than that with Mg2+. With greater than or equal to 0.5 microM cGMP or 20 to 300 microM cAMP and for cAMP-stimulated cGMP or cGMP-stimulated cAMP hydrolysis, activity with Mg2+ greater than Mn2+ greater than Co2+. In the presence of Mg2+, the purified enzyme hydrolyzed cGMP and cAMP with kinetics suggestive of positive cooperativity. Apparent Km values were 15 and 33 microM, and maximal velocities were 200 and 170 mumol/min/mg of protein, respectively. Substitution of Mn2+ for Mg2+ increased apparent Km and reduced Vmax for cGMP with little effect on Km or Vmax for cAMP. Co2+ increased Km and reduced Vmax for both. cGMP stimulated cAMP hydrolysis approximately 32-fold in the presence of Mg2+, much less with Mn2+ or Co2+. In the presence of Mg2+, Mn2+ and Co2+ at concentrations that increased activity when present singly inhibited cGMP-stimulated cAMP hydrolysis. It appears that divalent cations as well as cyclic nucleotides affect cooperative interactions of this enzyme. Whereas Co2+ effects were observed in the presence of either cyclic nucleotide, Mn2+ effects were especially prominent when cGMP was present (either as substrate or effector).     

Cyclic nucleotide phosphodiesterase in rat neostriatum: properties of the enzyme in crude homogenates

Homogenates of rat neostriatum hydrolysed cGMP faster than cAMP at both high (100 microM) and low (1 microM) substrate concentrations, although the hydrolysis of both nucleotides exhibited similar kinetic properties. Kinetic analysis of the effect of substrate concentration on the rate of cAMP and cGMP hydrolysis gave results characteristic of a negatively cooperative enzyme species, with two apparent Km's for each nucleotide. The ratio between the Vmax of the high Km form and the Vmax of the low Km form was similar in various subcellular fractions of neostriatal tissue, in a preparation of synaptic membranes from whole brain, and in homogenates of other brain regions, including both neural-rich and glial-rich tissues. In homogenates of neostriatum cAMP could almost completely block cGMP hydrolysis and vice versa. The kinetics of this inhibition were competitive at low (1 microM) substrate concentrations, and non-competitive at high (100 microM) substrate concentrations. Various phosphodiesterase inhibitors failed to preferentially inhibit the hydrolysis of either nucleotide at high or low nucleotide concentrations. Preliminary studies of the effect of a Ca(2+)-dependent endogenous activator preparation on the hydrolysis of cyclic nucleotides in homogenates of rat neostriatum showed a specific activation of cGMP hydrolysis at low nucleotide concentrations. The rate of cGMP hydrolysis at 1 microM substrate concentration was doubled in the presence of the activator preparation and 100 microM-CaCl2, while cGMP hydrolysis at 100 microM or cAMP hydrolysis at both 1 microM and 100 microM remained unaffected. These observations raise the possibility that cAMP and cGMP may be hydrolysed by the same enzyme in rat neostriatum, and that an endogenous activating factor may determine the relative affinities of the enzyme for the two nucleotides.     

The purification and characterization of glutathione S-transferase from the hepatopancreas of the blue crab, Callinectes sapidus

High glutathione S-transferase activity was found in the cytosol of F-cells from the hepatopancreas of the blue crab (Callinectes sapidus). Purification of glutathione S-transferase from hepatopancreas extracts by Sephadex G-200, DEAE-Sephacel, and chromatofocusing resulted in the isolation of two isozymes with isoelectric points of 5.9 and 5.7, as determined by analytical isoelectric focusing. Using 1-chloro-2,4-dinitrobenzene as the substrate the specific activities of the two purified isozymes were 222 and 182 mumol/min/mg, respectively. There was no evidence for basic transferase isozymes. In addition to 1-chloro-2,4-dinitrobenzene the purified glutathione S-transferase isozymes showed activity with p-nitrophenyl acetate, p-nitrobenzyl chloride, bromosulfophthalein, and benzopyrene oxide. Thus, both substitution and addition reactions associated with vertebrate glutathione S-transferase were found in the crab transferases. There was no when ethacrynic acid, methyl iodide, trans-4-phenyl-3-buten-2-one, 1,2-epoxy-(p-nitrophenoxy)propane, cumene hydroperoxide, and t-butyl hydroperoxide were used as substrates. The lack of peroxidase activity is of interest since this activity is commonly found in vertebrate transferase isozymes. The two transferases had a dimeric Mr of 40,800 with similar amino acid compositions and similar kinetic parameters (Vmax, Km, and pH maxima) with 1-chloro-2,4-dinitrobenzene as substrate. The two transferases could be distinguished by their isoelectric points, molecular mass of the monomers (22,300 for GST 1 and 22,300 and 22,400 for GST 2), and different inhibitor mechanisms with hematin and bromosulfophthalein.     

Substrate specificity of Acanthamoeba myosin I heavy chain kinase as determined with synthetic peptides

Phosphorylation of a single threonine (myosin IA) or serine (myosins IB and IC) in the heavy chains of the Acanthamoeba myosin I isozymes is required for expression of their actin-activated Mg2(+)-ATPase activities. We now report that the synthetic peptide Gly-Arg-Gly-Arg-Ser-Ser-Val-Tyr-Ser, which corresponds to the phosphorylated region of Acanthamoeba myosin IC, is a good substrate for myosin I heavy chain kinase: Km = 54 microM, and Vmax = 15 mumols/min.mg. The same serine is phosphorylated as in the native substrate (residue 6 in the above sequence), and kinase activity with the synthetic peptide as substrate is also stimulated by phosphatidylserine-enhanced autophosphorylation of the kinase. These results indicate that all of the essential sequence determinants of kinase specificity are contained within this 9-residue peptide. With the peptide as substrate, we found that another acidic phospholipid, phosphatidylinositol, also enhances autophosphorylation of the kinase whereas the neutral phospholipids phosphatidylcholine and phosphatidylethanolamine do not. By comparing the Km and Vmax values for a series of synthetic peptide substrates, we established that 1 basic amino acid is essential on the NH2-terminal side of the phosphorylation site, and two are preferable, and that a tyrosine is essential 2 residues away on the COOH-terminal side. There is a slight preference for arginines over lysines. All of these local sequence specificity determinants are present in the three native substrates, Acanthamoeba myosins IA, IB, and IC, and in two Dictyostelium myosin I isozymes that are putative substrates for the kinase. Similar sequences do not occur in the myosins I from intestinal brush border, which is not a substrate for the Acanthamoeba kinase.     

Purification and properties of a pig heart thiolase with broad chain length specificity and comparison of thiolases from pig heart and Escherichia coli.

A thiolase (acetyl CoA acyltransferase, EC 2.3-1.16) which acts on substrates of various chain lengths (thiolase I) has been purified from pig heart muscle 366-fold to near homogeneity as judged by gel electrophoresis. Its molecular weight was estimated to be 200,000 in the absence and 46,000 in the presence of sodium dodecyl sulfate. Kinetic measurements with acetoacetyl coenzyme A, 3-ketohexanoyl-CoA, 3-ketooctanoyl-CoA, and 3-ketodecanoyl-CoA yielded apparent Km values of 16, 8.3, 2.4, and 1.8 micron, respectively, whereas apparent Vmax values of 65 to 69 mumol/min/mg were obtained with all substrates except for acetoacetyl-CoA, with which a value of 26.5 mumol/min/mg was observed. Antibodies prepared against this thiolase were used to demonstrate that thiolase I and acetoacetyl-CoA thilase (thiolase II) from pig heart mitochondria are immunologically unrelated. The antibodies cross-reacted, however, with thiolase I from beef heart. Kinetic constants (Km, Vmax) were also determined for thiolases I and II from Escherichia coli, as were the native and subunit molecular weights of E. coli thiolase II. Although the E. coli thiolases were found to be immunologically distinct from the pig heart enzymes, their physical and kinetic properties are strikingly similar to those of the heart thiolases. In view of this finding and in view of the known physiological functions of the E. coli thiolases, it is proposed that thiolase I from pig heart is only involved in fatty acid metabolism, whereas thiolase II functions solely in ketone body degradation.     

The effect of pH, temperature, and D2O on the activity of porcine synovial collagenase and gelatinase

The pH dependence of Vmax and Vmax/Km for hydrolysis of Dnp-Pro-Leu-Gly-Leu-Trp-Ala-D-Arg-NH2 at the Gly-Leu bond by porcine synovial collagenase and gelatinase was determined in the pH range 5-10. Both enzymes exhibited bell-shaped dependencies on pH for these two kinetic parameters, indicating that activity is dependent on at least two ionizable groups, one of which must be unprotonated and the other protonated. For collagenase, Vmax/Km data indicate that in the substrate-free enzyme, these groups have apparent pK values of 7.0 and 9.5, while the Vmax profile indicates similar pK values of 6.8 and 10.1 for the enzyme-substrate complex. The corresponding pH profiles of gelatinase were similar to those of collagenase, indicating the importance of groups with apparent pK values of 5.9 and 10.0 for the free enzyme and 5.9 and 11.1 for the enzyme-substrate complex. When these kinetic constants were determined in D2O using the peptide substrate, there was no significant effect on Vmax or Km for collagenase or Km for gelatinase. However, there was a deuterium isotope effect of approximately 1.5 on Vmax for gelatinase. These results indicate that a proton transfer step is not involved in the rate-limiting step for collagenase, but may be limiting with gelatinase. The Arrhenius activation energies for peptide bond hydrolysis of the synthetic peptide as well as the natural substrates were also determined for both enzymes. The activation energy (81 kcal) for hydrolysis of collagen by collagenase was nine times greater than that determined for the synthetic substrate (9.2 kcal). In contrast, the activation energy for hydrolysis of gelatin by gelatinase (26.3 kcal) was only 2.4 times greater than that for the synthetic substrate (11 kcal).     

Identification and partial characterization of bovine heart cytosolic phosphorylase phosphatases

The properties of phosphatases in bovine heart cytosol were studied. Two isozymic forms of protein phosphatase H (H-1 and H-2) were resolved by chromatography on DEAE-Sephacel. The two isoenzymes had identical physical properties (Mr 260,000, 7.9 S). Treatment with 80% ethanol activated both isozymes and converted H-1 to a Mr 35,500 form and H-2 to Mr 67,000 and Mr 35,500 forms. Both H-1 and H-2 and their lower Mr activated forms had essentially identical Km values for phosphorylase a. The heart cytosol also contained a latent phosphatase (Fc) which could be activated by preincubation with either ATP X Mg and an activating factor (FA), or by Mn/trypsin treatment. The latter procedure converted the latent Fc (Mr 200,000) to a Mn2+-independent Mr 34,500 form. Both activated forms of Fc had similar Km values which were fourfold lower than the affinity of the protein phosphatase H forms for the phosphorylase a substrate.     

Nucleoside hydrolase from Crithidia fasciculata. Metabolic role, purification, specificity, and kinetic mechanism

Crithidia fasciculata cells grown on complex medium with added [8-14C, 5'-3H]inosine or [8-14C,5'-3H]adenosine metabolize greater than 50% of the salvaged nucleosides through a pathway involving N-glycoside bond cleavage. Cell extracts contain a substantial nucleoside hydrolase activity but an insignificant purine nucleoside phosphorylase. The nucleoside hydrolase has been purified 1000-fold to greater than 99% homogeneity from kilogram quantities of C. fasciculata. The enzyme is a tetramer of Mr 34,000 subunits to give an apparent holoenzyme Mr of 143,000 by gel filtration. All of the commonly occurring nucleosides are substrates. The Km values vary from 0.38 to 4.7 mM with purine nucleosides binding more tightly than the pyrimidines. Values of Vmax/Km vary from 3.4 x 10(3) M-1 s-1 to 1.7 x 10(5) M-1 s-1 with the pyrimidine nucleosides giving the larger values. The turnover rate for inosine is 32 s-1 at 30 degrees C. The kinetic mechanism with inosine as substrate is rapid equilibrium with random product release. The hydrolytic reaction can be reversed to give an experimental Keq of 106 M with H2O taken as unity. The product dissociation constants for ribose and hypoxanthine are 0.7 and 6.2 mM, respectively. Deoxynucleosides or 5'-substituted nucleosides are poor substrates or do not react, and are poor inhibitors of the enzyme. The enzyme discriminates against methanol attack from solvent during steady-state catalysis, indicating the participation of an enzyme-directed water nucleophile. The pH profile for inosine hydrolysis gives two apparent pKa values of 6.1 with decreasing Vmax/Km values below the pKa and a plateau at higher pH values. These effects are due to the pH sensitivity of the Vmax values, since Km is independent of pH. The pH profile implicates two negatively charged groups which stabilize a transition state with oxycarbonium character.     

The insulin- and glucagon-stimulated ''dense-vesicle'' high-affinity cyclic AMP phosphodiesterase from rat liver. Purification, characterization and inhibitor sensitivity.

The hormone-stimulated 'dense-vesicle' cyclic AMP phosphodiesterase was solubilized as a proteolytically 'clipped' species, and purified to apparent homogeneity from rat liver with a 2000-3000-fold purification and a 13-18% yield. It appeared to be a dimer (Mr 112,000), of two Mr-57,000 subunits. Solubilization of either a liver or a hepatocyte membrane fraction, with sodium cholate in the presence of the protein inhibitor benzamidine, identified three protein bands which could be immunoprecipitated by a polyclonal antibody raised against the pure enzyme. The major band at Mr 62,000 is suggested to be the native 'dense-vesicle' enzyme, having a Mr-5000 extension which serves to anchor this enzyme to the membrane and which is cleaved off during proteolytic solubilization; the Mr-200,000 band is an aggregate of the Mr-62,000 species, and the Mr-63,000 species is possibly a precursor. The purified 'clipped' enzyme hydrolysed cyclic AMP with kinetics indicative of apparent negative co-operativity, with a Hill coefficient (h) of 0.43 and limiting kinetic constants of Km1 = 0.3 +/- 0.05 microM, Km2 = 29 +/- 6 microM, Vmax.1 = 0.114 +/- 0.015 unit/mg of protein and Vmax.2 = 0.633 +/- 0.054 unit/mg of protein. It hydrolysed cyclic GMP with Michaelis kinetics, Km = 10 +/- 1 microM and Vmax. = 4.1 +/- 0.2 units/mg of protein. Cyclic GMP was a potent inhibitor of cyclic AMP hydrolysis, with an IC50 (concn. giving 50% inhibition) of 0.20 +/- 0.01 microM-cyclic GMP when assayed at 0.1 microM-cyclic AMP. This enzyme was inhibited potently by several drugs known to exert positive inotropic effects on the heart, was extremely thermolabile, with a half-life of 4.5 +/- 0.5 min at 40 degrees C, and was shown to be distinct from the rat liver insulin-stimulated peripheral-plasma-membrane cyclic AMP phosphodiesterase [Marchmont, Ayad & Houslay (1981) Biochem. J. 195, 645-652].     

cGMP-activated phosphodiesterase from human brain: kinetic and regulatory properties]

The kinetic and regulatory properties of cGMP-activated phosphodiesterase (PDE) from human brain were studied. In double reciprocal plots the enzyme activity is characterized by a linear dependence of cAMP and a nonlinear one for cGMP. Micromolar concentrations of cGMP accelerate cAMP hydrolysis (7-14-fold) with Ka for cGMP of 0.36 microM. Stimulation of cAMP hydrolysis is accompanied by a decrease of Km with no changes in Vmax. With a rise in the cGMP concentration above 5 microM PDE activation is changed by its inhibition. Both substrates act as competitive inhibitors towards each other. The Ki value for both cGMP and cAMP is 30 microM. After the increase in the cAMP (Bt)2 concentration the activation of 5 microM cAMP hydrolysis is accompanied by the enzyme inhibition. Both analogs competitively inhibit cGMP hydrolysis with Ki of 10 and 1500 microM for cGMP(Bt)2 and cAMP(Bt)2, respectively. The data obtained point to the existence of two binding sites for cyclic nucleotides, namely, a regulatory site which is highly specific for cGMP and a catalytic site responsible for the hydrolysis of the both substrates which displays no apparent specificity either for cAMP or for cGMP. The different affinity of natural and synthetic cyclic nucleotides for these sites is determined, to a large extent, by the amino groups in the 2nd and 6th positions of the purine ring.     

Purification and characterization of bovine heart phosphoinositide-specific phospholipase C: kinetic analysis of the Ca2+ requirement and La3+ inhibition

Bovine heart contains multiple phosphoinositide-specific phospholipase C (PIC) activities separable by ion-exchange chromatography. One PIC activity was purified to apparent homogeneity and migrated as a single band of Mr 85,000 on SDS-PAGE. The purified PIC was characterized with sonicated suspensions of either pure phosphatidylinositol 4,5-bisphosphate (PIP2) or phosphatidylinositol (PI) as substrates. At pH 7, apparent Vmax and Km values were higher for PIP2 than for PI, but the value of Vmax/Km was similar for the two substrates. PIC required Ca2+ for the hydrolysis of either PI or PIP2, and increasing free Ca2+ concentrations from 20 to 300 nM saturated PIC activity. The requirement of Ca2+ for PIC activity and the sensitivity of PIC to Ca2+ concentrations in the physiological range suggested the ion may be a cofactor. The PIC reaction mechanism was determined by two-substrate kinetic analysis; the data fit a model in which PIC contained single sites for Ca2+ and phosphoinositide, and utilized a rapid-equilibrium, random-order ternary mechanism for phosphoinositide hydrolysis. The KCa value for either PI or PIP2 hydrolysis was approximately 30 nM, suggesting resting intracellular free Ca2+ concentrations are sufficient to saturate the Ca2+ site of PIC. La3+ was used as a calcium analogue to modulate PIC activity. Low concentrations of LaCl3 (0.01-0.3 microM) inhibited PIC activity competitively with respect to calcium, consistent with a Ca2+ binding site on the enzyme.     

Dephosphorylation of Microtubule-Associated Protein 2, τ Factor, and Tubulin by Calcineurin

Calcineurin dephosphorylated microtubule-associated protein 2 (MAP2) and tau factor phosphorylated by cyclic AMP-dependent and Ca2+, calmodulin-dependent protein kinases from the brain. Tubulin, only phosphorylated by the Ca2+, calmodulin-dependent protein kinase, served as substrate for calcineurin. The concentrations of calmodulin required to give half-maximal activation of calcineurin were 21 and 16 nM with MAP2 and tau factor as substrates, respectively. The Km and Vmax values were in ranges of 1-3 microM and 0.4-1.7 mumol/mg/min, respectively, for MAP2 and tau factor. The Km value for tubulin was in a similar range, but the Vmax value was lower. The peptide map analysis revealed that calcineurin dephosphorylated MAP2 and tau factor universally, but not in a site-specific manner. The autophosphorylated Ca2+, calmodulin-dependent protein kinase was not dephosphorylated by calcineurin. These results suggest that calcineurin plays an important role in the functions of microtubules via dephosphorylation.     

Human aldehyde dehydrogenase. Activity with aldehyde metabolites of monoamines, diamines, and polyamines

Two isozymes (E1 and E2) of human aldehyde dehydrogenase (EC 1.2.1.3) were purified to homogeneity 13 years ago and a third isozyme (E3) with a low Km for gamma-aminobutyraldehyde only recently. Comparison with a variety of substrates demonstrates that substrate specificity of all three isozymes is broad and similar. With straight chain aliphatic aldehydes (C1-C6) the Km values of the E3 isozyme are identical with those of the E1 isozyme. All isozymes dehydrogenate naturally occurring aldehydes, 5-imidazoleacetaldehyde (histamine metabolite) and acrolein (product of beta-elimination of oxidized polyamines) with similar catalytic efficiency. Differences between the isozymes are in the Km values for aminoaldehydes. Although all isozymes can dehydrogenate gamma-aminobutyraldehyde, the Km value of the E3 isozyme is much lower: the same appears to apply to aldehyde metabolites of cadaverine, agmatine, spermidine, and spermine for which Km values range between 2-18 microM and kcat values between 0.8-1.9 mumol/min/mg. Thus, the E3 isozyme has properties which make it suitable for the metabolism of aminoaldehydes. The physiological role of E1 and E2 isozymes could be in dehydrogenation of aldehyde metabolites of monoamines such as 3,4-dihydroxyphenylacetaldehyde or 5-hydroxyindoleacetaldehyde; the catalytic efficiency with these substrates is better with E1 and E2 isozymes than with E3 isozyme. Isoelectric focusing of liver homogenates followed by development with various physiological substrates together with substrate specificity data suggest that aldehyde dehydrogenase (EC 1.2.1.3) is the only enzyme in the human liver capable of catalyzing dehydrogenation of aldehydes arising via monoamine, diamine, and plasma amine oxidases. Although the enzyme is generally considered to function in detoxication, our data suggest an additional function in metabolism of biogenic amines.     

cGMP transport by vesicles from human and mouse erythrocytes

cGMP secretion from cells can be mediated by ATP-binding cassette (ABC) transporters ABCC4, ABCC5, and ABCC11. Indirect evidence suggests that ABCC4 and ABCC5 contribute to cGMP transport by erythrocytes. We have re-investigated the issue using erythrocytes from wild-type and transporter knockout mice. Murine wild-type erythrocyte vesicles transported cGMP with an apparent Km that was 100-fold higher than their human counterparts, the apparent Vmax being similar. Whereas cGMP transport into human vesicles was efficiently inhibited by the ABCC4-specific substrate prostaglandin E1, cGMP transport into mouse vesicles was inhibited equally by Abcg2 and Abcc4 inhibitors/substrates. Similarly, cGMP transport into vesicles from Abcc4-/- and Abcg2-/- mice was 42% and 51% of that into wild-type mouse vesicles, respectively, whereas cGMP transport into vesicles from Abcc4(-/-)/Abcg2(-/-) mice was near background. The knockout mice were used to show that Abcg2-mediated cGMP transport occurred with lower affinity but higher Vmax than Abcc4-mediated transport. Involvement of Abcg2 in cGMP transport by Abcc4-/- erythrocyte vesicles was supported by higher transport at pH 5.5 than at pH 7.4, a characteristic of Abcg2-mediated transport. The relative contribution of ABCC4/Abcc4 and ABCG2/Abcg2 in cGMP transport was confirmed with a new inhibitor of ABCC4 transport, the protease inhibitor 4-(2-aminoethyl)benzenesulfonyl fluoride.     

Human placental 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase: purification from mitochondria and kinetic profiles, biophysical characterization of the purified mitochondrial and microsomal enzymes

In human placenta, 3 beta-hydroxy-5-ene-steroid dehydrogenase and steroid 5----4-ene-isomerase, an enzyme complex found in microsomes and mitochondria, synthesizes progesterone from pregnenolone and androstenedione from fetal dehydroepiandrosterone sulfate. The dehydrogenase and isomerase activities of the mitochondrial enzyme were copurified (733-fold) using sequential cholate solubilization, ion exchange chromatography (DEAE-Toyopearl 650S), and hydroxylapatite chromatography (Bio-Gel HT). Enzyme homogeneity was demonstrated by a single protein band in SDS-polyacrylamide gel electrophoresis (monomeric Mr = 41,000), gel filtration at constant specific enzyme activity (Mr = 77,000), and a single NH2-terminal sequence. Kinetic constants were determined for the oxidation of pregnenolone (Km = 1.6 microM, Vmax = 48.6 nmol/min/mg) and dehydroepiandrosterone (Km = 2.4 microM, Vmax = 48.5 nmol/min/mg) and for the isomerization of 5-pregnene-3,20-dione (Km = 9.3 microM, Vmax = 914.2 nmol/min/mg) and 5-androstene-3,17-dione (Km = 27.6 microM, Vmax = 888.4 nmol/min/mg. Mixed substrate studies showed that the dehydrogenase and isomerase activities utilize their respective pregnene and androstene substrates competitively. Dixon analysis demonstrated that the product steroids, progesterone and androstenedione, are competitive inhibitors of the C-21 and C-19 dehydrogenase activities. Enzyme purified from mitochondria and microsomes had similar kinetic profiles with respect to substrate utilization, product inhibition, and cofactor (NAD+) reduction (mean Km +/- SD using C-19 and C-21 dehydrogenase substrates = 26.4 +/- 0.8 microM, mean Vmax = 73.2 +/- 1.3 nmol/min/mg). Pure enzyme from both organelles exhibited identical biophysical properties in terms of molecular weight and subunit composition, pH optima (pH 9.8, dehydrogenase; pH 7.5, isomerase), temperature optimum (37 degrees C), stability in storage and solution, effects of divalent cations, and the single NH2-terminal sequence of 27 amino acids. These results suggest that the mitochondrial and microsomal enzymes are the same protein localized in different organelles.     

Activation of brain calcineurin phosphatase towards nonprotein phosphoesters by Ca2+, calmodulin, and Mg2+

Calcineurin purified from bovine brain was found to be active towards beta-naphthyl phosphate greater than p-nitrophenyl phosphate greater than alpha-naphthyl phosphate much greater than phosphotyrosine. In its native state, calcineurin shows little activity. It requires the synergistic action of Ca2+, calmodulin, and Mg2+ for maximum activation. Ca2+ and Ca2+ X calmodulin exert their activating effects by transforming the enzyme into a potentially active form which requires Mg2+ to express the full activity. Ni2+, Mn2+, and Co2+, but not Ca2+ or Zn2+, can substitute for Mg2+. The pH optimum, and the Vm and Km values of the phosphatase reaction are characteristics of the divalent cation cofactor. Ca2+ plus calmodulin increases the Vm in the presence of a given divalent cation, but has little effect on the Km for p-nitrophenyl phosphate. The activating effects of Mg2+ are different from those of the transition metal ions in terms of effects on Km, Vm, pH optimum of the phosphatase reaction and their affinity for calcineurin. Based on the Vm values determined in their respective optimum conditions, the order of effectiveness is: Mg2+ greater than or equal to Ni2+ greater than Mn2+ much greater than Co2+. The catalytic properties of calcineurin are markedly similar to those of p-nitrophenyl phosphatase activity associated with protein phosphatase 3C and with its catalytic subunit of Mr = 35,000, suggesting that there are common features in the catalytic sites of these two different classes of phosphatase.     

Five enzymes of the glycolytic pathway serve as substrates for purified epidermal-growth-factor-receptor kinase.

Several enzymes of the glycolytic pathway are phosphorylated in vitro and in vivo by retroviral transforming protein kinases. These substrates include the enzymes phosphoglycerate mutase (PGM), enolase and lactate dehydrogenase (LDH). Here we show that purified EGF (epidermal growth factor)-receptor kinase phosphorylates the enzymes PGM and enolase and also the key regulatory enzymes of the glycolytic pathway, phosphofructokinase and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in an EGF-dependent manner. Stoichiometry of phosphate incorporation into GAPDH (calculated from native Mr) is the highest, reaching approximately 1. LDH and other enzymes of the glycolytic pathway are not phosphorylated by the purified EGF-receptor kinase. These enzymes are phosphorylated under native conditions, and the Km values of EGF-receptor kinase for their phosphorylation are close to the physiological concentrations of these enzymes in the cell. EGF stimulates the reaction by 2-5-fold by increasing the Vmax. without affecting the Km of this process. Phosphorylation is rapid at 22 degrees C and at higher temperatures. However, unlike the self-phosphorylation of EGF-receptor, which occurs at 4 degrees C, the glycolytic enzymes are poorly phosphorylated at this temperature. Some enzymes, in particular enolase, increase the receptor Km for ATP in the autophosphorylation process and thus may act as competitive inhibitors of EGF-receptor self-phosphorylation. On the basis of the Km values of EGF receptor for the substrate enzymes and for ATP in the phosphorylation reaction, these enzymes may also be substrates in vivo for the EGF-receptor kinase.     

Characterization of the phosphotyrosyl protein phosphatase activity of calmodulin-dependent protein phosphatase

Calmodulin-dependent protein phosphatase from bovine brain and heart was assayed for phosphotyrosine and phosphoserine phosphatase activity using several substrates: 1) smooth muscle myosin light chain (LC20) phosphorylated on tyrosine or serine residues, 2) angiotensin I phosphorylated on tyrosine, and 3) synthetic phosphotyrosine- or phosphoserine-containing peptides with amino acid sequences patterned after the autophosphorylation site in Type II regulatory subunit of the cAMP-dependent protein kinase. The phosphatase was activated by Ni2+ and Mn2+, and stimulated further by calmodulin. In the presence of Ni2+ and calmodulin, it exhibited similar kinetic constants for the dephosphorylation of phosphotyrosyl LC20 (Km = 0.9 microM, and Vmax = 350 nmol/min/mg) and phosphoseryl LC20 (Km = 2.6 microM, Vmax = 690 nmol/min/mg). Dephosphorylation of phosphotyrosyl LC20 was inhibited by phosphoseryl LC20 with an apparent Ki of 2 microM. Compared to the reactions with phosphotyrosyl LC20 as the substrate, reactions with phosphotyrosine-containing oligopeptides exhibited slightly higher Km and lower Vmax values. The reaction with the phosphoseryl peptide based on the Type II regulatory subunit sequence exhibited a slightly higher Km (23 microM), but a much higher Vmax (4400 nmol/min/mg) than that with its phosphotyrosine-containing counterpart. Micromolar concentrations of Zn2+ inhibited the phosphatase activity; vanadate was less potent, and 25 mM NaF was ineffective. The study provides quantitative data to serve as a basis for comparing the ability of the calmodulin-dependent protein phosphatase to act on phosphotyrosine- and phosphoserine-containing substrates.     

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.