Opioid peptides are substrates for the bifunctional enzyme LTA4 hydrolase/aminopeptidase.

We determined if any naturally occurring peptides could act as substrates or inhibitors of the bifunctional, Zn2+ metalloenzyme LTA4 hydrolase/aminopeptidase (E.C.3.3.2.6). Several opioid peptides including met5-enkephalin, leu5-enkephalin, dynorphin1-6, dynorphin1-7, and dynorphin1-8 competitively inhibited the hydrolysis of L-proline-p-nitroanilide by leukotriene A4 hydrolase/aminopeptidase, consistent with an interaction at its active site. The enzyme catalyzed the N-terminal hydrolysis of tyrosine from met5-enkephalin with Km = 450 +/- 58 microM and Vmax = 4.9 +/- 0.6 nmol-hr-1-ug-1 and from leu5-enkephalin with Km = 387 +/- 90 microM and Vmax = 6.2 +/- 2.5 nmol-hr-1-ug-1. Bestatin, captopril and carnosine inhibited the hydrolysis of the enkephalins. It is noteworthy that the bifunctional catalytic traits of this enzyme include generation of an hyperalgesic substance, LTB4, and inactivation of analgesic opioid peptides.     

Purification and characterization of leukotriene A4 epoxide hydrolase from dog lung

Leukotriene A4 epoxide hydrolase from dog lung, a soluble enzyme catalyzing the hydrolysis of leukotriene A4 (LTA4) to leukotriene B4 (LTB4) was partially purified by anion exchange HPLC. The enzymatic reaction obeys Michaelis- Menten kinetics. The apparent Km ranged between 15 and 25 microM and the enzyme exhibited an optimum activity at pH 7.8. An improved assay for the epoxide hydrolase has been developed using bovine serum albumin and EDTA to increase the conversion of LTA4 to LTB4. This method was used to produce 700 mg of LTB4 from LTA4 methyl ester. The partial by purified enzyme was found to be uncompetitively inhibited by divalent cations. Ca+2, Mn+2, Fe+2, Zn+2 and Cu+2 were found to have inhibitor constants (Ki) of 89 mM, 3.4 mM, 1.1 mM, 0.57 mM, and 28 microM respectively Eicosapentaenoic acid was shown to be a competitive inhibitor of this enzyme with a Ki of 200 microM. From these inhibition studies, it can be theorized that the epoxide hydrolase has at least one hydrophobic and one hydrophilic binding site.     

Guinea-pig liver leukotriene A4 hydrolase. Purification, characterization and structural properties

Leukotriene A4 hydrolase from perfused guinea-pig liver was purified 1200-fold to near homogeneity with a yield of about 20%. Apparent values of Km and Vmax at 37 degrees C (27 microM and 68 mumol x mg-1 x min-1), turnover number, and activation energy for the conversion of leukotriene A4 into leukotriene B4 were estimated from kinetic data obtained at -10 degrees C, 0 degree C and +10 degrees C (Arrhenius plots). Physical properties including Mr (67,000-71,000), pH optimum, isoelectric point and Stokes' radius were determined. The amino acid composition and N-terminal amino acid sequence were established after carboxymethylation of the enzyme. Unlike liver cytosolic epoxide hydrolase, the purified enzyme did not catalyze the conversion of leukotriene A4 into (5S,6R)-5,6-dihydroxy-7,9-trans-11,14-cis-icosatetraenoic acid.     

Purification and properties of mitochondrial malate dehydrogenase from unfertilized eggs of the sea urchin, Anthocidaris crassispina

Purification and characterization of mitochondrial malate dehydrogenase [EC 1.1.1.37] from unfertilized eggs of the sea urchin, Anthocidaris crassispina, are described. The purification method consisted of dextran sulfate fractionation, Blue Dextran Sepharose chromatography, Phenyl-Sepharose hydrophobic chromatography and DEAE-cellulose chromatography. The enzyme was purified 771-fold with a 7% yield from the crude extract. The purified enzyme appeared homogeneous on polyacrylamide gel electrophoresis under both native and denatured conditions. After incubation at 45 degrees C for 50 min, the enzyme lost about 90% of its activity. In the presence of NADH, however, the enzyme was protected against the heat denaturation. The native enzyme had a molecular weight of about 65,000 and probably consisted of two identical subunits. In the reduction of oxaloacetate with NADH, a broad optimum pH ranging from 8.2 to 9.4 was found with 50 mM Tris-HCl and glycine-NaOH buffers. Sodium phosphate buffer apparently activated the enzyme. The apparent Km values for oxaloacetate and NADH were 19 microM and 30 microM, respectively. The optimum pH for malate oxidation with NAD+ was 10.2 in 50 mM NaHCO3-Na2CO3 buffer. The apparent Km values for malate and NAD+ were 7.0 mM and 0.6 mM, respectively. Zinc ion, sulfite ion, p-chloromercuriphenylsulfonate and adenine nucleotides strongly inhibited the enzyme.     

Leukotriene A4 hydrolase. Inhibition by bestatin and intrinsic aminopeptidase activity establish its functional resemblance to metallohydrolase enzymes.

Bestatin, an inhibitor of aminopeptidases, was also a potent inhibitor of leukotriene (LT) A4 hydrolase. On isolated enzyme its effects were immediate and reversible with a Ki = 201 +/- 95 mM. With erythrocytes it inhibited LTB4 formation greater than 90% within 10 min; with neutrophils it inhibited LTB4 formation by only 10% during the same period, increasing to 40% in 2 h. Bestatin inhibited LTA4 hydrolase selectively; neither 5-lipoxygenase nor 15-lipoxygenase activity in neutrophil lysates was affected. Purified LTA4 hydrolase exhibited an intrinsic aminopeptidase activity, hydrolyzing L-lysine-p-nitroanilide and L-leucine-beta-naphthylamide with apparent Km = 156 microM and 70 microM and Vmax = 50 and 215 nmol/min/mg, respectively. Both LTA4 and bestatin suppressed the intrinsic aminopeptidase activity of LTA4 hydrolase with apparent Ki values of 5.3 microM and 172 nM, respectively. Other metallohydrolase inhibitors tested did not reduce LTA4 hydrolase/aminopeptidase activity, with one exception; captopril, an inhibitor of angiotensin-converting enzyme, was as effective as bestatin. The results demonstrate a functional resemblance between LTA4 hydrolase and certain metallohydrolases, consistent with a molecular resemblance at their putative Zn2(+)-binding sites. The availability of a reversible, chemically stable inhibitor of LTA4 hydrolase may facilitate investigations on the role of LTB4 in inflammation, particularly the process termed transcellular biosynthesis.     

Purification and characterization of a novel form of 20 alpha-hydroxysteroid dehydrogenase from Clostridium scindens.

We have purified a steroid-inducible 20 alpha-hydroxysteroid dehydrogenase from Clostridium scindens to apparent homogeneity. The final enzyme preparation was purified 252-fold, with a recovery of 14%. Denaturing and nondenaturing polyacrylamide gradient gel electrophoresis showed that the native enzyme (Mr, 162,000) was a tetramer composed of subunits with a molecular weight of 40,000. The isoelectric point was approximately pH 6.1. The purified enzyme was highly specific for adrenocorticosteroid substrates possessing 17 alpha, 21-dihydroxy groups. The purified enzyme had high specific activity for the reduction of cortisone (Vmax, 280 nmol/min per mg of protein; Km, 22 microM) but was less reactive with cortisol (Vmax, 120 nmol/min per mg of protein; Km, 32 microM) at pH 6.3. The apparent Km for NADH was 8.1 microM with cortisone (50 microM) as the cosubstrate. Substrate inhibition was observed with concentrations of NADH greater than 0.1 mM. The purified enzyme also catalyzed the oxidation of 20 alpha-dihydrocortisol (Vmax, 200 nmol/min per mg of protein; Km, 41 microM) at pH 7.9. The apparent Km for NAD+ was 526 microM. The initial reaction velocities with NADPH were less than 50% of those with NADH. The amino-terminal sequence was determined to be Ala-Val-Lys-Val-Ala-Ile-Asn-Gly-Phe-Gly-Arg. These results indicate that this enzyme is a novel form of 20 alpha-hydroxysteroid dehydrogenase.     

Recombinant mouse leukotriene A4 hydrolase: a zinc metalloenzyme with dual enzymatic activities

Recombinant mouse leukotriene A4 hydrolase was expressed in Escherichia coli as a fusion protein with ten additional amino acids at the amino terminus and was purified to apparent homogeneity by means of precipitation, anion exchange, hydrophobic interaction and chromatofocusing chromatographies. By atomic absorption spectrometry, the enzyme was shown to contain one mol of zinc/mol of enzyme. Apparent kinetic constants (Km and Vmax) for the conversion of leukotriene A4 to leukotriene B4 (at 0 degree C, pH 8) were 5 microM and 900 nmol/mg per min, respectively. The purified enzyme also exhibited significant peptidase activity towards the synthetic amide alanine-4-nitroanilide. Km and Vmax for this reaction (at 37 degrees C, pH 8) were 680 microM and 365 nmol/mg per min, respectively. Apo-leukotriene A4 hydrolase, prepared by treating the enzyme with 1,10-phenanthroline, was virtually inactive with respect to both enzymatic activities, but could be reactivated by addition of stoichiometric amounts of zinc or cobalt. Exposure of the enzyme to leukotriene A4 resulted in a dose-dependent inactivation of both enzyme activities.     

Purification and initial characterization of peptidyl-tRNA hydrolase from rabbit reticulocytes.

We have identified an activity in rabbit reticulocyte lysate as peptidyl-tRNA hydrolase, based upon its ability to hydrolyze native reticulocyte peptidyl-tRNA, isolated from polyribosomes, and N-acylaminoacyl-tRNA, and its inability to hydrolyze aminoacyl-tRNA, precisely the same substrate specificity previously reported for peptidyl-tRNA hydrolase from bacteria or yeast. The physiological role of the reticulocyte enzyme may be to hydrolyze and recycle peptidyl-tRNA that has dissociated prematurely from elongating ribosomes, as suggested for the bacterial and yeast enzymes, since reticulocyte peptidyl-tRNA hydrolase is completely incapable of hydrolyzing peptidyl-tRNA that is still bound to polyribosomes. We have purified reticulocyte peptidyl-tRNA hydrolase over 5,000-fold from the postribosomal supernatant with a yield of 14%. The purified product shows a 72-kDa band upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis that has co-purified with enzyme activity and comprises about 90% of the total stained protein, strongly suggesting that the 72-kDa protein is the enzyme. Sucrose density gradient analysis indicates an apparent molecular mass for the native enzyme of 65 kDa, implying that it is a single polypeptide chain. The enzyme is almost completely inactive in the absence of a divalent cation: Mg2+ (1-2 mM) promotes activity best, Mn2+ is partly effective, and Ca2+ and spermidine are ineffective. The hydrolase shows a Km of 0.60 microM and Vmax of 7.1 nmol/min/mg with reticulocyte peptidyl-tRNA, a Km of 60 nM and Vmax of 14 nmol/min/mg with Escherichia coli fMet-tRNA(fMet), and a Km of 100 nM and Vmax of 2.2 nmol/min/mg with yeast N-acetyl-Phe-tRNA(Phe). The enzyme has a pH optimum of 7.0-7.25, it is inactivated by heat (60 degrees C for 5 min), and its activity is almost completely inhibited by pretreatment with N-ethylmaleimide or incubation with 20 mM phosphate. The fact that the enzyme hydrolyzes E. coli but not yeast or reticulocyte fMet-tRNA(fMet) may be explained, at least in part, by structural similarities between prokaryotic tRNA(fMet) and eukaryotic elongator tRNA that are not shared by eukaryotic tRNA(fMet).     

Enzymatic formation of 5,6-dihydroxy-7,9,11,14-eicosatetraenoic acid: kinetics of the reaction and stereochemistry of the product

The enzymatic conversion of leukotriene A4 into 5,6-dihydroxy-7,9,11,14-eicosatetraenoic acid, catalyzed by mouse liver cytosolic epoxide hydrolase (EC 3.3.2.3), was recently described (Haeggstr?m, J., Meijer, J. and R?dmark, O. (1986) J. Biol. Chem. 261, 6332-6337). In the present study, we report analytical data confirming the stereochemistry of this novel enzymatic metabolite of leukotriene A4. By steric analysis of the vicinal diol and comparison with synthetic material, the structure was established as (5S,6R)-dihydroxy-7,9-trans-11,14-cis-eicosatetraenoic acid. Apparent kinetic constants of this reaction were determined and found to be 5 microM and 550 nmol.mg-1.min-1, for Km and Vmax, respectively. Also, a semipurified preparation of human liver cytosolic epoxide hydrolase avidly catalyzed the same hydrolysis of leukotriene A4 (apparent Km was 8 microM). The enzyme was not inactivated by leukotriene A4, as judged by time-course experiments with a second substrate addition.     

Differences between microsomal and mitochondrial-matrix palmitoyl-coenzyme A hydrolase, and palmitoyl-L-carnitine hydrolase from rat liver.

Palmitoyl-CoA hydrolase (EC 3.1.2.2) and palmitoyl-L-carnitine hydrolase (EC 3.1.1.28) activities from rat liver were investigated. 1. Microsomal and mitochondrial-matrix palmitoyl-CoA hydrolase activities had similar pH and temperature optima, although the activities showed different temperature stability. They were inhibited by Pb2+ and Zn2+. The palmitoyl-CoA hydrolase activities in microsomal fraction and mitochondrial matrix were differently affected by the addition of Mg2+, Ca2+, Co2+, K+ and Na+ to the reaction mixture. ATP, ADP and NAD+ stimulated the microsomal activity and inhibited the mitochondrial-matrix enzyme. The activity of both the microsomal and mitochondrial-matrix hydrolase enzymes was specific for long-chain fatty acyl-CoA esters (C12-C18), with the highest activity for palmitoyl-CoA. The apparent Km for palmitoyl-CoA was 47 microM for the microsomal enzyme and 17 microM for the mitochondrial-matrix enzyme. 2. The palmitoyl-CoA hydrolase and palmitoyl-L-carnitine hydrolase activities of microsomal fraction had similar pH optima and were stimulated by dithiothreitol, but were affected differently by the addition of Pb2+, Mg2+, Ca2+, Mn2+ and cysteine. The two enzymes had different temperature-sensitivities. 3. The data strongly suggest that palmitoyl-CoA hydrolase and palmitoyl-L-carnitine hydrolase are separate microsomal enzymes, and that the hydrolysis of palmitoyl-CoA in the microsomal fraction and mitochondria matrix was catalysed by two different enzymes.     

Isolation of D-myo-inositol 1:2-cyclic phosphate 2-inositolphosphohydrolase from human placenta

We have isolated D-myo-inositol 1:2-cyclic phosphate 2-inositolphosphohydrolase (EC 3.1.4.36) from human placenta. This enzyme catalyzes the conversion of inositol 1:2-cyclic phosphate to inositol 1-phosphate. The enzyme was purified 1300-fold to apparent homogeneity from the soluble fraction of human placenta. The enzyme requires Mn2+ or Mg2+ ions for activity, has an apparent Km for inositol 1:2-cyclic phosphate of 0.15 mM and forms 2.2 mumol of inositol 1-phosphate/min/mg protein. The enzyme does not utilize the cyclic esters of inositol polyphosphates as substrates. The molecular weight determined by gel filtration chromatography is approximately 55,000. Upon electrophoresis in polyacrylamide gels in sodium dodecyl sulfate, the molecular weight was found to be 29,000 both in the presence and absence of beta-mercaptoethanol. The enzyme was inhibited by inositol 2-phosphate (IC50 = 4 microM) and to a lesser degree by inositol 1-phosphate (IC50 = 2 mM) and inositol (IC50 = 4 mM). Zn2+ is a potent inhibitor of enzyme activity (IC50 = 10 microM). Neither Li+ nor Ca2+ had any effect on enzyme activity. This enzyme may serve to generate inositol from inositol cyclic phosphate metabolites produced by the phosphoinositide signaling pathway in cells.     

Purification and characterisation of adenosine nucleosidase from Coffea arabica young leaves

An adenosine nucleosidase (ANase) (EC 3.2.2.7) was purified from young leaves of Coffea arabica L. cv. Catimor. A sequence of fractionating steps was used starting with ammonium sulphate salting-out, followed by anion exchange, hydrophobic interaction and gel filtration chromatography. The enzyme was purified 5804-fold and a specific activity of 8333 nkat mg-1 protein was measured. The native enzyme is a homodimer with an apparent molecular weight of 72 kDa estimated by gel filtration and each monomer has a molecular weight of 34.6 kDa, estimated by SDS-PAGE. The enzyme showed maximum activity at pH 6.0 in citrate-phosphate buffer (50 mM). The calculated Km is 6.3 microM and Vmax 9.8 nKat.     

Purification and characterization of bile salt hydrolase from Clostridium perfringens

Bile salt hydrolase (cholylglycine hydrolase, EC 3.5.1.24) has been purified to homogeneity (792-fold) from Clostridium perfringens using high performance DEAE-chromatography. The purified enzyme showed a single detectable protein band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with a relative molecular weight ca. 56,000. The intact enzyme had a relative molecular weight (Mr) of ca. 250,000 as determined by nondenaturing PAGE. The NH2-terminal sequence of bile salt hydrolase was determined to be Met-(Ser/Cys)-Arg-Thr-Lys-Leu-Val-Ileu-Thr-Ileu-Gly-Ala-Ser. The purified enzyme was active towards both glycine and taurine conjugates of cholate. The apparent Km and Vmax of the enzyme for glycocholate was estimated to be 0.5 mM and 107 nmol/min.mg protein, respectively. The pH optimum was in the range of 5.8 to 6.4. The enzyme was inhibited 85%, 81%, and 83% by 2 mM iodoacetate, p-chloromercuribenzoate, and phenylmethanesulfonylfluoride, respectively. Rabbit polyclonal antibody was prepared and used to demonstrate a single form of the enzyme in crude cell extracts.     

Purification and some properties of ATP-sulfurylase from developing sea urchin embryos

ATP-sulfurylase (ATP:sulfate adenylyltransferase, EC 2.7.7.4), purified about 200-fold from sea urchin embryos, was free of ATPase and inorganic pyrophosphatase. The molecular weight of the enzyme was approx. 280 000 measured by gel filtration. The enzyme was activated by Mg2+, Ca2+ or Zn2+; EDTA and p-chloromercuriphenylsulfonate inhibited the enzyme activity. The inhibition was reversed by addition of Mg2+ and dithiothreitol, respectively. The enzyme activity increased continuously as the pH was raised from 5.6 to 10.6. The Km values for the enzyme were calculated to be 13 microM for adenosine 5'-phosphosulfate and 23 microM for pyrophosphate.     

Purification, characterization, and amino acid composition of rabbit pulmonary bleomycin hydrolase

Bleomycin (BLM) hydrolase, a protective enzyme that inactivates the antitumor antibiotic BLM, was purified (6000-fold) to homogeneity from rabbit lungs by DEAE-Sephacel, phenyl-Sepharose chromatography, BLM-Sepharose affinity chromatography, and Mono Q fast protein liquid chromatography. The enzyme had a molecular mass of 250,000 daltons as demonstrated by Superose gel permeation chromatography and polyacrylamide gel electrophoresis (PAGE) under native conditions. Sodium dodecyl sulfate-PAGE revealed a single band of 50,000 daltons, suggesting a pentameric structure. The Km and Vmax for BLM A2 were 1.3 mM and 5.9 mumol mg-1 h-1, respectively. BLM hydrolase activity was labile, had a half-life of 25 min at 56 degrees C, 10 h at 37 degrees C, and 5 days at 4 degrees C, and was stabilized by 2 mM dithiothreitol. The enzyme had a pH optimum of 7.0-7.5 and was inhibited by N-ethylmaleimide, leupeptin, puromycin, and divalent cations such as Cu2+, Cd2+, Zn2+, and Co2+ but was unaffected by chelating agents. On the basis of Mono P chromatofocusing chromatography, three isoforms of BLM hydrolase (apparent pI's of 5.3, 4.5, and 4.3) were present in rabbit pulmonary cytosol. The elution profiles of BLM hydrolase from phenyl-Sepharose and Mono P chromatofocusing indicated that this enzyme is hydrophobic and acidic. This was confirmed by amino acid composition analysis, which demonstrated that 48% of the total amino acids of bleomycin hydrolase were hydrophobic and 37% were acidic.     

Thiamine Triphosphatase in the Membranes of the Main Electric Organ of Electrophorus electricus: Substrate-Enzyme Interactions

The main electric organ of Electrophorus electricus is particularly rich in thiamine triphosphate (TTP). Membrane fractions prepared from this tissue contain a thiamine triphosphatase that is strongly activated by anions and irreversibly inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), an anion transport inhibitor. Kinetic parameters of the enzyme are markedly affected by the conditions of enzyme preparation: In crude membranes, the apparent Km is 1.8 mM and the pH optimum is 6.8, but trypsin treatment of these membranes or their purification on a sucrose gradient decreases both the apparent Km (to 0.2 mM) and the pH optimum (to 5.0). Anions such as NO3- (250 mM) have the opposite effect, i.e., even in purified membranes, the pH optimum is now 7.8 and the Km is 1.1 mM; at pH 7.8, NO3- increases the Vmax 24-fold. TTP protects against inhibition by DIDS, and the KD for TTP could be estimated to be 0.25 mM, a value close to the apparent Km measured in the same purified membrane preparation. Thiamine pyrophosphate (0.1 mM) did not protect against DIDS inhibition. At lower (10(-5)-10(-6) M) substrate concentrations, Lineweaver-Burk plots of thiamine triphosphatase activity markedly deviate from linearity, with the curve being concave downward. This suggests either anticooperative binding or the existence of binding sites with different affinities for TTP. The latter possibility is supported by binding data obtained using [gamma-32P]TTP. Our data suggest the existence of a high-affinity binding site (KD of approximately 0.5 microM) for the Mg-TTP complex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Purification and characterization of phosphomannose isomerase-guanosine diphospho-D-mannose pyrophosphorylase. A bifunctional enzyme in the alginate biosynthetic pathway of Pseudomonas aeruginosa

We report here the purification and characterization of phosphomannose isomerase-guanosine 5'-diphospho-D-mannose pyrophosphorylase, a bifunctional enzyme (PMI-GMP) which catalyzes both the phosphomannose isomerase (PMI) and guanosine 5'-diphospho-D-mannose pyrophosphorylase (GMP) reactions of the Pseudomonas aeruginosa alginate biosynthetic pathway. The PMI and GMP activities co-eluted in the same protein peak through successive fractionation on hydrophobic interaction, ion exchange, and gel filtration chromatography. The purified enzyme migrated as a 56,000 molecular weight protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and the native protein migrated as a monomer of 54,000 molecular weight upon gel filtration chromatography. The apparent Km for D-mannose 6-phosphate was 3.03 mM, and the Vmax was 830 nmol/min/mg of enzyme. For the GMP forward reaction, apparent Km values of 20.5 microM and 29.5 microM for D-mannose 1-phosphate and GTP, respectively, were obtained from double reciprocal plots. The GMP forward reaction Vmax (5,680 nmol/min/mg of enzyme) was comparable to the reverse reaction Vmax (5,170 nmol/min/mg of enzyme), and the apparent Km for GDP-D-mannose was determined to be 14.2 microM. Both reactions required Mg2+ activation, but the PMI reaction rate was 4-fold higher with Co2+ as the activator. PMI (but not GMP) activity was sensitive to dithiothreitol, indicating the involvement of disulfide bonds to form a protein structure capable of PMI activity. DNA sequencing of a cloned mutant algA gene from P. aeruginosa revealed that a point mutation at nucleotide 961 greatly decreased the levels of both PMI and GMP in a crude extract.     

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.     

Cysteine synthase of an extremely thermophilic bacterium,Thermus thermophilus HB8

O-Acetyl-L-serine sulfhydrylase (EC 4.2.99.8) was first purified from an extremely thermophilic bacterium, Thermus thermophilus HB8, in order to ascertain that it is responsible for the cysteine synthesis in this organism cultured with either sulfate or methionine given as a sole sulfur source. Polyacrylamide gel electrophoreses both with and without SDS found high purity of the enzyme preparations finally obtained, through ammonium sulfate fractionation, ion exchange chromatography, gel filtration, and hydrophobic chromatography (or affinity chromatography). The enzyme activity formed only one elution curve in each of the four different chromatographies, strongly suggesting the presence of only one enzyme species in this organism. Molecular masses of 34,000 and 68,000 were estimated for dissociated subunit and the native enzyme, respectively, suggesting a homodimeric structure. The enzyme was stable at 70 degrees C at pH 7.8 for 60 min, and more than 90% of the activity was retained after incubation of its solution at 80 degrees C with 10 mm dithiothreitol. The enzyme was also quite stable at pH 8-12 (50 degrees C, 30 min). It had an apparent Km of 4.8 mM for O-acetyl-L-serine (with 1 mM sulfide) and a Vmax of 435 micromol/min/mg of protein. The apparent Km for sulfide was approximately 50 microM (with 20 mM acetylserine), suggesting that the enzyme can react with sulfide liberated very slowly from methionine. The absorption spectrum of the holo-enzyme and inhibition of the activity by carbonyl reagents suggested the presence of pyridoxal 5'-phosphate as a cofactor. The apo-enzyme showed an apparent Km of 29 microM for the cofactor at pH 8. Monoiodoacetic acid (1 mM) almost completely inactivated the enzyme. The meaning of a very high enzyme content in the cell is discussed.     

Purification and properties of an NADPH-linked aldehyde reductase from rat kidney

Rat kidney was shown to contain two NADPH-linked aldehyde reductases (alcohol:NADP+) oxidoreductase, EC 1.1.1.2) with different substrate affinities. The high-Km aldehyde reductase, which was purified to apparent homogeneity, had a molecular weight of 32 000 as determined by Sephadex G-100 gel filtration, and of 37 000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The purified enzyme reduced various aliphatic aldehydes of different carbon-chain lengths besides many chemicals containing aldehyde groups. The Km values for n-hexadecanal and n-octadecanal were 8 microM and 4 microM, respectively. Bovine serum albumin (1.8 mM) stimulated the reduction of n-hexadecanal and n-octadecanal, and increased the Vmax values by about 15-fold without changing the Km values. The kidney enzyme was not distinguishable from the brain and liver high-Km aldehyde reductases in mobility on polyacrylamide gel electrophoresis, immunological properties, peptide maps or substrate specificity.