The protease problem in Neurospora. Variable stability of enzymes in aromatic amino acid metabolism.
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Anthranilate Synthase↗
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[Properties of chymotrypsin proteinase from Aspergillus oryzae].
Chymotrypsin-type proteinase is detected in the proteolytic system of Asp. oryzae. The action of it and chymotrypsin is shown to depend on formaldehyde. Hydrolysis of substrates, p-nitrophenyl acetate (p-NPA) and N-benzoyl-tyrosine methyl ether (BTME), by both preparations is almost the same. The obtained activity pH-optimum for the studied proteinase esterolytic activity is located in the alkaline zone as well as for crystalline chymotrypsin (substrate p-NPA). It concerns pH of both enzymes stability as well. The enzyme under study is relatively labile. At 50 degrees C there are only traces of the activity in the medium with p-NPG. Its considerable decrease is observed at 40 degrees C. This type activity is more stable on the substrate BTME. 10 min later it disappears completely in the enzymic preparation at a temperature of 60 degrees C at 40 degrees C it is 96.8%. For 24 h at 25 degrees C the activity lowers only by 8%. Crystalline chymotrypsin is stable under these conditions. DEAE-cellulose chromatography (different types of elution) detected multiple forms of proteinase differing in solubility chromatographic properties and specific activity when splitting the substrates p-NPA, BTME and casein.
Aspergillus↗
Hydrolysis of organophosphate insecticides by an immobilized-enzyme system.
An enzyme preparation that could detoxify parathion and eight other organophosphate pesticides was covalently bound to either porous glass or porous silica beads. This immobilized-enzyme system was examined for its use in detoxification of pesticides in production wastewaters. The kinetics of parathion hydrolysis were examined at flow rates up to 96 liter/hr and at influent substrate concentrations ranging from 10--250 mg/liter. The enzyme reactor was able to hydrolyze 95% or more of the parathion added to industrial wastewaters generated during its production, thus reducing the effluent parathion concentration to below 500 ppb. Laboratory continuous-flow experiments were conducted for 70 days with industrial wastewater and indicated no loss in immobilized-enzyme activity. The influence of pH, temperature, solvents, and detergents on enzyme stability and activity and enzyme reactor kinetics will be discussed.
Enzymes, Immobilized↗
Conformational stabilization of enzymes in covalent catalysis.
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Aspartate Aminotransferases↗
Stability of enzymes in post-mortem rat brain.
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Acetylcholinesterase↗
Formation of stable anhydrides from CoA transferase and hydroxamic acids.
Acetohydroxamic acid reacts with the enzyme-CoA form of succinyl-CoA:3-ketoacid coenzyme A transferase to give an inactive product with a rate constant of 860 M-1 min-1 at pH 8.1, 25 degrees C. The reaction is reversible in the presence of coenzyme A and has an equilibrium constant of 0.040. The product is an anhydride that is an analog of the intermediate that has been postulated in the normal catalytic pathway; it is inactive because coenzyme A does not react with the acyl group of the hydroxamic acid. The equilibrium constant for formation of the anhydride from the thil ester of enzyme and methyl 3-mercaptopropionate is 75 times larger than the equilibrium constant of 2.2 for the formation of N,O-diacetylhydroxylamine from acetohydroxamic acid and acetyl-CoA. This shows that the enzyme stabilizes the anhydride at the active site by at least -2.6 kcal mol-1. Succinomonohydroxamic acid reacts with enzyme-CoA as both a substrate and an inactivator, with relative rate constants of 25:1. The inactivation is irreversible, indicating that the enzyme provides a larger stabilization of at least -5.9 kcal mol-1 for the anhydride of an analog of the specific substrate, succinate. The results are consistent with the hypothesis that the enzyme stabilizes an anhydride that is formed at the active site during turnover of normal substrates through a stepwise reaction mechanism.
Acetoacetates↗
Kinetic and spectroscopic evidence of cation-induced conformation changes in yeast K+ -activated aldehyde dehydrogenase.
The activity, stability and spectroscopic properties of yeast K+ -activated aldehyde dehydrogenase were measured at various times after removal from, and after returning to a solution containing K+. Enzyme activity is rapidly lost on removal of most of the K+ and rapidly regained if K+ is replaced immediately. These activity changes are slower than likely rates of K+ dissociation and association. These rapid changes in concentration result in altered enzyme stability with enzyme in K+ the more stable. U.v. difference spectra are produced whenever enzyme in an activating environment (K+ or Tl+) is compared with enzyme in a non-activating environment (Tris+ or Li+). These spectral changes occur within 10s. The saturation characteristics with K+ are hyperbolic for all three phenomena of activation, stabilization and spectral change, with estimated apparent dissociation constants (Ks) for K+ of 7.5 mM, 5.5 mM and 6 mM respectively. Continued incubation of enzyme in the absence of K+ results in the accumulation of an enzyme form that re-activates only slowly on replacing K+. Stability characteristics in various concentrations of K+ over equivalent time scales are consistent with the existence of additional conformations. Spectroscopic evidence also indicates such additional slow conformation changes. Results have been interpreted in terms of two separate conformation transitions induced or stabilized by K+.
Aldehyde Oxidoreductases↗
Levansucrase of Bacillus subtilis. Characterization of a stabilized fructosyl-enzyme complex and identification of an aspartly residue as the binding site of the fructosyl group.
A covalently linked fructosyl-enzyme complex was isolated from a reaction mixture of enzyme and sucrose submitted to the quenching effect of a large decrease of the pH. The fructosyl-enzyme bond was shown to be stable under acidic and neutral conditions in the presence of high concentration of urea and of sodium dodecyl sulfate. This intermediate did not transfer at a measurable rate its fructosyl group to the usual fructosyl acceptors of the enzyme reaction under the usual conditions of enzyme activity. However stability measurements of the fructosyl-enzyme bond indicated a marked lability at pH values above 8.5. The apparent rate constant of the hydrolytic reaction of this bond evaluated under the standard state of molar concentration of hydroxide ion was of the same order of magnitude as the apparent rate constant of the hydrolytic reaction of the transient fructosyl-enzyme postulated from the kinetic analysis of levansucrase. Furthermore, nucleophilic agents like imidazole enhanced the hydrolytic reaction of the fructosyl-enzyme bond. Identification of the fructosyl binding site on the enzyme was accomplished by proteolytic hydrolysis of the trapped complex. Peptic digestion followed by pronase digestion released a fructosyl-aspartate compound that we have isolated in a high state of purity. The lability of the fructosyl-aspartate bond under mild alkaline conditions suggested that the fructosyl was linked through an ester bond involving the beta-carboxyl of the aspartate residue. Treatment of the trapped complex with cyanogen bromide released only one fructosylated peptide. The apparent molecular weight of this peptide was estimated to be lower than 10000.
Aspartic Acid↗
Immobilized flounder muscle glyceraldehyde 3-phosphate dehydrogenase.
Partially purified flounder muscle (Pseudopleuronectus americanus) glyceraldehyde 3-phosphate dehydrogenase was immobilized on cyanogen bromide-activated Sepharose. The catalytic properties of the immobilized preparation were studied to determine if immobilization alters the kinetic properties of the native holoenzyme. The results indicate that the pH activity profile of immobilized glyceraldehyde 3-phosphate dehydrogenase did not differ from that of the native enzyme. The Michaelis constants (Km) for NAD and glyceraldehyde 3-phosphate were somewhat altered. The enzyme stability toward various inactivation treatments in the presence and absence of NAD was characterized and compared to that of he native enzyme. When either form of the enzyme was incubated with urea at concentrations greater than 2M, inactivation occurred very rapidly. Incubation in 0.1% trypsin for 60 minutes decreased the activity of immobilized glyceraldehyde 3-phosphate dehydrogenase by 45% and of the native soluble enzyme by 70%. The immobilized enzyme also exhibited considerably more stability than the native soluble enzyme when exposed to a temperature of 50 degrees or to 20 mM ATP. In all cases NAD either greatly reduced the rate of inactivation or completely protected the enzyme from inactivation.
Adenosine Triphosphate↗
Irreversible stimulation of adenylate cyclase activity of fat cell membranes of phosphoramidate and phosphonate analogs of GTP.
The ability of 5'-guanylylimidodiphosphate (Gpp(NH)p) to stimulate irreversibly the adenylate cyclease activity of fat cell membranes has been studied by preincubating the membranes with this or related analogs followed by assaying after thoroughly washing the membranes. Activation can occur in a simple Tris-HCl buffer, in the absence of added divalent cations and in the presence of EDTA. Dithiothreitol enhances the apparent degree of activation, perhaps by stabilization. The importance of utilizing optimal conditions for stabilizing enzyme activity, and of measuring the simultaneous changes in the control enzyme, is illustrated. The organomercurial, p-aminophenylmercuric acetate, inhibits profoundly the activity of the native as well as the Gpp(NH)p-stimulated adenylate cyclase, but in both cases subsequent exposure to dithiothreitol restores fully the original enzyme activity. However, the mercurial-inactivated enzyme does not react with Gpp(NP)p, as evidenced by the subsequent restoration of only the control enzyme activity upon exposure to dithiothreitol. Thus, reaction with Gpp(NH)p requires intact sulfhydryl groups, but the activated state is not irreversibly destroyed by the inactivation caused by sulfhydryl blockade. GTP and, less effectively, GDP and ATP inhibit activation by Gpp(NH)p, but interpretations are complicated by the facts that this inhibition is overcome with time and that GTP and ATP can protect potently from spontaneous inactivation. These two nucleotides can be used in the Gpp(NH)p preincubation to stabilize the enzyme. The Gpp(NH)p-activated enzyme cannot be reversed spontaneously during prolonged incubation at 30 degrees C in the absence or presence of GTP, ATP, MgCl2, glycine, dithiothreitol, NaF or EDTA. The strong nucleophile, neutral hydroxylamine, decreases the Gpp(NH)p-activated enzyme activity and no subsequent activation is detected upon re-exposure to the nucleotide.
Adenosine Triphosphate↗
The stability of immobilized enzymes.
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Drug Stability↗
The mechanism of action of glycosidases.
The factors that may contribute to the rate enhancement observed with enzymatic versus non-enzymatic hydrolysis of glycosides are discussed. The nature of the active site as deduced from labelling studies with beta-glucosidases is described. A two-step mechanism involving either an enzyme stabilized glycosyl ion or a covalent glycosyl-enzyme intermediate is proposed. Experiments with a beta-glucosidase from almonds show that even with 2-deoxy glucosides with good leaving groups as aglycon which are hydrolyzed 1000 times more slowly than the corresponding glucosides, the deglucosylation step is faster than the cleavage of the glycosidic bond.
Aspergillus↗
8-Mercaptoflavins as active site probes of flavoenzymes.
Representative examples of the various classes of flavoproteins have been converted to their apoprotein forms and the native flavin replaced by 8-mercapto-FMN or 8-mercapto-FAD. The spectral and catalytic properties of the modified enzymes are characteristically different from one group to another; the results suggest that flavin interactions at positions N(1) or N(5) of the flavin chromophore have profound influences on the properties of the flavoprotein. 1. The 8-thiolate anion form of 8-mercaptoflavin has an absorption maximum in the region 520 to 550 nm epsilon approximately 30 mM-1 cm-1). This form is retained on binding to flavoproteins whose physiological reactions involve obligatory one-electron transfers (e.g. flavodoxin, NADPH-cytochrome P-450 reductase). In the native form these enzymes stabilize the blue neutral radical of the flavin. A radical form of 8-mercaptoflavin is also stabilized by these proteins. 2. The p-quinoid form of 8-mercaptoflavin has an absorption maximum in the range 560 to 600 nm (epsilon approximately 30 mM-1 cm-1). This form is stabilized on binding to flavoproteins of the dehydrogenase-oxidase class (e.g. glucose oxidase, D-amino acid oxidase, lactate oxidase, Old Yellow Enzyme). These same enzymes in their native flavin form stabilize the red semiquinone, and have a pronounced reactivity with sulfite to form flavin N(5)-sulfite adducts. These properties of the native enzyme, including the ability to react with nitroalkane carbanions, are not exhibited by the 8-mercaptoflavoproteins. 3. A group of flavoenzymes fails to conform strictly to the above classification, exhibiting some properties of both classes. These include the examples of flavoprotein hydroxylases and transhydrogenases studied. 4. The riboflavin-binding protein of hen egg whites binds 8-mercaptoriboflavin preferentially in the unionized state, resulting in a shift in pK from 3.8 with free 8-mercaptoriboflavin to greater than or equal to 9.0 with the protein-bound form.
Binding Sites↗
Comparative studies on immobilization of human prostatic acid phosphatase.
Acid phosphatase (othophosphoric monoester phosphohydrolase (acid optimum), EC 3.1.3.2) from the human prostate was immobilized by its protein moiety on cyanogen bromide-activated Sepharose, by carbohydrate moiety on Concanavalin-A-Sepharose, and by Schiff base formation with partially oxidized carbohydrate groups on ethylenediamine-Sepharose. The highest retention of enzyme activity, 80%, was found for the noncovalent immobilization on Concanavalin-A-Sepharose. It was demonstrated that the optimal pH changes for the Concanavalin-A-Sepharose and CNBr-Sepharose-enzyme complexes are electrostratic in character. In all cases of immobilization the enzyme has higher thermostability than that for the native enzyme under the same conditions. The effects of the enzyme stabilization were interpreted in terms of the multipoint interaction between the enzyme molecule and the carrier.
Acid Phosphatase↗
Multispecific aspartate and aromatic amino acid aminotransferases in Escherichia coli.
Two aminotransferases from Escherichia coli were purified to homogeneity by the criterion of gel electrophoresis. The first (enzyme A) is active on L-aspartic acid, L-tyrosine, L-phenylalanine, and L-tryptophan; the second (enzyme B) is active on the aromatic amiono acids. Enzyme A is identical in substrate specificity with transaminase A and is mainly an aspartate aminotransferase; enzyme B has never been described before and is an aromatic amino acid aminotransferase. The two enzymes are different in the Vmax and Km values with their common substrates and pyridoxal phosphate, in heat stability (enzyme A being heat-stable and enzyme B being heat-labile at 55 degrees) and in pH optima with the amino acid substrates. They are similar in their amino acid composition, each enzyme appears to consist of two subunits, and enzyme B may be converted to enzyme A by controlled proteolysis with subtilsin. The conversion was detected by the generation of new aspartate aminotransferase activity from enzyme B and was further verified by identification by acrylamide gel electrophoresis of the newly formed enzyme A. The two enzymes appear to be products of two genes different in a small, probably terminal, nucleotide sequence.
Amino Acids↗
L-glycerol-3-phosphate dehydrogenase from the insect Ceratitis capitata. Purfication, physicochemical and enzymic properties.
Soluble L-glycerol-3-phosphate dehydrogenase (sn-glycerol-3-phosphate: NAD+ 2-oxidoreductase, EC 1.1.1.8) from the mediterranean fruit fly Ceratitis capitata has been purified 130-fold with an overall yield of about 40%. The final preparation had a specific activity of about 200 mumol NADH/min/mg protein. The enzyme preparation has been shown to be homogeneous throughout disc gel electrophoresis, dodecyl sulphate gel electrophoresis, isoelectric focusing and ultracentrifugation. The Km values for dihydroxyacetone phosphate, NADH, L-glycerol-3-phosphate and NAD+ were respectively 0.33, 0.018, 0.74 and 0.26 mM. L-glycerol-3-phosphate dehydrogenase from the insect had a maximal activity around pH 6.6 for the oxidation of NADH and pH 10.0 for the reduction of NAD+. It was stable from pH 6.0 to pH 9.0 at 20 degrees C for 1 h and remained active after incubating at 30 degrees C for 30 min at pH 6.6. The enzyme was completely inactivated by incubating at 60 degrees C for 5 min. Enzyme stability versus ionic strength as well as the dependence of the reaction velocity on temperature are also reported. The active enzyme was found to have a minimum molecular weight of approx. 63 000. Molecular weight determinations by sodium dodecyl sulphate gel electrophoresis gave subunit weights of 33 500. The isoelectric point of the protein was determined by electrofocusing and found to be 5.75 +/- 0.05. The extinction coefficient at 278 nm was calculated by dry weight measurements to be E1cm 1mg/ml = 0.42 +/- 0.1. Sedimentation velocity studies on ultracentrifuge indicated a dependence of the sedimentation coefficient on the enzyme concentration. The amino acid composition of the enzyme was determined. The protein has no free N-terminal residue and the digestion with carboxypeptidases gave the C-terminal sequence: -ala-gly-ser. All these data are discussed in relation to the properties of the enzyme from other sources.
Amino Acids↗
Stabilization of rat liver mitochondrial F1-adenosine triphosphatase during chloroform-induced solubilization.
1. Isolation of ATPase from rat liver submitochondrial particles by chloroform treatment requires the presence of ATP or ADP during enzyme solubilization. In the absence of adenine nucleotides the enzyme activity is very low although all protein components of F1-ATPase are released. The low concentrations of ATP or ADP required (5 microM) indicate that the high affinity nucleotide-binding sites are involved in enzyme stabilization. Other nucleotides tested (ITP, GTP, UTP, CTP) were found to be less effective. 2. Polyacrylamide gel electrophoresis and immunodiffusion in agar plates revealed that in the absence of adenine nucleotides a fraction of F1-ATPase released by chloroform treatment is split into fragments. The part of the dissociated enzyme molecule has a molecular weight identical with that of a beta-subunit of F1-ATPase. 3. Dissociation of the F1-ATPase molecule could also be prevented by aurovertin. 4. Crude F1-ATPase solubilized by chloroform treatment can be further purified by Sepharose 6B gel filtration. Specific ATPase activity of the purified enzyme was 90 mumol Pi/min per mg protein and the enzyme was composed of five protein subunits (alpha, beta, gamma, delta, epsilon) with molecular weights 58 000, 55 000, 28 000, 13 000 and 8000, respectively. 5. Chloroform-released F1-ATPase from rat liver mitochondria displayed immunochemical cross-reactivity with that isolated from beef heart mitochondria.
Adenosine Triphosphatases↗
Differential stabilities of soil enzymes. Assay and properties of phosphatase and arylsulphatase.
Methods have been refined for the assay of phosphatase and arylsulphatase activities in soil, based on the chromogenic p-nitrophenyl ester substrates. Basic assay conditions have been defined, and pH optima and kinetic parameters have been determined. The enzymes follow Michaelis-Menten kinetics; this conclusion is based on three methods of analysis of data determined over a wide range of substrate concentrations. The enzyme activities are very stable to storage of wet soil for up to 4 weeks at soil temperatures and above. For example, phosphatase had a half-life of approximately 2 weeks at 50 degrees C; arylsulphatase was rather less stable. Both enzymes retained 80% of activity after incubation with pronase for 1 week at 25 degrees C. On the basis of this work and studies on other soil enzymes, it is concluded that remarkable stability is a general feature of soil enzymes.
Arylsulfatases↗