Kinetic and allosteric properties of highly-purified, biosynthetic L-threonine dehydrogenase from brewer's yeast Saccharomyces carlsbergensis

Kinetic and allosteric propeties of highly purified "biosynthetic" L-threonine dehydratase from brewer's yeast S. carlbergensis were studied at three pH values, using L-threonine and L-serine as substrates. It was shown that the plot of the initial reaction rate (v) versus initial substrate concentrations ([S]0 pH 6.5 is hyperbolic (Km=5.0.10-2M), while these at pH 7.8 and 9.5 have a faintly pronounced sigmoidal shape with fast occurring saturation plateaus ([S]0.5= 1.0.10-2 and 0.9.10-2M, respectively). the ratios between L-threonine and L-serine dehydratation rates depend on pH. The kinetic properties and the dependence of substrate specificity on pH suggest that the enzyme molecule undergoes pH-induced (at pH 7.0) conformational changes. The determination of pK values of the enzyme functional groups involved in L-threonine binding demonstrated that these groups have pK is approximately equal to 7.5 and 9.5. The latter group was hypothetically identified as a epsilon-NH2-group of the lysine residue. High concentrations of the allosteric inhibitor (L-isoleucine) decrease the rates of L-threonine and L-serine dehydratation and induce the appearance (at pH 6.5) or increase (at pH 7.9 and 9.5) of homotropic cooperative interactions between the active sites in the course of L-threonine dehydratation. The enzyme inhibition by L-isoleucine increases with a decrease of L-threonine concentrations. Low L-isoleucine concentrations, as well as the enzyme activator (L-valine) stimulate the enzyme at non-saturating substrate concentrations (when L-threonine or L-serine are used as substrates) without normalization of (v) versus [S]0 plots. The maximal activation of the enzyme is observed at pHG 8.5--9.0. It is assumed that the molecule of "biosynthetic" L-threonine dehydratase from brewer's yeast contains two types of sites responsible for the effector binding, i.e., "activatory" and "inhibitory" ones.     

Detection and properties of L-threonine-L-serine dehydratase in human liver]

It has been shown that in liver extract of men deceased by different causes, L-threonine and L-serine dehydratase activities probably, belonging to only one enzyme--L-threonine-L-serine dehydratase--are found. Both activities and their ratios depend on K+ concentration both in the buffer used for enzyme extraction and in the reaction medium. Before extraction of active and stable forms of enzyme the liver is to homogenized in a buffer containing 0.15 M KCl. Both enzymatic activities have a pH-optimum at pH 9.6--10.0. It was shown that D-isomers of threonine and serine are not dehydratated and do not inhibit dehydratation of L-isomers. Studies of dependence of L-threonine and L-serine dehydratase reaction rates on temperature showed that at any temperature ranges the energy activation values are higher for the L-threonine dehydratase reaction than for the L-serine dehydratase reaction and that the ratio reaction rates for both reactions depends on temperature.     

Purification and characterization of serine racemase from a hyperthermophilic archaeon, Pyrobaculum islandicum

Pyrobaculum islandicum is an anaerobic hyperthermophilic archaeon that is most active at 100 degrees C. A pyridoxal 5'-phosphate-dependent serine racemase called Srr was purified from the organism. The corresponding srr gene was cloned, and recombinant Srr was purified from Escherichia coli. It showed the highest racemase activity toward L-serine, followed by L-threonine, D-serine, and D-threonine. Like rodent and plant serine racemases, Srr is bifunctional, showing high L-serine/L-threonine dehydratase activity. The sequence of Srr is 87% similar to that of Pyrobaculum aerophilum IlvA (a putative threonine dehydratase) but less than 32% similar to any other serine racemases and threonine dehydratases. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration analyses revealed that Srr is a homotrimer of a 44,000-molecular-weight subunit. Both racemase and dehydratase activities were highest at 95 degrees C, while racemization and dehydration were maximum at pH 8.2 and 7.8, respectively. Unlike other, related Ilv enzymes, Srr showed no allosteric properties: neither of these enzymatic activities was affected by either L-amino acids (isoleucine and valine) or most of the metal ions. Only Fe2+ and Cu2+ caused 20 to 30% inhibition and 30 to 40% stimulation of both enzyme activities, respectively. ATP inhibited racemase activity by 10 to 20%. The Km and Vmax values of the racemase activity of Srr for L-serine were 185 mM and 20.1 micromol/min/mg, respectively, while the corresponding values of the dehydratase activity of L-serine were 2.2 mM and 80.4 micromol/min/mg, respectively.     

Behavior of L-threonine-degrading enzymes during liver regeneration

We examined the effects of a two-thirds hepatectomy in the adult rat on the activities of the three L-threonine-degrading enzymes, L-threonine dehydratase, L-threonine aldolase and L-threonine dehydrogenase. Noticeable variations were observed which did not occur in either sham-operated or turpentine-treated rats and were not linked to food intake. They were considered specific to the regenerating liver. When the reactions were followed in vitro, L-threonine deaminase and L-threonine aldolase were significantly lower for the first 12-24 h: L-threonine dehydrogenase decreased only after 48 h. These results are linked to a decrease in the enzyme concentration in the tissue. L-Serine and L-threonine liver concentrations increased 2-3-fold during the same periods. When the activities were evaluated in vivo, the levels of the first two enzymes remained constant for 24 h, but increased after 48 h; L-threonine dehydrogenase increased between 12 and 48 h. The in vivo activity of the enzymes was reflected by total L-threonine degradation, which had a single sharp peak at 48 h. The asynchronous variations in enzyme activity are related to the differences in protein metabolism which occur in the regenerating liver, and are the consequence of a new transient differential control. The changes observed are significant in liver regeneration; they regulate the consumption and the serum and liver levels of L-serine and L-threonine, setting them aside for protein synthesis. They minutely control the flux of amino acids toward gluconeogenesis, since, during the first 48 h after partial hepatectomy, the production of glucose is ensured principally by lactate; the contribution of L-threonine seems to be more significant only at 48 h. These findings are useful in the study of the regulation of the enzymes involved in amino acid metabolism during liver regeneration.     

The iron-sulfur-cluster-containing L-serine dehydratase from Peptostreptococcus asaccharolyticus. Stereochemistry of the deamination of L-threonine.

The stereochemistry of the deamination of L-threonine to 2-oxobutyrate, catalyzed by purified L-serine dehydratase of Peptostreptococcus asaccharolyticus, was elucidated. For this purpose the enzyme reaction was carried out with unlabelled L-threonine in 2H2O and in 3HOH, as well as with L-[3-3H]threonine in unlabelled water. Isotopically labelled 2-oxobutyrate thus formed was directly reduced in a coupled reaction with L- or D-lactate dehydrogenase and NADH. The (2R)- or (2S)-2-hydroxybutyrate species obtained were then subjected to configurational analyses of their labelled methylene group. The results from 1H-NMR spectroscopy and, after degradation of 2-hydroxybutyrate to propionate, the transcarboxylase assay consistently indicated that the deamination of L-threonine catalyzed by L-serine dehydratase of P. asaccharolyticus proceeds with inversion and retention in a 2:1 ratio. This partial racemization is the first ever to be observed for a reaction catalyzed by serine dehydratase, therefore confirming the distinction of the L-serine dehydratase of P. asaccharolyticus as an iron-sulfur protein from those dehydratases dependent on pyridoxal phosphate. For the latter enzymes exclusively, retention has been reported.     

Fluorometric study on the interaction of amino acids and ATP with valyl-tRNA synthetase from Bacillus stearothermophilus

Interactions of several amino acids and nucleotides with valyl-tRNA synthetase [EC 6.1.1.9] (VRS) from Bacillus stearothermophilus were investigated using as a probe the ligand-induced quenching of protein fluorescence (lambda ex = 295 nm, lambda em = 340 nm) of VRS. L-Valine, L-threonine, L-isoleucine, L-glutamic acid, L-leucine, and D-valine caused fluorescence quenching. Among them, L-threonine had a Kd value comparable to that for the cognate substrate, L-valine, but the other amino acids were bound more weakly as estimated by the fluorescence titration method. L-Alanine, L-histidine, and L-serine did not cause any fluorescence change. Among the nucleotides tested (ATP, ADP, AMP, GTP, ITP, CTP, and UTP), only ATP caused the fluorescence change. In the presence of an excess amount of ATP, only L-valine and L-threonine, among the tested amino acids, induced the fluorescence quenching, and the binding of L-valine was greatly favored under this condition. This is consistent with the results of the ATP-PP1 exchange reaction by VRS, in which only L-valine and L-threonine, of these 9 amino acids tested, could serve as substrates, and the Km value for L-valine was much smaller than that for L-threonine. Thus the binding of ATP to VRS enhances the substrate specificity of VRS towards amino acids.(ABSTRACT TRUNCATED AT 250 WORDS)     

Solution X-ray scattering studies of the yeast phosphofructokinase allosteric transition. Characterization of an ATP-induced conformation distinct in quaternary structure from the R and T states of the enzyme

The allosteric transition of yeast phosphofructokinase has been studied by solution x-ray scattering. The scattering curves corresponding to the native enzyme (T conformation) were found to be similar to the curves recorded in the presence of saturating concentrations of fructose 6-phosphate (R conformation) or AMP (R or R' conformation). However, the curves obtained in the presence of ATP are clearly different: the radius of gyration increases and the secondary minima and maxima are systematically shifted to lower angles, suggesting a swelling of the enzyme in the presence of ATP. These results give the first direct evidence for the existence of an ATP-induced T' conformation, distinct in quaternary structure from the R and T states of the enzyme oligomer, in agreement with our previous modeling of yeast phosphofructokinase regulation. X-ray scattering data are discussed in relation to the distinct molecular mechanisms of the ATP and fructose 6-phosphate allosteric effects involving, respectively, sequential and concerted conformational changes of the enzyme oligomer.     

Functional linkage of adenine nucleotide binding sites in mammalian muscle 6-phosphofructokinase

6-Phosphofructokinases (Pfk) are homo- and heterooligomeric, allosteric enzymes that catalyze one of the rate-limiting steps of the glycolysis: the phosphorylation of fructose 6-phosphate at position 1. Pfk activity is modulated by a number of regulators including adenine nucleotides. Recent crystal structures from eukaryotic Pfk revealed several adenine nucleotide binding sites. Herein, we determined the functional relevance of two adenine nucleotide binding sites through site-directed mutagenesis and enzyme kinetic studies. Subsequent characterization of Pfk mutants allowed the identification of the activating (AMP, ADP) and inhibitory (ATP, ADP) allosteric binding sites. Mutation of one binding site reciprocally influenced the allosteric regulation through nucleotides interacting with the other binding site. Such reciprocal linkage between the activating and inhibitory binding sites is in agreement with current models of allosteric enzyme regulation. Because the allosteric nucleotide binding sites in eukaryotic Pfk did not evolve from prokaryotic ancestors, reciprocal linkage of functionally opposed allosteric binding sites must have developed independently in prokaryotic and eukaryotic Pfk (convergent evolution).     

The role of glyoxylate in the regulation of biodegradative threonine dehydratase of Escherichia coli.

The activity of biodegradative threonine dehydratase of Escherichia coli K12 was reversibly inhibited by glyoxylate in the presence of AMP. Kinetic analysis showed that the inhibition was mixed with respect to L-threonine and competitive in terms of AMP; the inhibitory effect of glyoxylate was less pronounced at high protein concentrations. Incubation of dehydratase with L-threonine shifted the absorption maximum of the enzyme-bound pyridoxal phosphate from 413 to 425 nm; addition of glyoxylate completely prevented the threonine-mediated spectral shift. In addition to the inhibitory effect, incubation of purified enzyme with glyoxylate resulted in a progressive, irreversible inactivation of the enzyme and formation of inactive protein aggregates. The rates of inactivation were decreased with increasing concentrations of protein and AMP. During inactivation by glyoxylate, the 413-nm absorption maximum of the native enzyme was replaced by a new peak at 385 nm. Experiments with [14C]glyoxylate showed a rapid binding of 1 mol of glyoxylate per 147,000 g followed by a slow binding of 3 additional mol of glyoxylate; the glyoxylate-protein linkage was stable to acid precipitation and protein denaturants. Competition binding experiments revealed that pyruvate (which also inactivated the E. coli enzyme, Feldman, D.A., and Datta, P. (1975) Biochemistry 14, 1760-1767) did not interfere with the binding of glyoxylate or vice versa, suggesting that the two keto acids may occupy separate sites on the enzyme molecule. Nevertheless, experiments on enzyme inactivation using glyoxylate plus pyruvate reveal mutual interactions between these ligands in terms of lack of additive effect, retardation in the spectral shift due to glyoxylate, and stabilization of the enzyme in the presence and absence of AMP. We conclude from these results that the control of biodegradative threonine dehydratase is governed by a complex set of regulatory events resulting from reversible and irreversible association of these effectors with the enzyme molecule.     

Regulation of AMP deaminase from chicken erythrocytes. A kinetic study of the allosteric interactions.

The allosteric properties of AMP deaminase [EC 3.5.4.6] from chicken erythrocytes have been qualitatively and quantitatively accounted for by the concerted transition theory of Monod et al., on the assumption that this enzyme has different numbers of binding sites for each ligand. Theoretical curves yield a satisfactory fit for all experimental saturation functions with respect to activation by alkali metals and inhibition by Pi, assuming that the numbers of binding sites for AMP, alkali metals, and Pi are 4, 2, and 4, respectively. The enzyme was inhibited by concentrations of ATP and GTP below 0.1 and 0.25 mM, respectively, whereas activation of the enzyme was observed at ATP and GTP concentrations above 0.4 and 1.5 mM, respectively. These unusual kinetics with respect to ATP and GTP could be also accounted for by assuming 2 inhibitory and 4 activating sites for each ligand.     

Modulation of phosphofructokinase behavior by chemical modifications during the immobilization process

Phosphofructokinase was immobilized within a protein membrane or on soluble protein polymers using glutaraldehyde as cross-linking reagent. The native enzyme was also modified chemically, using the cross-linking reagent alone. A comparative kinetic investigation of these preparations was carried out. The catalytic activity of the chemically modified enzyme and its affinity towards fructose 6-phosphate decreased significantly; the modified enzyme lost its cooperative properties and the allosteric regulation by AMP was affected. When the chemical treatment was performed in the presence of effectors (AMP or ATP) the allosteric transition induced by AMP was restored, suggesting that the cross-linking reagent modified the AMP regulatory sites, albeit no higher-substrate-affinity enzyme conformation was frozen. Molecular data showed that glutaraldehyde produced intramolecular then intermolecular bonds as its concentration increased. When the enzyme was immobilized into protein membranes or on soluble polymers, the enzyme behavior was quite similar: decrease of affinity towards fructose 6-phosphate but no changes in cooperative properties and modifications of allosteric transition induced by AMP. When AMP was present during the immobilisation process, the enzyme immobilized in this way was no longer sensitive to effectors, either AMP or ATP. It showed Michaelian behavior and higher substrate affinity quite similar to that of the native enzyme. The data suggested that a higher-substrate-affinity enzymatic form was most probably stabilized by immobilization.     

Interaction of calmodulin with muscle phosphofructokinase. Interplay with metabolic effectors of the enzyme under physiological conditions

The hysteretic calmodulin-induced inactivation of muscle phosphofructokinase and the calmodulin-mediated reactivation are essentially dependent on environmental conditions. The interplay of calmodulin during these reactions and at allosteric conditions with Mg . ATP, fructose 6-phosphate, adenosine 5'-[beta, gamma-imido]triphosphate and with the allosteric effectors AMP, ADP, fructose 1,6-bisphosphate, fructose 2,6-bisphosphate and glucose 1,6-bisphosphate was studied by two techniques. (a) A two-step technique with a preincubation of enzyme, calmodulin and effectors in close to physiological concentrations before dilution into an optimal activity assay. It reveals aggregation and slowly reversible conformation changes. (b) A direct assay of dilute enzyme at allosteric conditions. Dominating in the interplay of calmodulin with metabolic effectors is the competitive-like action of calmodulin on Mg . ATP binding to the regulatory sites of the enzyme. At high enzyme concentrations in the absence of hexose phosphates, i.e. at noncatalytic conditions calmodulin counteracts the stabilization of the highly active tetrameric form caused by Mg . ATP. In the allosteric assay it counteracts the ATP-induced allosteric inhibition. In both cases calmodulin acts synergistic with AMP and ADP. To a minor degree calmodulin also counteracts the stabilization of the tetrameric form caused by fructose 6-phosphate and hexose bisphosphate, now however antagonistically to AMP and ADP. By the demonstrated interactions the enzyme can be slowly and hysteretically shifted between an active tetrameric and an inactive dimeric state under control metabolic conditions and of Ca2+ and calmodulin. Resting conditions will inactivate and high contractile activity reactivate available enzyme.     

Effects of amino acids, adenine nucleotides and inorganic pyrophosphate on glutamine synthetase from Anabaena cylindrica.

Glutamine synthetase (L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2) from Anabaena cylindrica was inhibited by alanine, glycine, serine and aspartate. The effects of alanine and serine were uncompetitive with respect to glutamate, while those of glycine and asparatate were uncompetitive with respect to glutamate, while those of glycine and aspartate were non-competitive and mixed type respectively. Different pairs of amino acids and their various combinations caused a cumulative inhibition of the enzyme activity. Glutamine synthetase was also inhibited by ADP and AMP and both nucleotides affected the enzyme competitively with respect to ATP and non-competitively for glutamate. Inorganic pyrophosphate, between 2 and 3 mM, produced a very pronounced inhibiton of enzyme activity. The inhibition by PPi was uncompetitive for ATP. Various combinations of the adenine nucleotides, PPi and Pi exerted a cumulative inhibitory effect on the enzyme activity, as did the amino acids, in different combinations with either adenine nucleotides, PPi or Pi. The effects of the adenine nucleotides and the amino acids were more pronounced at higher concentrations of ammonia. Except for serine similar responses of these effectors were obtained with increasing concentrations of Mg2+. It is proposed that changes in the free concentrations of Mg2+ are important in energy-dependent regulation of the enzyme activity in this alga.     

Affinity labeling of the allosteric site of fructose 1,6-bisphosphatase with an AMP analog

D-Fructose 1,6-bisphosphatase [EC 3.1.3.11, FBPase] is one of the key enzymes in glyconeogenesis and its activity is controlled by various effectors such as substrate, AMP and ATP. To analyze this complex regulation system, we tried an affinity labeling of FBPase with an AMP derivative, since AMP is a potent allosteric inhibitor of this enzyme. The results obtained are as follows. 1. To determine the functional groups which are essential for AMP as an inhibitor, inhibitory activities of some AMP derivatives were examined. These derivatives modified at the purine ring or phosphate group lost the activity while one modified at the ribose ring retained the ability to inhibit FBPase. This shows that an affinity labeling reagent should be an AMP derivative in which the ribose ring is modified. 2. 2',3'-Dialdehyde AMP (dial-AMP) was prepared by periodate oxidation of AMP and was reacted with FBPase. Under appropriate conditions, 1 mol of the reagent was incorporated per mol of enzyme subunit with a concomitant loss of enzyme activity. The reaction was prevented by the presence of AMP but not of ATP. The heat-stability, the kinetic parameters and the UV-absorption spectrum of the modified enzyme were all the same as those of native FBPase in the presence of AMP. Thus it was concluded that the allosteric AMP site in FBPase was modified specifically.     

L-serine dehydratase from Arthrobacter globiformis.

1. L-Serine dehydratase (EC 4.2.1.13) was purified 970-fold from glycine-grown Arthrobacter globiformis to a final specific activity of 660micronmol of pyruvate formed/min per mg of protein. 2. The enzyme is specific for L-serine; D-serine, L-threonine and L-cysteine are not attacked. 3. The time-course of pyruvate formation by the purified enzyme, in common with enzyme in crude extracts and throughout the purification, is non-linear. The reaction rate increases progressively for several minutes before becoming constant. The enzyme is activated by preincubation with L-serine and a linear time-course is then obtained. 4. The substrate-saturation curve for L-serine is sigmoid. The value of [S]0.5 varies with protein concentration, from 6.5mM at 23microng/ml to 20mM at 0.23microng/ml. The Hill coefficient remains constant at 2.9.5 The enzyme shows a non-specific requirement for a univalent or bivalent cation. Half-maximal activity is produced by 1.0mM-MgCl2 or by 22.5mM-KCl. 6. L-Cysteine and D-serine act as competitive inhibitors of L-serine dehydratase, with Ki values of 1.2 and 4.9mM respectively. L-Cysteine, at higher concentrations, also causes a slowly developing irreversible inhibition of the enzyme. 7. Inhibition by HgCl2 (5micronM)can be partially reversed in its initial phase by 1mM-L-cysteine, but after 10 min it becomes irreversible. 8. In contrast with the situation in all cell-free preparations, toluene-treated cells of A. globiformis form pyruvate from L-serine at a constant rate from the initiation of the reaction, show a hyperbolic substrate-saturation curve with an apparent Km of 7mM and do not require a cation for activity.     

AMP deaminase from yeast. Role in AMP degradation, large scale purification, and properties of the native and proteolyzed enzyme

Eukaryotes have been proposed to depend on AMP deaminase as a primary step in the regulation of intracellular adenine nucleotide pools. This report describes 1) the role of AMP deaminase in adenylate metabolism in yeast cell extracts, 2) a method for large scale purification of the enzyme, 3) the kinetic properties of native and proteolyzed enzymes, 4) the kinetic reaction mechanism, and 5) regulatory interactions with ATP, GTP, MgATP, ADP, and PO4. Allosteric regulation of yeast AMP deaminase is of physiological significance, since expression of the gene is constitutive (Meyer, S. L., Kvalnes-Krick, K. L., and Schramm, V. L. (1989) Biochemistry 28, 8734-8743). The metabolism of ATP in cell-free extracts of yeast demonstrates that AMP deaminase is the sole pathway of AMP catabolism in these extracts. Purification of the enzyme from bakers' yeast yields a proteolytically cleaved enzyme, Mr 86,000, which is missing 192 amino acids from the N-terminal region. Extracts of Escherichia coli containing a plasmid with the gene for yeast AMP deaminase contained only the unproteolyzed enzyme, Mr 100,000. The unproteolyzed enzyme is highly unstable during purification. Substrate saturation plots for proteolyzed AMP deaminase are sigmoidal. In the presence of ATP, the allosteric activator, the enzyme exhibits normal saturation kinetics. ATP activates the proteolyzed AMP deaminase by increasing the affinity for AMP from 1.3 to 0.2 mM without affecting VM. Activation by ATP is more efficient than MgATP, with half-maximum activation constants of 6 and 80 microM, respectively. The kinetic properties of the proteolyzed and unproteolyzed AMP deaminase are similar. Thus, the N-terminal region is not required for catalysis or allosteric activation. AMP deaminase is competitively inhibited by GTP and PO4 with respect to AMP. The inhibition constants for these inhibitors decrease in the presence of ATP. ATP, therefore, tightens the binding of GTP, PO4, and AMP. The products of the reaction, NH3 and IMP, are competitive inhibitors against substrate, consistent with a rapid equilibrium random kinetic mechanism. Kinetic dissociation constants are reported for the binary and ternary substrate and product complexes and the allosteric modulators.     

The allosteric properties of beef-liver fructose bisphosphatase.

1. The activity of beef liver fructose bisphosphatase has been shown to respond cooperatively to increasing concentrations of the activating cations Mg2+ and Mn2+. The allosteric inhibitor AMP caused an increase in this cooperativity and a decrease in the apparent affinity of the enzyme for the activating cation. 2. The cooperative response of the enzyme to AMP is similarly increased by increasing cation concentrations with a concomitant decrease in the apparent affinity. 3. Direct binding experiments indicated that in the absence of either Mg2+ or Mn2+ the enzyme bound AMP non-cooperatively up to a maximum of two molecules per molecule of enzyme, a result that is indicative of half-sites reactivity. The binding became increasingly cooperative as the concentration of the activating cation was increased. 4. The substrate fructose bisphosphate had no effect on any of these cooperative responses. 5. These results may be most simply interpreted in terms of concerted model in which the activating cation functions both as an allosteric activator and as an essential cofactor for the reaction.     

Purification and physicochemical, kinetic and immunological properties of allosteric serine hydroxymethyltransferase from monkey liver.

The homogeneous serine hydroxymethyltransferase purified from monkey liver, by the use of Blue Sepharose affinity chromatography, exhibited positive homotropic co-operative interactions (h = 2.5) with tetrahydrofolate and heterotropic interactions with L-serine and nicotinamide nucleotides. The enzyme had an unusually high temperature optimum of 60 degrees C and was protected against thermal inactivation by L-serine. The allosteric effects were abolished when the monkey liver enzyme was purified by using a heat-denaturation step in the presence of L-serine, a procedure adopted by earlier workers for the purification of this enzyme from mammalian and bacterial sources. The enzyme activity was inhibited completely by N5-methyltetrahydrofolate, N5-formyltetrahydrofolate, dichloromethotrexate, aminopterin and D-cycloserine, whereas methotrexate and dihydrofolate were partial inhibitors. The insoluble monkey liver enzyme-antibody complex was catalytically active and failed to show positive homotropic co-operative interactions with tetrahydrofolate (h = 1) and heterotropic interactions with NAD+. The enzyme showed a higher heat-stability in a complex with its antibody than as the free enzyme. These results highlight the pitfalls in using a heat-denaturation step in the purification of allosteric enzymes.     

Effect of higher alcohols on the activity of biosynthetic L-threonine dehydratase from Saccharomyces carlsbergensis brewer's yeast]

The effects of low (1 . 10(-4) M) and high (1 . 10(-3) M) concentrations of n-propanol, isobutanol and isoamylols on the kinetic behaviour of "biosynthetic" L-threonine dehydratase from brewer's yeast S. carlsbergensis 776 were studied. It was concluded that these alcohols control the activity of the first enzyme of the L-threonine biosynthetic pathway.