Some kinetic properties of gamma-glutamyltransferase from rabbit liver

gamma-Glutamyltransferase ((5-glutamyl)-peptide: amino-acid 5-glutamyltransferase, EC 2.3.2.2) of rabbit liver (detergent form) was purified 1100-fold in order to study its kinetic properties. Kinetic studies were conducted from pH 6.0 to 12.0 in the absence and presence of the acceptor substrate glycylglycine using gamma-glutamyl-3-carboxy-4-nitroanilide as the donor. The existence of more than one binding site for both donor and acceptor is postulated on kinetic evidence such as donor substrate activation, donor substrate inhibition and acceptor substrate activation. Homotropic interaction is also observed, in the form of negative cooperativity, in donor substrate binding, in the absence of acceptor at pH less than 9.0 and positive cooperativity (n = 2), in the absence or presence of acceptor at pH greater than 9.0. Hydrolase reaction reaches a maximum of activity at pH 10 (pK 8.6). Transferase activity under conditions of maximal velocity is maximal at pH 9.0 (pK 7.1). The ratio of transferase activity/hydrolase activity is maximal at pH 7.0-7.5. At low donor substrate concentrations, maximal activity is attained at pH 7.5.     

A steady-state kinetic study of the reaction catalysed by the secondary-amine mono-oxygenase of Pseudomonas aminovorans.

1. Secondary-amine mono-oxygenase (proposed EC group 1.14.99.-) was partially purified from trimethylamine-grown Pseudomonas aminovorans by (NH4)2SO4 fractionation, gel filtration, hydrophobic chromatography on 5-aminopentylamino-Sepharose, and affinity chromatography on Sepharose-bound NADH. 2. Some problems in the affinity-chromatography step are discussed. 3. A steady-state kinetic analysis varying substrate, oxygen and electron-donor concentrations was performed, which, over the concentration range studied, gave a series of families of approximately parallel double-reciprocal plots. From secondary and tertiary plots, Michaelis constants of 0.160 mM, 0.086 mM and 0.121 mM were obtained for dimethylamine, NADPH and oxygen respectively. 4. Product-inhibition studies supported the postulated Hexa Uni Ping Pong (triple-transfer) reaction mechanism.     

Influence of temperature on the determination of enzyme activities in human serum. gamma-Glutamyl transferase.

Optimum reaction conditions for determination of gamma-glutamyl transferase were studied at 25, 30 and 37 degrees C using a kinetic test and gamma-glutamyl-3-carboxy-4-nitranilide as substrate. There was no dependence on temperature of half saturation constants of gamma-glutamyl-3-carboxy-4-nitranilide. The corresponding constants for glycylglycine were influenced by temperature and the pH. The optimum showed a dependence upon temperature. In Arrhenius' plot, a deviation from straight line can be observed only above 35 degrees C. The influence of temperature on the determination of enzyme activities in human serum are discussed.     

The kinetic properties of human placental choline acetyltransferase

1. Michaelis constants for human placental choline acetyltransferase were shown to be dependent on the concentration of the second substrate present. The primary plots indicate a sequential rather than a Ping Pong mechanism and are of the same type with 300mm- and 500mm-sodium chloride. 2. Similar results have been obtained with rabbit brain choline acetyltransferase. 3. Product inhibition of the forward reaction has been studied. CoA inhibits competitively with respect to acetyl-CoA and non-competitively with respect to choline. Acetylcholine inhibits competitively with respect to choline and non-competitively with respect to acetyl-CoA. No inhibition is given by acetylcholine when the enzyme is saturated with choline. 4. It is concluded that human placental choline acetyltransferase has an ordered mechanism of the Theorell-Chance type.     

Some properties of serine: glyoxylate aminotransferase from rye seedlings (Secale cereale L.).

Serine: glyoxylate aminotransferase (EC 2.6.1.45) from rye seedlings catalysed transamination between L-serine and glyoxylate according to the Ping Pong Bi Bi mechanism with double substrate inhibition. As judged from the Km values, L-serine, L-alanine, and L-asparagine served as substrates for the enzyme with glyoxylate, whereas L-alanine and L-asparagine underwent transamination with hydroxypyruvate as acceptor. Pyridoxal phosphate (PLP) seems to be rather loosely bound to the enzyme protein. Aminooxyacetate and D-serine were found to be pure competitive inhibitors of the enzyme, with Ki values of 0.12 microM and 1.6 mM, respectively. Among the PLP inhibitors isonicotinic acid hydrazide and hydroxylamine were far less effective than aminooxyacetate (20% and 70% inhibition at 0.1 mM concentration, respectively). Inhibition by the SH group inhibitors at 1 mM concentration did not exceed 50%. L-Serine distinctly diminished the inhibitory effect of this type inhibitors. Preincubation of the enzyme with glyoxylate distinctly diminished transamination. Glyoxylate limited the inhibitory action of formaldehyde probably by competing for the reactive groups present in the active centre.     

Kinetic studies with cytosol and mitochondrial phosphoenolpyruvate carboxykinases

1. Measurements of Michaelis constants for oxaloacetate in the reaction catalysed by liver phosphoenolpyruvate carboxykinase give values much lower than previously reported. With Mg(2+) as bivalent cation, the Michaelis constant was approx. 2.5x10(-5)m whether the enzyme used was the mitochondrial phosphoenolpyruvate carboxykinase purified from sheep liver or chicken liver or the cytosol enzyme purified from rat liver or sheep liver. 2. When Mn(2+) replaced Mg(2+) in the reaction a lower Michaelis constant of 9x10(-6)m was found, but only with the mitochondrial enzymes. 3. With all enzymes malate at high concentration was a competitive inhibitor with respect to oxaloacetate when Mn(2+) was the added cation. With Mg(2+) the inhibition by malate was competitive with the mitochondrial enzymes and non-competitive with the cytosol enzymes.     

MODULATION OF γ-GLUTAMYL TRANSPEPTIDASE ACTIVITY FROM BOVINE CHOROID PLEXUS

Abstract— The catalytic activity of γ-glutamyl transpeptidase (γ-GTP) from bovine choroid plexus has been shown to be subject to modulation by a variety of effectors. L-Alanine and L-serine not only functioned as acceptor substrates to which γ-glutamyl moieties could be transferred, but also as noncom-petitive inhibitors of the reaction in the presence of the dipeptide acceptor substrate glycylglycine. In contrast, D-alanine does not function as an acceptor substrate, but does noncompetitively inhibit the transfer of γ-glutamyl groups to glycylglycine. Similarly, borate ions inhibited y-GTP noncompetitively, while a mixture of L-serine and borate were potent uncompetitive inhibitors of the reaction with a K _i of 0.6 mM. Several dicarboxylic acids, including maleate, maleylglycine, and malonate, inhibited γ-GTP; this inhibition was acceptor substrate-dependent. The inhibition of γ-GTP by maleate was competitive with respect to the acceptor substrate glycylglycine.     

Kinetic studies of the phenol sulphate-phenol sulphotransferase of Aspergillus oryzae.

Arylsulphatase II of Aspergillus oryzae exhibits both hydrolytic and sulphotransferase activities. The kinetic data suggest the formation of an intermediate covalent enzyme-sulphate complex with transfer of sulphate from donor to acceptor proceeding via a Ping Pong mechanism. The unusual kinetic behaviour when 2-hydroxy-5-nitrophenyl sulphate is the substrate is also consistent with this mechanism.     

A direct continuous spectrophotometric assay for transglutaminase activity

Herein we report the development of a direct and continuous spectrophotometric method for determining transglutaminase (TGase) activity by using N,N-dimethyl-1,4-phenylenediamine (DMPDA) as a gamma-glutamyl acceptor substrate and carbobenzyloxy-l-glutamylglycine (Z-Gln-Gly) as a typical peptide gamma-glutamyl donor substrate. The transamidation activity of TGase can thus be followed by monitoring the increase of absorbance of the resulting anilide product at 278 nm. The extinction coefficient of the authentic, independently synthesized anilide was determined to be epsilon = 8940 +/- 55 M(-1) cm(-1). Using this assay, we determined the apparent K(M) of DMPDA to be 0.25 mM, which compares favorably to the apparent K(M) values determined for other acceptor substrates under conditions where Z-Gln-Gly is also used as the donor substrate, such as N-acetyl-l-lysine methyl ester (9.6 mM) and methylamine (13.1 mM). Finally, the sensitivity of this assay technique was established through the measurement of irreversible inhibition constants for iodoacetamide, determined to be K(I) = 75 +/- 11 nM and k(inact) = (120 +/- 1) x 10(5) M(-1) min(-1).     

Partial purification and characterization of 3-hydroxy-3-methylglutaryl coenzyme a reductase from the leaves of guayule (Parthenium argentatum)

3-Hydroxy-3-methylglutaryl coenzyme A reductase has been isolated and was partially purified from the leaves of Parthenium argentatum. The enzyme was found to be associated both with the cytosol and the chloroplasts. Ten mM dithiothreitol was essential to prevent loss of activity. Optimum activities of cytosolic and chloroplastic fractions were observed at pH 7.0 and 7.5 respectively. Preincubation of the reaction mixtures with CoA, acetyl-CoA, σ-phenanthroline and iodoacetamide resulted in the progressive loss of enzyme activity. 3-Hydroxybutyrate and mevalonate also inhibited the enzyme. The Michaelis constants of the enzyme for HMG-CoA and NADPH were 0.25 and 0.31 mM respectively for the cytosolic enzyme, while those for the chloroplastic enzyme were 0.018 and 0.42 mM respectively. Inhibition studies indicated that hydroxybutyrate was a competitive inhibitor with respect to HMG-CoA. The inhibition of mevalonate was competitive with HMG-CoA and non-competitive with NADPH.     

Inhibitory effects in vitro of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-glutathione,a proposed metabolite of l-histidine,on γ-glutamyltransferase activity

A transfer of the γ-glutamyl moiety of S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]glutathione (I), an adduct of glutathione and l-histidine metabolite urocanic acid, has been investigated by using γ-glutamyltransferase preparation from bovine kidney. When an equimolar mixture of two diastereomers of compound I in a phosphate buffer was allowed to react with glycylglycine in the presence of the transferase, two diastereomers of N-{S-[2-carboxy-1-(1H-imidazol-4-yl)ethyl]-l-cysteinyl}glycine (II) were formed in the same yield with each other and this was accompanied by a formation of γ-glutamylglycylglycine. Kinetics of compound I with the transferase indicated high affinity between the two materials, while the maximal reaction velocity of the γ-glutamyl transfer was low. Effects of compound I in vitro on the transfer of γ-glutamyl moiety of γ-glutamyl-p-nitroanilide to glycylglycine with the transferase were also studied, and the results indicated that the transfer was suppressed by compound I based on its competitive and non-competitive inhibitions. These results suggest that little variation in reactivities of two diastereomers of compound I as the substrate is given by the difference in stereomerism of their asymmetric carbon atoms and that inhibitory effects of compound I on the catalytic action of the transferase is of sufficient physiological importance to decrease the degradation of natural γ-glutamyl compounds, such as glutathione and its analogs.     

Serine: glyoxylate, alanine:glyoxylate, and glutamate:glyoxylate aminotransferase reactions in peroxisomes from spinach leaves

Two different aminotransferases, that have glyoxylate as the amino acceptor, have specific activities of 1 to 2 mumol . min-1 . mg of protein-1 in the isolated peroxisomal fraction from spinach leaves. Their properties were evaluated after separation on a hydroxylapatite column. Both enzymes had a Km for glyoxylate of 0.15 mM and an amino acid Km of 2 to 3 mM. Reactions proceeded by a Ping Pong Bi Bi mechanism. Serine:glyoxylate aminotransferase was relatively specific for both substrates and could only be slightly reversed with 100 mM glycine, although the Ki of glycine was 33 mM. The glutamate:glyoxylate amino-transferase protein was equally active in catalyzing an alanine:glyoxylate aminotransferase reaction, but the reverse reactions with 100 mM glycine were hardly measureable, although the Ki (glycine) was 8.7 mM. Protection against hydroxylamine inhibition from reaction with pyridoxal phosphate was used to investigate the specificity of amino acid binding. Substrate amino acids protected at about the same concentration as their Km, while glycine protected at its Ki concentration. Thus, the nearly irreversible catalysis with glycine is not due to a failure to bind glycine. The significance of a peroxisomal alanine:glyoxylate aminotransferase activity has not been incorporated into schemes for the oxidative photosynthetic carbon cycle.     

Species differences in the inhibition of glutathione S-aryltransferase by phthaleins and dicarboxylic acids

1. Glutathione-S-aryltransferase activity from grass grubs (Costelytra zealandica) was inhibited by phthaleins, sulphonphthaleins and some dicarboxylic acids. 2. These compounds had no detectable action on the enzyme from sheep liver. 3. In insect enzyme the inhibition was competitive with respect to glutathione and non-competitive with respect to the aromatic substrate. 4. Michaelis constants and inhibitor constants were measured for sheep-liver or grass-grub enzyme between pH5 and pH10 and evidence was obtained for the presence of two groups with pK9.2 in the glutathione-binding site of the insect enzyme. 5. Only one such group was detected in the sheep-liver enzyme.     

Some kinetic properties of the tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase from Neurospora crassa.

The steady-state kinetic properties of purified tryptophan-sensitive 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase from Neurospora crassa were examined. The results suggest that the enzyme obeys a Rapid-Equilibrium Ordered mechanism, in which phosphoenolpyruvate is the first substrate to bind and 3-deoxy-D-arabino-heptulosonate 7-phosphate is the second product to be released, rather than a Ping Pong mechanism as has been reported previously. The inhibition by tryptophan was found to be parabolic competitive with respect to D-erythrose 4-phosphate and parabolic non-competitive with respect to phosphoenolpyruvate. The enzyme was inactivated by EDTA, and could be protected against this inactivation by phosphoenolpyruvate or 3-deoxy-D-arabino-heptulosonate 7-phosphate but not by D-erythrose 4-phosphate, tryptophan or Pi. This suggests that the enzyme may be a metalloenzyme.     

Purification and characteristics of a gamma-glutamyl kinase involved in Escherichia coli proline biosynthesis.

gamma-Glutamyl kinase, the first enzyme of the proline biosynthetic pathway, was purified to homogeneity from an Escherichia coli strain resistant to the proline analog 3,4-dehydroproline. The enzyme had a native molecular weight of 236,000 and was apparently comprised of six identical 40,000-dalton subunits. Enzymatic activity of the protein was detectable only in assays containing highly purified gamma-glutamyl phosphate reductase, the second enzyme of the proline pathway. Plots of gamma-glutamyl kinase activity as a function of glutamate concentration were sigmoidal, with a half-saturation value for glutamate of 33 mM, whereas plots of enzyme activity as a function of ATP concentration displayed typical Michaelis-Menten kinetics with a Km for ATP of 4 X 10(-4) M. Enzyme activity was insensitive to the glutamate analog L-methionine-DL-sulfoximine, but ADP was a potent competitive inhibitor. Characteristics of the enzyme were compared with those of a gamma-glutamyl kinase partially purified from a 3,4-dehydroproline-sensitive E. coli. These results indicated that the only major difference was that the enzyme from the 3,4-dehydroproline-resistant strain was 100-fold less sensitive to feedback inhibition by proline.     

Inhibition of human sperm acrosin by synthetic agents.

Twenty-two synthetic proteinase inhibitors were tested for their inhibitory properties towards human acrosin. p-Nitrophenyl-p1-guanidino benzoate (NPGB) was the most effective (K1 value of 1-5 X 10(-8) M), producing a non-competitive type of inhibition in contrast to all other inhibitors which showed a competitive type of inhibition. The Michaelis constant for human acrosin on BAEE at pH 8-1 was calculated to be 4-25 X 10(-5) M.     

Uptake and metabolism of glutamine in cultured kidney cells

The metabolism of glutamine was investigated in cultured rat kidney cells. Glutamine utilization and product formation were followed as a function of time at either 10 microM or 1 mM initial glutamine concentration. At 1 mM glutamine, glutamate and gamma-glutamylglutamate were the major products formed at the end of a 5-min incubation period; glutamate accounted for 46% while gamma-glutamylglutamate accounted for 33% of the glutamine utilized. With time, glutamate continued to accumulate while gamma-glutamyl peptide formation leveled off. The role of gamma-glutamyl transpeptidase was assessed by using hippurate, a physiological activator of gamma-glutamyl transpeptidase and acivicin, L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid, an inhibitor of gamma-glutamyl transpeptidase. Hippurate, 4 mM, increased the utilization of glutamine and the formation of glutamate, gamma-glutamyl peptides and ammonia. Exposure of cells to acivicin resulted in 98% inhibition of gamma-glutamyl transpeptidase without effecting phosphate-dependent glutaminase activity. Acivicin inhibition resulted in a decreased utilization of glutamine and product formation as compared to control; 5-oxoproline appearance fell 70%. The fractional distribution of glutamine carbon and nitrogen into its metabolic products in control, hippurate and acivicin-treated cells showed no change at the end of 60 min. The data provide evidence that gamma-glutamyl transpeptidase utilizes glutamine and forms gamma-glutamyl peptides in cultured kidney cells.     

Purification and properties of 4-methyleneglutaminase from the leaves of peanut (Arachis hypogaea)

The enzyme 4-methyleneglutaminase has been purified from Arachis hypogaea leaves. This enzyme also catalysed the deamidation of glutamine at 20% of the rate of 4-methyleneglutamine, exhibiting the same affinity for both substrates (K_m  20 mM), but was inactive with asparagine. The hydrolysis of 4-methyleneglutamine was subject to competitive inhibition by glutamine, glutamate-5-hydroxamate and phenol red and non-competitive inhibition by glutamate and 4-methyleneglutamate. The enzyme activity was insensitive to a variety of salts and carboxylic acids.     

A kinetic study of the alpha-keto acid dehydrogenase complexes from pig heart mitochondria.

The kinetic mechanisms of the 2-oxoglutarate and pyruvate dehydrogenease complexes from pig heart mitochondria were studied at pH 7.5 and 25 degrees. A three-site ping-pong mechanism for the actin of both complexes was proposed on the basis of the parallel lines obtained when 1/v was plotted against 2-oxoglutarate or pyruvate concentration for various levels of CoA and a level of NAD+ near its Michaelis constant value. Rate equations were derived from the proposed mechanism. Michaelis constants for the reactants of the 2-oxoglutarate dehydrogenase complex reaction are: 2-oxoglutarate, 0.220 mM; CoA, 0.025 mM; NAD+, 0.050 mM. Those of the pyruvate dehydrogenase complex are: pyruvate, 0.015 mM; CoA, 0.021 mM; NAD+, 0.079 mM. Product inhibition studies showed that succinyl-CoA or acetyl-CoA was competitive with respect to CoA, and NADH was competitive with respect to NAD+ in both overall reactions, and that succinyl-CoA or acetyl-CoA and NADH were uncompetitive with respect to 2-oxoglutarate or pyruvate, respectively. However, noncompetitive (rather than uncompetitive) inhibition patterns were observed for succinyl-CoA or acetyl-CoA versus NAD+ and for NADH versus CoA. These results are consistent with the proposed mechanisms.