Nucleoside-triphosphatase and hydrolysis of thiamin triphosphate in Escherichia coli

[PROPERTIES OF CHICKEN LIVER MEMBRANE-ASSOCIATED THIAMINE TRIPHOSPHATASE]

The enzymes involved in thiamine triphosphate (ThTP) metabolism in birds are not characterized so far. The aim of the present work was to study some properties of ThTPase in chicken liver. In liver homogenate, ThTPase activity has been found to display a bell-like pH-profile with a maximum of 5.5-6.0. Low activity was observed without divalent metal ions, while the addition of Mg2+ or Ca2+, each at 5 mM concentration, enhanced the rate of ThTP hydrolysis by a factor of 17-20. In the presence of 5 mM Mg2+ an apparent K(m) of the enzyme for ThTP was estimated by the method of non-linear regression as well as from the Hanes plot to be 1.7-2.2 mM. Monovalent anions such as I-, SCN-, NO3-, Br-, Cl- (at 150 mM concentration) showed inhibitory effect decreasing the rate of ThTPase reaction by 20-60%. After the homogenate was centrifuged, more than 85% of ThTPase activity was revealed in the fraction of insoluble particles indicating a membrane localization of the enzyme. The precipitate treatment with 1% sodium deoxycholate caused about 53% solubilization of the activity. During Toyopeal HW-55 chromatography, ThTPase activity was eluted simultaneously with ATPase and ITPase peaks in the void volume of the column. Thus, a non-specific high molecular mass protein complex seems to be involved in ThTP hydrolysis in the chicken liver. The chicken liver phosphatase is clearly distinguishable from all membrane-bound ThTPases reported previously.

Purification and characteristics of functional properties of soluble nucleoside triphosphatase (apyrase) from bovine brain

Soluble NTPase, differing in its properties from known proteins exhibiting NTPase activity, was purified from bovine brain to homogeneity. The enzyme has pH optimum at 7.5 and shows absolute dependence on bivalent cations and broad substrate specificity towards nucleoside-5 -tri- and -diphosphates, characteristics of apyrases. The NTPase follows Michaelis-Menten kinetics in the range of investigated substrate concentrations, the apparent K(m) values for UTP, ITP, GTP, CTP, CDP, and ATP being 86, 25, 41, 150, 500, and 260 microM, respectively. According to gel-filtration and SDS-PAGE data, the molecular mass of the enzyme is 60 kD. The NTPase is localized in the cytosol fraction and expressed in different bovine organs and tissues. Total NTPase activity of extracts of bovine organs and tissues decreases in the following order: liver > heart > skeletal muscle > lung > brain > spleen > kidney ~ small intestine. The enzyme activity can be regulated by acetyl-CoA, alpha-ketoglutarate, and fructose-1,6-diphosphate acting as activators in physiological concentrations, whereas propionate exhibits an inhibitory effect.

Pig tissues express a catalytically inefficient 25-kDa thiamine triphosphatase: insight in the catalytic mechanisms of this enzyme

Thiamine triphosphate (ThTP) is found in most organisms and may be an intracellular signal molecule produced in response to stress. We have recently cloned the cDNA coding for a highly specific mammalian 25-kDa thiamine triphosphatase. The enzyme was active in all mammalian species studied except pig, although the corresponding mRNA was present. In order to determine whether the very low ThTPase activity in pig tissues is due to the absence of the protein or to a lack of catalytic efficiency, we expressed human and pig ThTPase in E. coli as GST fusion proteins. The purified recombinant pig GST-ThTPase was found to be 2-3 orders of magnitude less active than human GST-ThTPase. Using site-directed mutagenesis, we show that, in particular, the change of Glu85 to lysine is responsible for decreased solubility and catalytic activity of the pig enzyme. Immunohistochemical studies revealed a distribution of the protein in pig brain very similar to the one reported in rodent brain. Thus, our results suggest that a 25-kDa protein homologous to hThTPase but practically devoid of enzyme activity is expressed in pig tissues. This raises the possibility that this protein may play a physiological role other than ThTP hydrolysis.

Human recombinant thiamine triphosphatase: purification, secondary structure and catalytic properties

Thiamine triphosphate (ThTP) is found in most living organisms and it may act as a phosphate donor for protein phosphorylation. We have recently cloned the cDNA coding for a highly specific mammalian 25 kDa thiamine triphosphatase (ThTPase; EC 3.6.1.28). As the enzyme has a high catalytic efficiency and no sequence homology with known phosphohydrolases, it was worth investigating its structure and catalytic properties. For this purpose, we expressed the untagged recombinant human ThTPase (hThTPase) in E. coli, produced the protein on a large scale and purified it to homogeneity. Its kinetic properties were similar to those of the genuine human enzyme, indicating that the recombinant hThTPase is completely functional. Mg2+ ions were required for activity and Ca2+ inhibited the enzyme by competition with Mg2+. With ATP as substrate, the catalytic efficiency was 10(-4)-fold lower than with ThTP, confirming the nearly absolute specificity of the 25 kDa ThTPase for ThTP. The activity was maximum at pH 8.5 and very low at pH 6.0. Zn2+ ions were inhibitory at micromolar concentrations at pH 8.0 but activated at pH 6.0. Kinetic analysis suggests an activator site for Mg2+ and a separate regulatory site for Zn2+. The effects of group-specific reagents such as Woodward's reagent K and diethylpyrocarbonate suggest that at least one carboxyl group in the active site is essential for catalysis, while a positively charged amino group may be involved in substrate binding. The secondary structure of the enzyme, as determined by Fourier-transform infrared spectroscopy, was predominantly beta-sheet and alpha-helix.

Solubilization of thiamine triphosphatase from the electric organ of Electrophorus electricus

The membrane-associated, anion-regulated thiamine triphosphatase from Electrophorus electricus electric organ can be solubilized by various neutral detergents. Polyoxyethylene ethers are the most effective. Anionic detergents readily inactivate the enzyme. A 6.4-fold increase in specific activity is obtained by successive treatment of crude membranes with octanoyl-N-methylglucamide, which solubilized other proteins, and Lubrol-PX with releases 60% of the thiamine triphosphatase (TTPase) activity. Solubilization by Lubrol-PX rapidly modifies kinetic parameters. The Km, Vmax and pH optimum are decreased. However, the solubilized TTPase may be kept at 0 degrees C for many hours without further change in specific activity. At 35 degrees C, the half-life is still 83 min at pH 5.0, but denaturation becomes rapid at pH greater than or equal to 7. Solubilization modifies anion effects on TTPase activity. The activating effect of nitrate is nearly lost, while inhibition by sulfate is no longer time-dependent.

High affinity of acid phosphatase encoded by PHO3 gene in Saccharomyces cerevisiae for thiamin phosphates

The enzymatic properties of acid phosphatase (orthophosphoric-monoester phosphohydrolase, EC 3.1.3.2) encoded by PHO3 gene in Saccharomyces cerevisiae, which is repressed by thiamin and has thiamin-binding activity at pH 5.0, were investigated to study physiological functions. The following results led to the conclusion that thiamin-repressible acid phosphatase physiologically catalyzes the hydrolysis of thiamin phosphates in the periplasmic space of S. cerevisiae, thus participating in utilization of the thiamin moiety of the phosphates by yeast cells: (a) thiamin-repressible acid phosphatase showed Km values of 1.6 and 1.7 microM at pH 5.0 for thiamin monophosphate and thiamin pyrophosphate, respectively. These Km values were 2-3 orders of magnitude lower than those (0.61 and 1.7 mM) for p-nitrophenyl phosphate; (b) thiamin exerted remarkable competitive inhibition in the hydrolysis of thiamin monophosphate (Ki 2.2 microM at pH 5.0), whereas the activity for p-nitrophenyl phosphate was slightly affected by thiamin; (c) the inhibitory effect of inorganic phosphate, which does not repress the thiamin-repressible enzyme, on the hydrolysis of thiamin monophosphate was much smaller than that of p-nitrophenyl phosphate. Moreover, the modification of thiamin-repressible acid phosphatase of S. cerevisiae with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide resulted in the complete loss of thiamin-binding activity and the Km value of the modified enzyme for thiamin monophosphate increased nearly to the value of the native enzyme for p-nitrophenyl phosphate. These results also indicate that the high affinity of the thiamin-repressible acid phosphatase for thiamin phosphates is due to the thiamin-binding properties of this enzyme.