Molecular-kinetic parameters of thiamine enzymes and the mechanism of antivitamin action of hydroxythiamine in animal organisms]

The molecula-kinetic parameters (Km, Ki) of three thiamine enzymes, e. g. thiamine pyrophosphokinase (EC 2.7.6.2), pyruvate dehydrogenase (EC 1.2.4.1) and transketolase (EC 2.2.1.1) with respect to the effects of the thiamine antimetabolite hydroxythiamine in the whole animal organism have been compared. It has been shown that only the first two enzymes, which interact competitively with the vitamin, antivitamin or their pyrophosphate ethers, obey the kinetic parameters obtained for the purified enzymes in vitro. The anticoenzymic effect of hydroxythiamine pyrophosphate with respect to transketolase is not observed in vivo at maximal concentration of the anticoenzyme in tissues due to the absence of competitive interactions with thiamine pyrophosphate. The incorporation of the true and false coenzymes into transketolase occurs only during de novo transketolase synthesis (the apoform is absent in tissues, with the exception of erythrocytes) and proceeds slowly with a half-life time equal to 24--30 hrs. After a single injection of hydroxythiamine at a large dose (70--400 mg/kg) the maximal inhibition of the transketolase activity in tissues (liver, heart, kidney, muscle, spleen, lungs adrenal grands) manifests itself by the 48th--72nd hour, when the concentration of free hydroxythiamine and its pyrophosphate is minimal and the whole anticoenzyme is tightly bound to the protein, forming the false holoenzyme. The use of hydroxythiamine for inhibition of pyruvate dehydrogenase or transketolase in animal organism is discussed.     

Biosynthesis of thiamine triphosphate and identification of thiamine diphosphate-binding proteins in the rat liver hyaloplasm

The nature of the thiamine diphosphate binding proteins from rat liver hyaloplasm was studied. When [14C]thiamine was used as a marker, a [14C]thiamine diphosphate-containing electrophoretically homogeneous protein preparation was isolated from the liver soluble fraction and classified as transketolase. No other non-enzymatic proteins which bind thiamine diphosphate and can serve as substrates in the reaction of thiamine diphosphate synthesis in the hyaloplasm were found. It was shown that the phosphate group is transferred by rat liver thiamine diphosphate kinase to the free (but not to the protein-bound) thiamine diphosphate as it was believed earlier.     

Examination of donor substrate conversion in yeast transketolase

The cleavage of the donor substrate d-xylulose 5-phosphate by wild-type and H263A mutant yeast transketolase was studied using enzyme kinetics and circular dichroism spectroscopy. The enzymes are able to catalyze the cleavage of donor substrates, the first half-reaction, even in the absence of any acceptor substrate yielding d-glyceraldehyde 3-phosphate as measured in the coupled optical test according to Kochetov (Kochetov, G. A. (1982) Methods Enzymol. 90, 209-223) and compared with the H263A variant. Overall, the H263A mutant enzyme is less active than the wild-type. However, an increase in the rate constant of the release of the enzyme-bound glycolyl moiety was observed and related to a stabilization of the "active glycolaldehyde" (alpha-carbanion) by histidine 263. Chemically synthesized dl-(alpha,beta-dihydroxyethyl)thiamin diphosphate is bound to wild-type transketolase with an apparent K(D) of 4.3 +/- 0.8 microm (racemate) calculated from titration experiments using circular dichroism spectroscopy. Both enantiomers are cleaved by the enzyme at different rates. In contrast to the enzyme-generated alpha-carbanion of (alpha,beta-dihydroxyethyl)thiamin diphosphate formed by decarboxylation of hydroxylactylthiamin diphosphate after incubation of transketolase with beta-hydroxypyruvate, the synthesized dl-(alpha,beta-dihydroxyethyl)thiamin diphosphate did not work as donor substrate when erythrose 4-phosphate is used as acceptor substrate in the coupled enzymatic test according to Sprenger (Sprenger, G. A., Sch?rken, U., Sprenger, G., and Sahm, H. (1995) Eur. J. Biochem. 230, 525-532).     

Thiamine diphosphate pool and its biosynthesis in the liver in galactosamine hepatitis

The hepatitis-like changes were induced in the liver of albino female rats weighing 120-150 g and fed on the appropriate vivarium diet by single parenteral administration of hydrochloride galactosamine in a dose of 0.9 or 1.8 mmol per 1 kg of body weight. The thiamine diphosphate level in the cytosol fraction of the liver decreased 24 h after the preparation administration, the same in blood but with the higher dose used. The activity of pyruvate dehydrogenase, a thiamine diphosphate dependent enzyme, decreased similarly. The cytosol transketolase activity lowered by 38-39%. The coenzyme biosynthesis disturbance due to a fall by 49-58% in the thiamine pyrophosphatase activity is considered to be responsible for hydrochloride galactosamine-induced decrease in the thiamine diphosphate pool. Specificity of the thiamine diphosphate pool disturbance and discoordination of thiamine diphosphate dependent enzymes in the liver are observed under administration of hydrochloride galactosamine.     

Reduced Stability of Rat Brain Transketolase After Conversion to the Apo Form

The stability of rat brain transketolase, whether measured at 37 or 0°C, was reduced after conversion to the apo form by removal of thiamine diphosphate, as shown by a decline in the activity recovered when assayed in the presence of thiamine diphosphate. Both the shape of the breakdown curve and the failure to recover the full activity, even after incubation with thiamine diphosphate, showed that the breakdown of the apotrans-ketolase was complex. The initial rate of breakdown of the apoenzyme was sharply pH dependent, being minimal at 37°C at a pH value of 7.6, close to that likely to exist in vivo. The rate rose sharply with deviation of the pH in either direction. The stability of the enzyme on storage at 0°C showed a similar pattern of pH dependence, provided that allowance is made for temperature effects on dissociation constants. These findings provide further support for the hypothesis that differences in brain transketolase may play a part in the etiology of Wernicke-Korsakoff's syndrome.     

The binding of thiamine pyrophosphate with transketolase in equilibrium conditions

The binding between thiamine pyrophosphate and transketolase, purified from baker's yeast, in equilibrium conditions has been studied. In the presence of Ca²⁺, the enzyme molecule has been shown to possess two binding sites for the coenzyme, whose dissociation constants are 3.2 × 10^?8 and 2.5 × 10^?7M; besides, there are site(s) where the binding of the coenzyme is less firm. In the presence of Mg²⁺, a positive cooperative interaction between the binding sites of thiamine pyrophosphate has been observed. Regardless of the cation used, the major part of the catalytic activity of the transketolase molecule manifests itself in the binding of one molecule of the coenzyme.     

Difference in thiamine metabolism between extensor digitorum longus and soleus muscles

1. Thiamine diphosphate level was higher in soleus muscle than in extensor digitorum longus muscle in various animals, whereas thiamine triphosphate level was less in the former muscle than in the latter except for mouse. 2. 2-Oxoglutarate dehydrogenase, transketolase and thiamine pyrophosphokinase activities were higher in soleus muscle than in extensor digitorum longus in rat and guinea pig. 3. The differences between rat two muscle phenotypes in thiamine diphosphate, but not thiamine triphosphate, level and the thiamine-related enzyme activities disappeared after denervation. 4. Tenotomy had little effect on thiamine phosphate levels and the thiamine-related enzyme activities in rat skeletal muscles.     

Optimization of hydroxythiamine administration regimen in vitaminological research

A correlation between the different doses and inhibitory effect of hydroxythiamine relative to the activity of main thiamine pyrophosphate-dependent enzymes is studied in experiments on animals. It is established that the maximal inhibitory effect on the transketolase and oxoglutarate dehydrogenase activities is at a dose of antivitamin of 0.35 mmol/kg and that of pyruvate dehydrogenase of 0.55 mmol/kg. At lower doses of hydroxythiamine the inhibition of enzyme activities occurred in a dose-dependent manner.     

Isolation and properties of noncovalent complex of transketolase with RNA

A method for isolation of homogenous transketolase from baker's yeast using immunoaffinity chromatography was significantly simplified. It was demonstrated that transketolase could be isolated from fresh yeast in the form of a complex with a high molecular weight RNA. Storage of yeast led to the dissociation of the complex to a low molecular weight complex and then to the free enzyme. Conditions were chosen for complex dissociation and free enzyme isolation. In comparison to the free enzyme, the specific activities of the high and low molecular weight complexes were decreased 20-25- and 3-5.5-fold, respectively. The affinity to the cofactor thiamine diphosphate and to xylulose-5-phosphate (donor substrate) did not change for the low molecular weight complex, while the time of binding to calcium increased. The latter was necessary for the complete manifestation of the enzymatic activity. Changes in the circular dichroism spectrum between 300 and 360 nm after the addition of thiamine diphosphate, which characterize the formation of the catalytically active holoenzyme, were significantly lower for the low molecular weight complex than for the free enzyme.     

Paraquat and menadione exposure of rainbow trout (Oncorhynchus mykiss)--studies of effects on the pentose-phosphate shunt and thiamine levels in liver and kidney

Possible xenobiotic interactions with thiamine were studied in salmonid fish, by repeatedly injecting two model substances, paraquat and menadione, into juvenile rainbow trout (Oncorhynchus mykiss). These two substances were chosen because of their well-known ability to redox-cycle and cause depletion of NADPH in several biological systems. Depletion of NADPH increases metabolism through the pentose-phosphate shunt and may thereby increase the need for thiamine diphosphate by heightened transketolase activity. A special food was produced with lower thiamine content than commercial food, usually enriched with thiamine, which could mask an effect on the thiamine level. After 9 weeks of exposure, glucose-6-phosphate dehydrogenase, transketolase, glutathione reductase and ethoxyresorufin O-deethylase were analysed in liver and kidney cellular sub-fractions as well as analysis of total thiamine concentrations in liver, kidney and muscle. The results showed that paraquat caused a large increase in hepatic glutathione reductase activity and induced hepatic glucose-6-phosphate dehydrogenase activity, i.e., the rate-limiting enzyme in the oxidative part of the pentose-phosphate shunt. Despite this paraquat exposure did not affect transketolase activity and total thiamine concentration.     

Certain characteristics of the rat liver transketolase

Modification of the method for transketolase purification in the rat liver was used to reveal the existence of two molecular forms of this enzyme. One of the forms does not react to development of avitaminosis in animals caused by oxythiamine in different doses. The method is developed for isolation of the basic transketolase form in the rat liver with the 50-80% yield of activity. Km values of two sites for coenzyme binding on protein do not depend on the extent of holoenzyme reconstruction from apoenzyme.     

Isolation of transketolase from rabbit liver and comparison of some of its kinetic properties with transketolase from other sources

1. Rabbit liver transketolase activity was purified 56-fold using the following steps: ammonium sulfate precipitation, chromatography on DEAE-Sephadex A-25, concentration through an Amicon ultrafiltration cell and rechromatography on DEAE-Sephadex A-25. 2. The enzyme showed an optimum PH for activity at 7.8-8.0. 3. The optimum temperature was around 40 degrees C and the activation energy calculated from the Arrhenius plot was found to be 11.4 kcal/mole. 4. The molecular weight of the enzyme, as determined by gel filtration, was found to be approximately 162,000, while the content of thiamin diphosphate was between 1.8 and 2 mumole per mole protein. 5. Addition of thiamin diphosphate and magnesium chloride did not influence the activity. 6. From the kinetic studies of the enzyme, the Km values for xylulose-5-phosphate, ribose-5-phosphate and fructose-6-phosphate were 3.8 x 10(-5) M, 9.5 x 10(-5) M and 1.1 x 10(-2) M, respectively.     

Tumor and body interrelationships in 14C-thiamine utilization

Variations in the content of 14C-thiamine, thiamine-dependent enzyme transketolase (TK), and thiamine diphosphate (TDP) in mouse tissues were studied during Ehrlich's ascites tumour growth. The concentration of TDP in the liver of tumour-bearing animals continuously drops during 10 days after inoculation, TDP level in the tumour itself decreases more abruptly by the terminal period of tumour growth (the 10th day). At the same time the tumour demonstrates sharp deficiency of the coenzyme and redundance of apoenzymatic forms of TK. The growing deficiency of thiamine in the tumour favours greater tension of thiamine metabolism and appearance of competitive tumour-host interrelationships that are most pronounced in the presence of hypovitaminosis.     

Three-dimensional structure of transketolase, a thiamine diphosphate dependent enzyme, at 2.5 A resolution.

The crystal structure of Saccharomyces cerevisiae transketolase, a thiamine diphosphate dependent enzyme, has been determined to 2.5 A resolution. The enzyme is a dimer with the active sites located at the interface between the two identical subunits. The cofactor, vitamin B1 derived thiamine diphosphate, is bound at the interface between the two subunits. The enzyme subunit is built up of three domains of the alpha/beta type. The diphosphate moiety of thiamine diphosphate is bound to the enzyme at the carboxyl end of the parallel beta-sheet of the N-terminal domain and interacts with the protein through a Ca2+ ion. The thiazolium ring interacts with residues from both subunits, whereas the pyrimidine ring is buried in a hydrophobic pocket of the enzyme, formed by the loops at the carboxyl end of the beta-sheet in the middle domain in the second subunit. The structure analysis identifies amino acids critical for cofactor binding and provides mechanistic insights into thiamine catalysis.     

Effect of coenzyme modification on the structural and catalytic properties of wild-type transketolase and of the variant E418A from Saccharomyces cerevisiae

Transketolase from baker's yeast is a thiamin diphosphate-dependent enzyme in sugar metabolism that reconstitutes with various analogues of the coenzyme. The methylated analogues (4'-methylamino-thiamin diphosphate and N1'-methylated thiamin diphosphate) of the native cofactor were used to investigate the function of the aminopyrimidine moiety of the coenzyme in transketolase catalysis. For the wild-type transketolase complex with the 4'-methylamino analogue, no electron density was found for the methyl group in the X-ray structure, whereas in the complex with the N1'-methylated coenzyme the entire aminopyrimidine ring was disordered. This indicates a high flexibility of the respective parts of the enzyme-bound thiamin diphosphate analogues. In the E418A variant of transketolase reconstituted with N1'-methylated thiamin diphosphate, the electron density of the analogue was well defined and showed the typical V-conformation found in the wild-type holoenzyme [Lindqvist Y, Schneider G, Ermler U, Sundstrom M (1992) EMBO J11, 2373-2379]. The near-UV CD spectrum of the variant E418A reconstituted with N1'-methylated thiamin diphosphate was identical to that of the wild-type holoenzyme, while the CD spectrum of the variant combined with the unmodified cofactor did not overlap with that of the native protein. The activation of the analogues was measured by the H/D-exchange at C2. Methylation at the N1' position of the cofactor activated the enzyme-bound cofactor analogue (as shown by a fast H/D-exchange rate constant). The absorbance changes in the course of substrate turnover of the different complexes investigated (transient kinetics) revealed the stability of the alpha-carbanion/enamine as the key intermediate in cofactor action to be dependent on the functionality of the 4-aminopyrimidine moiety of thiamin diphosphate.     

Purification and characterization of, and preparation of an antibody to, transketolase from human red blood cells

Transketolase (sedoheptulose-7-phosphate: D-glyceraldehyde-3-phosphate glycolaldehydetransferase, EC 2.2.1.1) was purified 16 000-fold from human red blood cells, using DEAE-Sephadex A-50, Sephadex G-150, FPLC on Mono P, and Sephadex G-100. The purified enzyme migrated as a single protein band on SDS-polyacrylamide gel electrophoresis. The FPLC step resolved transketolase into three peaks, designated I, II and III. From results of re-FPLC on Mono P, SDS-polyacrylamide gel electrophoresis, gel filtration, catalytic studies, amino acid analysis and immunological studies, it was concluded that I, II and III were originally the same protein, modified during storage and purification. Transketolase had a subunit (Mr 70 000) and appeared to be composed of two identical subunits. 1 mol of subunit contained 0.9 mol of thiamine pyrophosphate. The pH optimum of the reaction lay within the range 7.6-8.0, and the Km values were determined to be 1.5 X 10(-4) M for xylulose 5-phosphate and 4.0 X 10(-4) M for ribose 5-phosphate. Hg2+ and p-chloromercuribenzoate inhibited the enzyme reaction, and the inhibition of the latter disappeared upon the addition of cysteine. Thiamine and its phosphate esters did not, but cysteine (1 X 10(-2) M) and ethanol (10% and 1% v/v) did activate the enzyme reaction. Antibody prepared to II bound all forms of transketolase in the hemolysate, but inhibited the reaction only about 20%.     

Thiamine intestinal transport and phosphorylation : a study in vitro of potential inhibitors of small intestinal thiamine-pyrophosphokinase using a crude enzymatic preparation.

Using as enzymatic source the cytoplasmatic fraction of enterocytes isolated from the rat small intestine, thiamine-pyrophosphokinase activity was studied with a radiometric method using [thiazole-2-(14)C] thiamine. The Km value for thiamine was 2.14 X 10(-6) M and V 0.87 nmol of thiamine pyrophosphate mg-1 protein h-1. Eleven thiamine structural analogs and derivatives were assayed for their inhibitory action on the small intestine thiamine-pyrophosphokinase activity. Their Ki values were : pyrithiamine, 2.25 X 10(-6) M; thiamine monophosphate, 4 X 10(-6) M; 2'-ethylthiamine, 8 X 10(-6) M; 2'-butylthiamine, 6 X 10(-6) M; chloroethylthiamine and dimethalium, 1.5 X 10(-5) M; amprolium, 1.8 X 10(-4) M; L-582571, 1.65 X 10(-4) M; oxythiamine, 4.2 X 10(-3) M. Of the miscellaneous compounds tested (toxopyrimidine, Na-pyrophosphate, choline, L-phenylalanine, ethyl-urethane and 5-fluorouracil), none had any inhibitory action on intestinal thiamine-pyrophosphokinase activity, even if used at concentrations hundred times higher than that of labelled thiamine.     

Isolation and properties of human transketolase

Recombinant human (His)₆-transketolase (hTK) was obtained in preparative amounts by heterologous expression of the gene encoding human transketolase in Escherichia coli cells. The enzyme, isolated in the form of a holoenzyme, was homogeneous by SDS-PAGE; a method for obtaining the apoenzyme was also developed. The amount of active transketolase in the isolated protein preparation was correlated with the content of thiamine diphosphate (ThDP) determined in the same preparation. Induced optical activity, facilitating studies of ThDP binding by the apoenzyme and measurement of the transketolase reaction at each stage, was detected by circular dichroism spectroscopy. A single-substrate reaction was characterized, catalyzed by hTK in the presence of the donor substrate and in the absence of the acceptor substrate. The values of the Michaelis constant were determined for ThDP and a pair of physiological substrates of the enzyme (xylulose 5-phosphate and ribose 5-phosphate).     

Postnatal development of thiamine metabolism in rat skeletal muscle.

1. The activities of 2-oxoglutarate dehydrogenase, transketolase, thiamine pyrophosphokinase and thiamine triphosphatase and the concentrations of thiamine phosphates were almost the same between rat extensor digitorum longus and soleus muscles at 2 weeks of age. 2. These enzyme activities changed after 3 weeks of age in a different way depending on the muscle phenotype. 3. Thiamine diphosphate level and the activity of 2-oxoglutarate dehydrogenase increased only in soleus muscle and thiamine triphosphate level increased only in extensor digitorum longus during development.     

Cyclid 3':5'-nucleotide phosphodiesterase. Interconvertible multiple forms and their effects on enzyme activity and kinetics.

An extract of rat liver or human platelet displayed three cyclic 3':5'-nucleotide phosphodiesterase activity peaks (I, II, and III) in a continuous sucrose density gradient when assayed with millimolar adenosine 3':5'-monophosphate (cAMP) or guanosine 3':5'-monophosphate (cGMP). The three fractions obtained from each nucleotide were not superimposable. The molecular weights corresponding to the three activity peaks of cAMP phosphodiesterase in rat liver were approximately: I, 22,000; II, 75,000; and III, 140,000. In both tissues, fraction I was barely detectable when assayed with micromolar concentrations of either nucleotide, presumably because fraction I has low affinity for cAMP and cGMP. Any one of the three forms upon recentrifugation on the gradient generated the others, indicating that they were interconvertible. The multiple forms appear to represent different aggregated states of the enzyme. The ratio of the three forms of cAMP phosphodiesterase in the platelet was shifted by dibutyryl cAMP (B2cAMP) and by the enzyme concentration. B2cAMP enhanced the formation of fraction I. Low enzyme concentration favored the equilibrium towards fraction I, while high enzyme concentration favored fraction III. When phosphodiesterase activities in the extract of rat liver, human platelets, or bovine brain were examined as a function of enzyme concentration, rectilinear rates were observed with micromolar, but not with millimolar cAMP or cGMP. The specific activity with millimolar cAMP was higher with low than with high protein concentrations, suggesting that the dissociated form catalyzed the hydrolysis of cAMP faster than that of the associated form. In contrast, the specific activity with millimolar cGMP was lower with low than with high protein concentrations. Supplementing the reaction mixture with bovine serum albumin to a final constant protein concentration did not affect the activity, suggesting that the concentration of the enzyme rather than that of extraneous proteins affected the enzyme activity. A change in enzyme concentration affected the kinetic properties of phosphodiesterase. A low enzyme concentration of cAMP phosphodiesterase yielded a linear Lineweaver-Burk plot, and a Km of 1.2 X 10(-4) M (bovine), 3 X 10(-5) M (platelet), or 5 X 10(-4) M (liver), while a high enzyme concentration yielded a nonlinear plot, and apparent Km values of 1.4 X 10(-4) M and 2 X 10(-5) M (brain), 4 X 10(-5) M and 3 X 10(-6) M (platelet), or 4 X 10(-5) M and 3 X 10(-6) (liver). Since a low enzyme concentration favored fraction I, the dissociated form, whereas a high enzyme concentration favored fraction III, the associated form, these kinetic constants suggest that the dissociated form exhibits a high Km and the associated form exhibits a low Km. In contrast, a high enzyme concentration gave a linear kinetic plot for cGMP phosphodiesterase, while a low enzyme concentration gave a nonlinear plot...