The involvement of superoxide anions in the autoxidation of various cofactors of cysteamine-oxygenase

The reduction of tetranitroblue tetrazolium with cysteamine, mediated by a number of dyes, elemental sulphur, elemental selenium and selenide, under aerobic conditions, was inhibited to various extent upon addition of superoxide dismutase. A strict parallelism between the ability to produce O2- ions and the property of those compounds to act as cofactors for cysteamine-oxygenase, to yield hypotaurine, has been observed. Based on the fact that the autoxidation of cysteamine also gives rise to O2- formation, though to a minor extent, we propose a mechanism for cysteamine-oxygenase action. This mechanism was derived from the data obtained in the model system studied.     

Treatment of YAC128 mice and their wild-type littermates with cystamine does not lead to its accumulation in plasma or brain: implications for the treatment of Huntington disease

Cystamine is beneficial to Huntington disease (HD) transgenic mice. To elucidate the mechanism, cystamine metabolites were determined in brain and plasma of cystamine-treated mice. A major route for cystamine metabolism is thought to be: cystamine --> cysteamine --> hypotaurine --> taurine. Here we describe an HPLC system with coulometric detection that can rapidly measure underivatized cystamine, cysteamine and hypotaurine, as well as cysteine and glutathione in the same deproteinized tissue sample. A method is also described for the coulometric estimation of taurine as its isoindole-sulfonate derivative. Using this new methodology we showed that cystamine and cysteamine are undetectable (< or = 0.2 nmol/100 mg protein) in the brains of 3-month-old HD transgenic (YAC128) mice (or their wild-type littermates) treated daily for 2 weeks with cystamine (225 mg/kg) in their drinking water. No significant changes were observed in brain glutathione and taurine but significant increases were observed in brain cysteine. Cystamine and cysteamine were not detected in the plasma of YAC128 mice treated daily with cystamine between the ages of 4 and 12 or 7 and 12 months. These findings suggest that cystamine is not directly involved in mitigating HD but that increased brain cysteine or uncharacterized sulfur metabolites may be responsible.     

Inhibition of choline oxidase by quinoid dyes

Choline oxidase catalyzes the oxidation of choline to glycine-betaine, with betaine-aldehyde as intermediate and molecular oxygen as primary electron acceptor. This study reports on the inhibitory effects of triarylmethanes (cationic malachite green; neutral leukomalachite green), phenoxazines (cationic, meldola blue and nile blue; neutral nile red) and a structurally-related phenothiazine (methylene blue) on choline oxidase, assayed at 25 degrees C in 50 mM MOPS buffer, pH 7, using choline as substrate. Methylene B acted as a competitive inhibitor with K(i) = 74 +/- 7.2 microM, pointing to the choline-binding site of the enzyme as a target site. Nile B caused noncompetitive inhibition of enzyme activity with K(i) = 20 +/- 4.5 microM. In contrast to methylene B and nile B, malachite G and meldola B caused complex, nonlinear inhibition of choline oxidase, with estimated K(i) values in the micromolar range. The difference in kinetic pattern was ascribed to the differential ability of the dyes to interact (and interfere) with the flavin cofactor, generating different perturbations in the steady-state balance of the catalytic process.     

Methylene blue directly oxidizes glutathione without the intermediate formation of hydrogen peroxide

Methylene blue stimulates the oxidation of glutathione in red blood cells in vitro and in vivo. This oxidation has been attributed to hydrogen peroxide that is generated from the autooxidation of leucomethylene blue arising from the reduction of methylene blue by NADPH. In this report we present evidence that methylene blue directly oxidizes glutathione and that oxidation of glutathione by hydrogen peroxide is a secondary reaction. Moreover, superoxide dismutase has no effect on the oxidation. Under aerobic conditions, methylene blue oxidizes glutathione 30 times faster than the spontaneous autooxidation of glutathione. Under anaerobic conditions the stoichiometry of the reaction of methylene blue with glutathione supports a direct chemical reaction. The reaction rates between glutathione and methylene blue suggest a second order reaction over the conditions tested. That neither oxygen radical formation nor significant amounts of hydrogen peroxide are produced by methylene blue, even in the presence of added glucose, is further confirmed by the failure to detect significant amounts of lipid peroxidation products, or hemolysis, in red blood cells incubated with the dye.     

Microsomal mixed-function oxidase-dependent renaturation of reduced ribonuclease

A purified hepatic microsomal mixed-function drug oxidase (EC 1.14.13.8) catalyzes oxidation of cysteamine to cystamine. Since cysteamine is a normal intracellular metabolite, this reaction could provide an enzymic mechanism for the continuous generation of disulfides required for formation of disulfide bonds in newly synthesized proteins. This hypothesis was tested by studying the renaturation of reduced ribonuclease in media containing glutathione reductase, purified microsomal oxidase, an NADPH-generating system, and physiological concentrations of glutathione and cysteamine. Under these conditions renaturation of reduced-disorganized ribonuclease is completely dependent upon the microsomal oxidase, and optimal renaturation rates are obtained when the relative activities of glutathione reductase and cysteamine oxidase approximate levels present in whole liver homogenates.     

Pyrroloquinoline quinone (coenzyme PQQ) and the oxidation of SH residues in proteins.

Pyrroloquinoline quinone (PQQ) catalyzes the oxidation of cysteamine at neutral pH with a second order rate constant K2 = 0.45 M-1 s-1. The reduction of PQQ was monitored by absorption and fluorescence spectroscopy, whereas the oxidation of cysteamine to cystamine was followed by titration with 5,5'-dithiobis(2-nitrobenzoic acid). PQQ also catalyzes the oxidation of thiol groups critically connected with the function of two proteins, i.e. thioredoxin and phosphoribulose kinase. The reaction of PQQ with reduced thioredoxin brings about the oxidation of two thiol groups of the oxireductase, whereas the enzyme phosphoribulose kinase is inactivated at 25 degrees C. The oxidized disulfide bond of phosphoribulose kinase is reduced by dithiothreitol and the enzyme recovers catalytic activity. The ability of PQQ to catalyze the oxidation of vicinal cysteinyl residues to generate disulfide bonds under mild experimental conditions can be exploited to define the precise role of modified thiol residues in either catalysis or stabilization of protein structure.     

Inhibition of choline oxidase by quinoid dyes

Choline oxidase catalyzes the oxidation of choline to glycine-betaine, with betaine-aldehyde as intermediate and molecular oxygen as primary electron acceptor. This study reports on the inhibitory effects of triarylmethanes (cationic malachite green; neutral leukomalachite green), phenoxazines (cationic, meldola blue and nile blue; neutral nile red) and a structurally-related phenothiazine (methylene blue) on choline oxidase, assayed at 25°C in 50 mM MOPS buffer, pH 7, using choline as substrate. Methylene B acted as a competitive inhibitor with K_i = 74 ± 7.2 μM, pointing to the choline–binding site of the enzyme as a target site. Nile B caused noncompetitive inhibition of enzyme activity with K_i = 20 ± 4.5 μM. In contrast to methylene B and nile B, malachite G and meldola B caused complex, nonlinear inhibition of choline oxidase, with estimated K_i values in the micromolar range. The difference in kinetic pattern was ascribed to the differential ability of the dyes to interact (and interfere) with the flavin cofactor, generating different perturbations in the steady-state balance of the catalytic process.     

Chemical and spectral properties of carbon monoxide: methylene blue oxidoreductase. The molybdenum-containing iron-sulfur flavoprotein from Pseudomonas carboxydovorans

Carbon monoxide:methylene blue oxidoreductase, the key enzyme of CO-oxidation in energy metabolism of the carboxydobacterium Pseudomonas carboxydovorans, has been isolated in good yield and purity and found to contain FAD, molybdenum, iron, and labile sulfide in the ratio of 1:1:4:4. The enzyme is, therefore, a new molybdenum-containing iron-sulfur flavoprotein, exhibiting chemical and spectral properties quite similar to those of xanthine oxidase. Analytical data on the spectral characteristics of the enzyme in the oxidized and various reduced states are presented. Carbon monoxide:methylene blue oxidoreductase turned out to be photoreducible in the presence of EDTA and urea and was subject to reoxidation by air oxygen; no flavoprotein semiquinone was formed. Unphysiological electron acceptors, e.g. methylene blue, were used as oxidizing substrates whereas NAD or NADP turned out to be ineffective. Methylene blue reduction with CO was not affected by the presence of allopurinol, and carbon monoxide:methylene blue oxidoreductase was not able to catalyze the reduction of methylene blue with xanthine, adenine, or aldehydes. CO was the only reducing substrate used by the enzyme. Carbon monoxide:methylene blue oxidoreductase formed no sulfite adduct, and the reactivity with ferricyanide or cytochrome c was significant but slow. As known for other molybdenum hydroxylases, carbon monoxide:methylene blue oxidoreductase was rapidly inactivated by methanol, but the enzyme exhibited no ability to catalyze the oxidation of NADH with methylene blue, and NAD was not able to overcome methanol inhibition.     

Sulphur metabolism in Thiorhodaceae. II. stoichiometric relationship of CO2 fixation to oxidation of hydrogen sulphide and intracellular sulphur inChromatium okenii

Stoichiometry of sulphide and intracellular sulphur oxidation in connection with CO₂ fixation was studied inChromatium okenii. The equipment used was a special stirred cuvette with a rapid-sampling arrangement, which allowed short-time experiments with illuminated bacterial suspensions under anaerobic conditions. Turnover of the sulphur compounds is controlled by a linear CO₂ fixation rate which amounts to 0.069µmoles of CO₂/min mg of cell protein at light saturation. Van Niel's equations for bacterial photosynthesis could be confirmed for short periods under the condition that sulphate is produced during increase of intracellular sulphur; i.e., oxidation of sulphide and of intracellular sulphur do not occur consecutively but simultaneously. The full oxidation rate of intracellular sulphur starts after complete consumption of sulphide. The time during which sulphide is oxidized to intracellular sulphur amounts to 1/3–1/4 of the time necessary for the complete quantitative oxidation of the sulphide to sulphate.     

Gamma-glutamylcysteine synthetase. Interactions of an essential sulfhydryl group

gamma-Glutamylcysteine synthetase (isolated from rat kidney) has one sulfhydryl group that reacts with 5,5'-dithiobis-(2-nitrobenzoate). This single exposed sulfhydryl group is not required for enzyme activity. The enzyme is potently inactivated by cystamine, which apparently interacts with a sulfhydryl group at the active site to form a mixed disulfide. 5,5'-Dithiobis-(2-nitrobenzoate) does not interact with the sulfhydryl group that reacts with cystamine. After the enzyme was 90% inactivated by reaction with cystamine, 3.4 mol of 5,5'-dithiobis-(2-nitrobenzoate) reacted per mol of enzyme, indicating that binding of cystamine exposes sulfhydryl groups which are apparently buried or unreactive in the native enzyme. L-Glutamate (but not D-glutamate or L-alpha-aminobutyrate) protected against inactivation by cystamine. In contrast, ATP enhanced the rate of inactivation by cystamine, and the apparent Km value for this effect is similar to that for ATP in the catalytic reaction. Studies on the structural features of cystamine that facilitate its interaction with the enzyme showed that selenocystamine, monodansylcystamine, and N-[2[2-aminoethyl)-dithio)ethyl]-4-azido-2-nitrobenzeneamine are also good inhibitors. Whereas S-(S-methyl)cysteamine-Sepharose does not interact with the enzyme (Seelig, G. F., and Meister, A. (1982) J. Biol. Chem. 257, 5092-5096), S-(S-methyl)cysteamine is a potent inhibitor; 1 mol of this compound completely inactivated 1 mol of enzyme. In the course of this work, a useful modification of the method for isolating this enzyme from kidney was developed.     

Enzymatic and non-enzymatic conversion of cystamine to thiotaurine and taurine

The disulfide-containing molecule cystamine and the thiosulfonate thiotaurine are of interest as therapeutics. Both are precursors of taurine, but the chemistry of their metabolism is not clear. The rates at which these molecules are metabolized is also unknown. The chemistry and rate constants have been determined for a process in which cystamine is converted in four reactions to thiotaurine. Cystamine is oxidized by diamine oxidase with a specificity constant comparable to other diamine substrates. The rapid hydrogen peroxide-mediated oxidation of cystaldimine yields reactive glyoxal and thiocysteamine, which quickly performs transsulfuration with hypotaurine. Thiotaurine reacts spontaneously with hydrogen peroxide to form taurine and sulfite, but it is 15-fold less reactive than hypotaurine as an antioxidant. An estimation of biological rates of reaction indicates that cystamine is likely to be oxidized by diamine oxidase in vivo, but its metabolic products will be diverted to molecules other than thiotaurine.     

Discovery and characterization of a second mammalian thiol dioxygenase, cysteamine dioxygenase

There are only two known thiol dioxygenase activities in mammals, and they are ascribed to the enzymes cysteine dioxygenase (CDO) and cysteamine (2-aminoethanethiol) dioxygenase (ADO). Although many studies have been dedicated to CDO, resulting in the identification of its gene and even characterization of the tertiary structure of the protein, relatively little is known about cysteamine dioxygenase. The failure to identify the gene for this protein has significantly hampered our understanding of the metabolism of cysteamine, a product of the constitutive degradation of coenzyme A, and the synthesis of taurine, the final product of cysteamine oxidation and the second most abundant amino acid in mammalian tissues. In this study we identified a hypothetical murine protein homolog of CDO (hereafter called ADO) that is encoded by the gene Gm237 and belongs to the DUF1637 protein family. When expressed as a recombinant protein, ADO exhibited significant cysteamine dioxygenase activity in vitro. The reaction was highly specific for cysteamine; cysteine was not oxidized by the enzyme, and structurally related compounds were not competitive inhibitors of the reaction. When overexpressed in HepG2/C3A cells, ADO increased the production of hypotaurine from cysteamine. Similarly, when endogenous expression of the human ADO ortholog C10orf22 in HepG2/C3A cells was reduced by RNA-mediated interference, hypotaurine production decreased. Western blots of murine tissues with an antibody developed against ADO showed that the protein is ubiquitously expressed with the highest levels in brain, heart, and skeletal muscle. Overall, these data suggest that ADO is responsible for endogenous cysteamine dioxygenase activity.     

Reversible covalent enzyme immobilization methods for reuse of carriers

Reversible immobilization techniques which allow for multiple use of the carrier are relevant for applications, such as enzymatic microreactors, biosensors with specific setups and for expensive carriers such as superparamagnetic particles. The activity of immobilized enzymes reduces with time, so that the introduction of fresh immobilized enzyme becomes necessary. Thus, methods for reversible immobilization and multiple carrier reuse can help to reduce purchase costs and facilitate reactor construction. In this work, we present a method that makes use of the reduction and oxidation of cystamine, a cleavable linker with disulfide bond and amine functionality. For a proof of principle, α-chymotrypsin was immobilized on polyethylene glycol with terminal epoxy groups using cystamine as a crosslinker. The enzyme was highly active and could be used in repeated cycles. After the enzymatic reaction was demonstrated, α-chymotrypsin was cleaved off the particle by reducing agents. The resulting thiols on the particle surface were oxidized to disulfides by means of cysteamine, the reduction product of cystamine. This way, an almost complete oxidation of surface thiols with cysteamine was possible, restoring amine functionalization for further reactions. Reduction and oxidation were repeated several times without a decrease in the extent of amine coupling. Finally, immobilization of α-chymotrypsin could be repeated with results comparable to first run.     

Cooperative behavior in the thiol oxidation of rabbit muscle glycogen phosphorylase in cysteamine/cystamine redox buffers

Glycogen phosphorylase a and b are irreversibly inactivated by oxidation with the disulfide cystamine. The mechanism is complex and involves oxidation of at least two classes of sulfhydryl groups. The oxidation of one or more of the first class of 4 +/- 1 sulfhydryl groups is reversible, but the equilibrium constant for the oxidation is so unfavorable (1 X 10(-4)) that the micromolar concentrations of cysteamine released stoichiometrically with enzyme oxidation are sufficient to prevent complete oxidation even in the presence of 100 mM cystamine. The rapid phase of inactivation of phosphorylase b, which is first order in cystamine (k = 2.9 +/- 0.3 M-1 min-1), is followed by the oxidation of 5 +/- 1 groups in an irreversible process that is second order in cystamine concentration (k = 3.9 +/- M-2 min-1). Similar behavior is observed for phosphorylase a, although the behavior is complicated by association/dissociation equilibrium. The second-order dependence of the rate of irreversible inactivation on cystamine concentration is interpreted in terms of a "cooperative" model in which a rapidly reversible thermodynamically unfavorable equilibrium oxidation of one or more sulfhydryl groups must precede the irreversible oxidation of one or more additional sulfhydryl groups. The thiol/disulfide oxidation equilibrium constant for the initial reversible reaction is estimated to be at least 10(4) less favorable than that for the reversible oxidation of phosphofructokinase.     

Photooxidation of methionine with immobilized methylene blue as photooxidizer.

Methylene blue immobilized on porous glass beads was used to catalyze the photooxidation of methionine alone and the methionine residues of lysozyme. A solution of 2 mM methionine in 50% acetic acid was oxidized to methionine sulfoxide in the presence of immobilized methylene blue after 6 h of photooxidation at 37 degrees C. Selective photooxidation of the methionyl residues in lysozyme was achieved after 26 h of reaction in 84% acetic acid at 4 degrees C. The specific activity of lysozyme exposed to light in the presence of methylene blue decreased by 94%, while that of a lysozyme solution in the presence of methylene blue not exposed to light decreased by 21%. The lysozyme solution exposed to light but not containing the methylene blue beads lost 33% of its specific activity after the same period of photooxidation. It was shown that the decrease in enzyme activity was not caused by adsorption of the enzyme onto the beads.     

Photooxidation of methionine with immobilized methylene blue photooxidizer

Methylene blue immobilized on porous glass beads was used to catalyze the photooxidation of methionine alone and the methionine residues of lysozyme. A solution of 2 mM methionine in 50% acetic acid was oxidized to methionine sulfoxide in the presence of immobilized methylene blue after 6 h of photooxidation at 37°C. Selective photooxidation of the methionyl residues in lysozyme was achieved after 26 h of reaction in 84% acetic acid at 4°C. The specific activity of lysozyme exposed to light in the presence of methylene blue decreased by 94% while that of a lysozyme solution in the presence of methylene blue not exposed to light decreased by 21%. The lysozyme solution exposed to light but not containing the methylene blue beads lost 33% of its specific activity after the same period of photooxidation. It was shown that the decrease in enzyme activity was not caused by adsorption of the enzyme onto the beads.     

An improved sulphur assay applied to a problem of isethionate metabolism in squid axon and other nerves

An assay has ken developed for total sulphur which is based on a wet oxidation and measurement with a spectrophotofluorometer of light scattering by barium sulphate. The method has been adapted to the measurement of isethionate in squid nerve and blood, in other cephalopod nerve, and in the nerve tissue of other species including mammals. A correlation has been found between isethionate contents and the activity in the same tissues of one kind of DFP-hydrolysing enzyme, the highest levels of both being in squid nerve. Squid nerve also took up cysteine rapidly and metabolized it predominantly to hypotaurine but not to isethionate. We speculate that a hypotaurine derivative is a reserve form of isethionate, and that the so-called DFPase is involved in the release of hypotaurine and its metabolism to isethionate as needed.     

Microbial metabolism of quinoline and related compounds. VIII. Xanthine dehydrogenase from a quinoline utilizing Pseudomonas putida strain.

The xanthine dehydrogenase from Pseudomonas putida 86 was purified 68-fold to homogeneity with 47% recovery. SDS-polyacrylamide gel electrophoresis of the enzyme revealed two protein bands corresponding to an Mr of 87,000 and 52,000. The Mr of the native enzyme was calculated to 550,000 by gel chromatography. The enzyme contained 4 atoms of molybdenum, 16 atoms of iron, 16 atoms of acidlabile sulphur and 4 molecules of FAD. Due to the composition of the cofactors the xanthine dehydrogenase belongs to the class of molybdo-iron/sulphur-flavoproteins. Form A, an oxidation product of the molybdenum cofactor, was identified. Methanol and cyanide were effective inhibitors.     

Methylene blue competes with paraquat for reduction by flavo-enzymes resulting in decreased superoxide production in the presence of heme proteins

Methylene blue competes 100 to 600 times more effectively than paraquat for reduction by three different flavo-containing enzymes; xanthine oxidase, NADH cytochrome c reductase, and NADPH cytochrome c reductase. Paraquat and methylene blue both interact with deflavo xanthine oxidase, indicating that neither electron acceptor reacted at the FAD site of the enzyme where molecular oxygen is reduced to superoxide. As the paraquat radical also directly reduced acetylated cytochrome c the hemeprotein could not be utilized for measuring superoxide production in the presence of the herbicide. In the presence of cytochrome c the methylene blue caused a sharp decrease in both paraquat-induced superoxide and hydroxyl radical production.     

Oxidation of reduced sulphur compounds by intact cells of Thiobacillus acidophilus

Oxidation of reduced sulphur compounds by Thiobacillus acidophilus was studied with cell suspensions from heterotrophic and mixotrophic chemostat cultures. Maximum substrate-dependent oxygen uptake rates and affinities observed with cell suspensions from mixotrophic cultures were higher than with heterotrophically grown cells. ph Optima for oxidation of sulphur compounds fell within the pH range for growth (pH 2–5), except for sulphite oxidation (optimum at pH 5.5). During oxidation of sulphide by cell suspensions, intermediary sulphur was formed. Tetrathionate was formed as an intermediate during aerobic incubation with thiosulphate and trithionate. Whether or not sulphite is an inter-mediate during sulphur compound oxidation by T. acidophilus remains unclear. Experiments with anaerobic cell suspensions of T. acidophilus revealed that trithionate metabolism was initiated by a hydrolytic cleavage yielding thiosulphate and sulphate. A hydrolytic cleavage was also implicated in the metabolism of tetrathionate. After anaerobic incubation of T. acidophilus with tetrathionate, the substrate was completely converted to equimolar amounts of thiosulphate, sulphur and sulphate. Sulphide- and sulphite oxidation were partly inhibited by the protonophore uncouplers 2,4-dinitrophenol (DNP) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) and by the sulfhydryl-binding agent N-ethylmaleimide (NEM). Oxidation of elemental sulphur was completely inhibited by these compounds. Oxidation of thiosulphate, tetrathionate and trithionate was only slightly affected. The possible localization of the different enzyme systems involved in sulphur compound oxidation by T. acidophilus is discussed.