Properties of an Escherichia coli rhodanese

A rhodanese enzyme of less than 20,000 molecular weight has been purified from Escherichia coli. The enzyme is accessible to substrates upon addition of whole cells to standard assay mixtures. This rhodanese has a Stokes radius of 17 A which for a globular protein corresponds to a molecular weight close to 14,000. It undergoes autoxidation to a polymeric form which is probably an inert dimer. Enzyme inactivated by oxidation can be reactivated by millimolar concentrations of cysteine. Steady-state initial velocity measurements indicate that the enzyme catalyzes the transfer of sulfane sulfur by way of a double displacement mechanism with formation of a covalent enzyme-sulfur intermediate. The turnover number for the enzyme-catalyzed reaction, with thiosulfate as donor substrate and cyanide ion as the sulfur acceptor, is 260 s-1. This value corresponds to a catalytic efficiency 60% of that measured for a previously characterized bovine liver enzyme of more than twice the molecular weight. Furthermore, KmCN is 24 mM which is 2 orders of magnitude higher than the value observed previously for the bovine enzyme. Evidence from chemical inactivation studies implicates an essential sulfhydryl group in the enzyme activity. It is proposed that this group is the site of substrate-sulfur binding in the obligatory enzyme-sulfur intermediate. Furthermore, a cationic site important for binding of the donor thiosulfate is tentatively identified from anion inhibition studies. Tests of alternate acceptor substrates indicate that the physiological dithiol, dihydrolipoate, is a more efficient acceptor than cyanide ion for the enzyme-bound sulfur. Of possibly greater physiological significance, it has been found that the enzyme catalyzes the formation of iron-sulfur centers. Other work indicates the E. coli rhodanese is subject to catabolite repression and suggests a physiological role for the enzyme in aerobic energy metabolism.     

Studies on the efficacy of lipoate and dihydrolipoate in the alteration of cadmium2+ toxicity in isolated hepatocytes

Lipoate (thioctic acid) is presently used in therapy of a variety of diseases such as liver and neurological disorders. However, nothing is known about the efficacy of lipoate and its reduced form dihydrolipoate in acute cadmium (Cd2+) toxicity which involves severe liver disturbances. Therefore, we investigated the effects of these redox compounds on Cd2(+)-induced injuries in isolated rat hepatocytes. The cells were coincubated with 150 microM Cd2+ and either 1.5-6.0 mM lipoate or 17-89 microM dihydrolipoate for up to 90 min and Cd2+ uptake as well as viability criteria were monitored. Both exposure regimens diminished Cd2+ uptake in correspondence to time and concentration. They also ameliorated Cd2(+)-induced cell deterioration as reflected by the decrease in Cd2(+)-induced membrane damage (leakage of aspartate aminotransferase), by the lessening of the Cd2(+)-stimulated lipid peroxidation (TBA-reactants) and by the increase in Cd2(+)-depleted cellular glutathione (GSH + 2 GSSG). Half-maximal protection was achieved at molar ratios of 9.9 to 19 (lipoate vs. Cd2+) and 0.25 to 0.74 (dihydrolipoate vs. Cd2+), indicating a 19.5 to 50.6 lower protective efficacy of lipoate as compared to dihydrolipoate. Lipoate induced an increase in extracellular acid-soluble thiols different from glutathione. It is suggested that dihydrolipoate primarily protects cells by extracellular chelation of Cd2+, whereas intracellular reduction of lipoate to the dihydro-compound followed by complexation of both intra- and extracellular Cd2+ contributes to the amelioration provided by lipoate.     

Transfer of persulfide sulfur from thiocystine to rhodanese.

THiocystine (bis-[2-amino-2-carboxyethyl]trisulfide) is a natural substrate for rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1). Analogs of thiocystine were prepared by eliminating the carboxyl or amino group or by lengthening the carbon chain. Of these only homothiocystine (bis-[2-amino-2-carboxypropyl]trisulfide) had appreciable activity as a substrate. At pH 8.6, the optimum for rhodanese, transfer of sulfane sulfur to cyanide in the presence of rhodanese was nonspecific. Only the sulfane sulfur of 35S-labeled thiocystine was transferred to rhodanese. Thus, thiocystine and thiosulfate both produce a rhodanese persulfide as a stable intermediate in sulfur transfer.     

Common themes and variations in the rhodanese superfamily

The rhodanese homology domain is a ubiquitous fold found in several phylogenetically related proteins encoded by eubacterial, archeal, and eukaryotic genomes. Although rhodanese-like proteins share evolutionary relationships, analysis of their sequences highlights that they are so heterogeneous to form the rhodanese superfamily. The variability occurs at different levels including sequence, active site loop length, presence of a critical catalytic Cys residue, and domain arrangement. Even within the same genome, multiple genes encode rhodanese-like proteins presenting with variably arranged rhodanese domain(s): as single or tandem domain(s), or combined with other protein domain(s). Given the highly variable organization of the rhodanese domain(s) and the context where it is found, here we review the structural organization and function of the rhodanese-like proteins. The overview of the most recent findings about rhodanese allow us to depict a superfamily of versatile proteins relying on persulfide chemistry to accomplish cellular functions spanning from resistance to environmental threats, such as cyanide, and key cellular reactions related to sulfur metabolism and progression of cell cycle.     

Properties of a soluble thiosulfate sulfur transferase (rhodanese) of the marine methanogen Methanosarcina frisia

Abstract: Cell-free extracts of Methanosarcina frisia contain high thiosulfate sulfur transferase (TST) (rhodanese), slight thiosulfate reductase but no thiosulfate: acceptor oxidoreductase activity. Neither adenylylsulfate reductase nor sulfite: acceptor oxidoreductase activity could be detected. TST is an acidic protein with an M _r of 25 000 and was enriched by ion-exchange chromatography and gel filtration. The enzyme has a temperature optimum at 60°C and a pH optimum at pH 11. The K _m values for thiosulfate and cyanide are 0.53 mM and 1.57 mM, respectively. Low concentrations of cysteine, glutathione, dithioerythritol, and dihydrolipoate increase the activity of the enzyme while unphysiological concentrations of these effectors cause a decrease. Sulfite and N -bromosuccinimide inhibit the energy activity extremely.     

Rhodanese-Mediated sulfur transfer to succinate dehydrogenase.

The interaction of the sulfurtransferase rhodanese (EC 2.8.1.1) with succinate dehydrogenase (EC 1.3.99.1), yeast alcohol dehydrogenase (EC 1.1.1.1) and bovine serum albumin was studied. Succinate dehydrogenase incorporates the sulfane sulfur of [35S]rhodanese and, in the presence of unlabelled rhodanese, also incorporates that of [35S]thiosulfate. Rhodanese releases most of its transferable sulfur and is re-loaded in the presence of thiosulfate. Rhodanese undergoes similar modifications with yeast alcohol dehydrogenase but this latter does not bind 35S in amounts comparable to those incorporated in succinate dehydrogenase: nearly all the 35S released by [35S]rhodanese is with low-molecular-weight compounds. Bovine serum albumin also binds very little sulfur and [35S]rhodanese present in the reaction mixture does not discharge its radioactive sulfur nor does it take up sulfur from thiosulfate. Sulfur release from rhodanese appears to depend on the presence of - SH groups in the acceptor protein. Sulfur incorporated into succinate dehydrogenase was analytically determined as sulfide. A comparison of the optical spectra of succinate dehydrogenase preparations incubated with or without rhodanese indicates that there is an effect of the sulfurtransferase on the iron-sulfur absorption of the flavorprotein. The interaction of rhodanese with succinate dehydrogenase greatly decreases the catalytic activity of rhodanese with respect to thiocyanate formation. This is attributed to modifications in rhodanese associated with the reduction of sulfane sulfur to sulfide. Thiosulfate in part protects from this deactivation. The reconstitutive capacity of succinate dehydrogenase increased in parallel with sulfur incorporated in that enzyme following its interaction with rhodanese.     

Identification of putative sulfurtransferase genes in the extremophilic Acidithiobacillus ferrooxidans ATCC 23270 genome: structural and functional characterization of the proteins

Eight nucleotide sequences containing a single rhodanese domain were found in the Acidithiobacillus ferrooxidans ATCC 23270 genome: p11, p14, p14.3, p15, p16, p16.2, p21, and p28. Amino acids sequence comparisons allowed us to identify the potentially catalytic Cys residues and other highly conserved rhodanese family features in all eight proteins. The genomic contexts of some of the rhodanese-like genes and the determination of their expression at the mRNA level by using macroarrays suggested their implication in sulfur oxidation and metabolism, formation of Fe-S clusters or detoxification mechanisms. Several of the putative rhodanese genes were successfully isolated, cloned and overexpressed in E. coli and their thiosulfate:cyanide sulfurtransferase (TST) and 3-mercaptopyruvate/cyanide sulfurtransferase (MST) activities were determined. Based on their sulfurtransferase activities and on structural comparisons of catalytic sites and electrostatic potentials between homology- modeled A. ferrooxidans rhodaneses and the reported crystal structures of E. coli GlpE (TST) and SseA (MST) proteins, two of the rhodanese-like proteins (P15 and P16.2) could clearly be defined as TSTs, and P14 and P16 could possibly correspond to MSTs. Nevertheless, several of the eight A. ferrooxidans rhodanese-like proteins may have some different functional activities yet to be discovered.     

Functional sulfurtransferase is associated with mitochondrial complex I from Yarrowia lipolytica, but is not required for assembly of its iron-sulfur clusters

Here, we report that in the obligate aerobic yeast Yarrowia lipolytica, a protein exhibiting rhodanese (thiosulfate:cyanide sulfurtransferase) activity is associated with proton pumping NADH:ubiquinone oxidoreductase (complex I). Complex I is a key enzyme of the mitochondrial respiratory chain that contains eight iron-sulfur clusters. From a rhodanese deletion strain, we purified functional complex I that lacked the additional protein but was fully assembled and displayed no functional defects or changes in EPR signature. In contrast to previous suggestions, this indicated that the sulfurtransferase associated with Y. lipolytica complex I is not required for assembly of its iron-sulfur clusters.     

Increased catalytic performance of the 2-oxoacid dehydrogenase complexes in the presence of thioredoxin, a thiol–disulfide oxidoreductase

Bacterial and mammalian pyruvate and 2-oxoglutarate dehydrogenase complexes undergo an irreversible inactivation upon accumulation of the dihydrolipoate intermediate. The first component of the complexes, 2-oxoacid dehydrogenase, is affected. Addition of thioredoxin protects from this inactivation, increasing catalytic rates and limiting degrees of the substrate transformation to products, acyl-CoA and NADH. Although the redox active cysteines of thioredoxin are essential for its interplay with the complexes, the effects are observed with both dithiol and disulfide forms of the protein. This indicates that thioredoxin affects an SH/S–S component of the system, which is present in the two redox states. The complex-bound lipoate is concluded to be the thioredoxin target, since (i) both dithiol and disulfide forms of the residue are available during the catalytic cycle and (ii) the thioredoxin reaction with the essential SH/S–S group of the terminal component of the complex, dihydrolipoyl dehydrogenase, is excluded. Thus, the thioredoxin disulfide interacts with the dihydrolipoate intermediate, while the thioredoxin dithiol reacts with the lipoate disulfide. Kinetic consequences of such interplay are consistent with the observed thioredoxin effects. Owing to the essential reactivity of the SH/S–S couple in thioredoxin, the thiol–disulfide exchange between thioredoxin and the lipoate residue is easy reversible, providing both protection (by the mixed disulfide formation) and catalysis (by the appropriate lipoate release). In contrast, non-protein SH/S–S compounds prevent the inactivatory action of dihydrolipoate intermediate only at a high excess over the complex-bound lipoate. This interferes with the catalysis-required release of the residue from its mixed disulfide. Therefore, only thioredoxin is capable to ‘buffer' the steady-state concentration of the reactive dithiol. Such action represents a new thioredoxin function, which may be exploited to protect other enzymes with exposed redox-active thiol intermediates.     

2-ketoacid dehydrogenase complexes of Escherichia coli: stereospecificities of the three components for (R)-lipoate

Stereospecificities of component enzymes in the pyruvate dehydrogenase complex and 2-ketoglutarate dehydrogenase complex from Escherichia coli for lipoate and dihydrolipoate are determined. Assays of the component enzymes using R,S-, R-, or S-lipoate or the enantiomers of dihydrolipoate show that only the R-enantiomers are substrates for these enzymes. Nonenzymatic reactions involving acetyl group transfer and coupled electron and acetyl group transfer between enantiomeric molecules of lipoate or/and dihydrolipoate proceed at significant rates. Coupled acetyl group and electron transfer from enzyme-bound acetyldihydrolipoyl moieties to free lipoate is also observed. The S-enantiomers are neither substrates nor inhibitors; however, products of S-enantiomers are slowly generated in enzymatic reactions owing to nonenzymatic reactions between enzyme-bound acetyldihydrolipoyl-groups and free S-lipoate or S-dihydrolipoate.     

Plant mercaptopyruvate sulfurtransferases: molecular cloning, subcellular localization and enzymatic activities.

Mercaptopyruvate sulfurtransferase (MST, EC 2.8.1.2) and thiosulfate sulfurtransferase (TST, rhodanese, EC 2.8.1.1) are evolutionarily related enzymes that catalyze the transfer of sulfur ions from mercaptopyruvate and thiosulfate, respectively, to cyanide ions. We have isolated and characterized two cDNAs, AtMST1 and AtMST2, that are Arabidopsis homologs of TST and MST from other organisms. Deduced amino-acid sequences showed similarity to each other, although MST1 has a N-terminal extension of 57 amino acids containing a targeting sequence. MST1 and MST2 are located in mitochondria and cytoplasm, respectively, as shown by immunoblot analysis of subcellular fractions and by green fluorescent protein (GFP) analysis. However, some regions of MST1 fused to GFP were found to target not only mitochondria, but also chloroplasts, suggesting that the regions on the targeting sequence recognized by protein import systems of mitochondria and chloroplasts are not identical. Recombinant proteins, expressed in Escherichia coli, exhibited MST/TST activity ratios determined from kcat/Km values of 11 and 26 for MST1 and MST2, respectively. This indicates that the proteins encoded by both AtMST1 and AtMST2 are MST rather than TST type. One of the hypotheses proposed so far for the physiological function of MST and TST concerns iron-sulfur cluster assembly. In order to address this possibility, a T-DNA insertion Arabidopsis mutant, in which the AtMST1 was disrupted, was isolated by PCR screening of T-DNA mutant libraries. However, the mutation had no effect on levels of iron-sulfur enzyme activities, suggesting that MST1 is not directly involved in iron-sulfur cluster assembly.     

Surface changes and role of buried water molecules during the sulfane sulfur transfer in rhodanese from Azotobacter vinelandii: a fluorescence quenching and nuclear magnetic relaxation dispersion spectroscopic study

The Azotobacter vinelandii rhodanese is a sulfurtransferase enzyme that catalyzes the transfer of the outer sulfur atom from thiosulfate to cyanide. Recently, investigations by NMR relaxation on the (15)N-enriched protein reported that interdomain contacts are rigidly maintained upon the sulfane sulfur transfer from the enzyme to the substrate. The modality of the enzymatic mechanism is then confined to a surface interaction, including dynamics of water molecules buried in the tertiary structure. Thus, investigations have been carried out by fluorescence, circular dichroism, and nuclear magnetic relaxation dispersion measurements. The comparison of circular dichroism spectra of the persulfurated enzyme and the sulfur-free form indicated that small changes occur. Fluorescence quenching studies have been performed to evaluate the conformational changes during catalysis using the fluorescent probe 8-anilinonaphthalene-2-sulfonic acid, and acrylamide, iodide, and cesium ions as quenchers. Changes in exchange dynamics of water molecules buried in the structure with bulk water, observed by nuclear magnetic relaxation dispersion, are due to local conformational transitions, likely involving residues around the active site, and are consistent with the global correlation time found by (15)N relaxation. These results, taken together, provide important information for elucidating the conformational features of the mechanism of action of the enzyme either in the role of a selective donor of a sulfur atom to small-sized substrates (i.e., to cyanide, transforming it into thiocyanate) or in the role of sulfur insertase for the formation of the Fe(2)S(2) iron-sulfur cluster in sulfur-deprived ferredoxins.     

Rhodanese from Thiobacillus A2: catalysis of reactions of thiosulphate with dihydrolipoate and dihydrolipoamide.

Rhodanese (thiosulphate:cyanide sulphurtransferase EC.2.8.1.1) was purified 25- to 30-fold from thiosulphate-grown Thiobacillus A2. It exhibited a pH optimum between pH 10-2 and 10-4 and apparent Km values of 0-36 mM-Na2S2O3 and 17 mM-KCN. Ultraviolet spectrophotometry and thin-layer chromatography showed that the enzyme catalysed the reaction of S2O3(2-) with dihydrolipoic acid or dihydrolipoamide, producing alpha-lipoate or lipoamide, with the intermediate production of the persulphides of dihydrolipoate and dihydrolipoamide, which were demonstrated chromatographically. This is the first demonstration of catalysis by a thiobacillus rhodanese of reactions which are likely to be physiologically important in the oxidative dissimilation of thiosulphate by a central energy-conserving pathway.     

Reducing sugars can induce the oxidative inactivation of rhodanese.

The enzyme rhodanese (thiosulfate sulfurtransferase, EC 2.8.1.1) is inactivated on incubation with reducing sugars such as glucose, mannose, or fructose, but is stable with non-reducing sugars or related polyhydroxy compounds. The enzyme is inactivated with (ES) or without (E) the transferable sulfur atom, although E is considerably more sensitive, and inactivation is accentuated by cyanide. Inactivation of E is accompanied by increased proteolytic susceptibility, a decreased sulfhydryl titer, a red-shift and quenching of the protein fluorescence, and the appearance of hydrophobic surfaces. Superoxide dismutase and/or catalase protect rhodanese. Inactive enzyme can be partially reactivated during assay and almost completely reactivated by incubation with thiosulfate, lauryl maltoside, and 2-mercaptoethanol. These results are similar to those observed when rhodanese is inactivated by hydrogen peroxide. These observations, as well as the cyanide-dependent, oxidative inactivation by phenylglyoxal, are explained by invoking the formation of reactive oxygen species such as superoxide or hydrogen peroxide from autooxidation of alpha-hydroxy carbonyl compounds, which can be facilitated by cyanide.     

Mitochondrial rhodanese: membrane-bound and complexed activity

We have proposed that phosphorylated and dephosphorylated forms of the mitochondrial sulfurtransferase, rhodanese, function as converter enzymes that interact with membrane-bound iron-sulfur centers of the electron transport chain to modulate the rate of mitochondrial respiration (Ogata, K., Dai, X., and Volini, M. (1989) J. Biol. Chem. 204, 2718-2725). In the present studies, we have explored some structural aspects of the mitochondrial rhodanese system. By sequential extraction of lysed mitochondria with phosphate buffer and phosphate buffer containing 20 mM cholate, we have shown that 30% of the rhodanese activity of bovine liver is membrane-bound. Resolution of cholate extracts on Sephadex G-100 indicates that part of the bound rhodanese is complexed with other mitochondrial proteins. Tests with the complex show that it forms iron-sulfur centers when incubated with the rhodanese sulfur-donor substrate thiosulfate, iron ions, and a reducing agent. Experiments on the rhodanese activity of rat liver mitochondria give similar results. Taken together, the findings indicate that liver rhodanese is in part bound to the mitochondrial membrane as a component of a multiprotein complex that forms iron-sulfur centers. The findings are consistent with the role we propose for rhodanese in the modulation of mitochondrial respiratory activity.     

Rhodanese-thioredoxin system and allyl sulfur compounds

Sodium 2-propenyl thiosulfate, a water-soluble organo-sulfane sulfur compound isolated from garlic, induces apoptosis in a number of cancer cells. The molecular mechanism of action of sodium 2-propenyl thiosulfate has not been completely clarified. In this work we investigated, by in vivo and in vitro experiments, the effects of this compound on the expression and activity of rhodanese. Rhodanese is a protein belonging to a family of enzymes widely present in all phyla and reputed to play a number of distinct biological roles, such as cyanide detoxification, regeneration of iron-sulfur clusters and metabolism of sulfur sulfane compounds. The cytotoxic effects of sodium 2-propenyl thiosulfate on HuT 78 cells were evaluated by flow cytometry and DNA fragmentation and by monitoring the progressive formation of mobile lipids by NMR spectroscopy. Sodium 2-propenyl thiosulfate was also found to induce inhibition of the sulfurtransferase activity in tumor cells. Interestingly, in vitro experiments using fluorescence spectroscopy, kinetic studies and MS analysis showed that sodium 2-propenyl thiosulfate was able to bind the sulfur-free form of the rhodanese, inhibiting its thiosulfate:cyanide-sulfurtransferase activity by thiolation of the catalytic cysteine. The activity of the enzyme was restored by thioredoxin in a concentration-dependent and time-dependent manner. Our results suggest an important involvement of the essential thioredoxin-thioredoxin reductase system in cancer cell cytotoxicity by organo-sulfane sulfur compounds and highlight the correlation between apoptosis induced by these compounds and the damage to the mitochondrial enzymes involved in the repair of the Fe-S cluster and in the detoxification system.     

Latest news about the sulfurtransferase protein family of higher plants

Sulfurtransferases/rhodaneses (Str) comprise a group of enzymes widely distributed in all phyla which catalyze in vitro the transfer of a sulfur atom from suitable sulfur donors to nucleophilic sulfur acceptors. The best characterized Str is bovine rhodanese (EC 2.8.1.1) which catalyses in vitro the transfer of a sulfane sulfur atom from thiosulfate to cyanide, leading to the formation of sulfite and thiocyanate. Plants as well as other organisms contain many proteins carrying a typical rhodanese pattern or domain forming multi-protein families (MPF). Despite the presence of Str activities in many living organisms, the physiological role of the members of this MPF has not been established unambiguously. While in mammals these proteins are involved in the elimination of toxic cyanogenic compounds, their ubiquity suggests additional physiological functions. In plants, Str are localized in the cytoplasm, mitochondria, plastids, and nucleus. Str probably also transfer reduced sulfur onto substrates as large as peptides or proteins. Several studies in different organisms demonstrate a protein–protein interaction with members of the thioredoxin MPF indicating a role of Str in maintenance of the cellular redox homeostasis. The increased expression of several members of the Str MPF in various stress conditions could be a response to oxidative stress. In summary, data indicate that Str are involved in various essential metabolic reactions.     

The differential functional stability of various forms of bovine liver rhodanese

Bovine liver rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) was prepared in dilute solutions and subjected to conditions that led to a time-dependent loss of enzyme activity. The rate of this activity loss was found to be dependent upon the sulfur substitution state of the enzyme, and the presence or absence of the substrates, thiosulfate and cyanide. In the absence of excess substrates, free enzyme (E), and the covalent intermediate form of the enzyme bearing a divalent sulfur atom in the active site (ES), are of approximately equal functional stability. In comparison, E, in the presence of excess cyanide, was markedly more labile, while ES, supported by 10-50 mM thiosulfate, showed no significant loss of activity under any of the conditions tested. All the enzyme solutions were shown to be losing assayable protein from solution. However, it was demonstrated that, for rhodanese in the E form, the amount of protein lost was insufficient to account for the activity lost, and a marked decline in specific activity was observed. Enzyme in the ES form, whether supported by additional thiosulfate or not, did not decline in the specific activity, though comparable protein loss did occur from these solutions. Intrinsic fluorescence measurements of rhodanese in the ES form, before and after removal of the persulfide sulfur through the addition of cyanide, indicated that loss of enzymic activity was not accompanied by loss of the bound sulfur atom. Therefore, the stabilizing effect observed with thiosulfate could not be explained simply by its ability to maintain enzyme in the sulfur-substituted state. Since the concentration of thiosulfate employed in these experiments was insufficient to maintain all the enzyme in ES.S2O3 form, thiosulfate was acting as a chemical reagent rather than a substrate in stabilizing enzyme activity.     

The inactivation of rhodanese by nitrite and inhibition by other anions in vitro

Cyanide detoxification in mammals occurs, in part, by sulfur transfer by rhodanese to form the less toxic thiocyanate. Thiosulfate and nitrite are often used in combination for the treatment of cyanide intoxication. This report shows that nitrite can inhibit the rate of sulfur transfer by rhodanese in vitro. Nitrate, chloride, sulfate, and acetate were also examined as inhibitors. Inhibition by nitrite appeared to be more complex than for the other anions tested. Closer examination showed that nitrite can inactivate the sulfur-free rhodanese. Our observation leads to the suggestion that, in vivo, either rhodanese is maintained in its more stable sulfur-substituted form or cellular compartmentalization prevents inactivation by nitrite.     

Enzymatic detoxification of cyanide: clues from Pseudomonas aeruginosa Rhodanese

Cyanide is a dreaded chemical because of its toxic properties. Although cyanide acts as a general metabolic inhibitor, it is synthesized, excreted and metabolized by hundreds of organisms, including bacteria, algae, fungi, plants, and insects, as a mean to avoid predation or competition. Several cyanide compounds are also produced by industrial activities, resulting in serious environmental pollution. Bioremediation has been exploited as a possible alternative to chemical detoxification of cyanide compounds, and various microbial systems allowing cyanide degradation have been described. Enzymatic pathways involving hydrolytic, oxidative, reductive, and substitution/transfer reactions are implicated in detoxification of cyanide by bacteria and fungi. Amongst enzymes involved in transfer reactions, rhodanese catalyzes sulfane sulfur transfer from thiosulfate to cyanide, leading to the formation of the less toxic thiocyanate. Mitochondrial rhodanese has been associated with protection of aerobic respiration from cyanide poisoning. Here, the biochemical and physiological properties of microbial sulfurtransferases are reviewed in the light of the importance of rhodanese in cyanide detoxification by the cyanogenic bacterium Pseudomonas aeruginosa. Critical issues limiting the application of a rhodanese-based cellular system to cyanide bioremediation are also discussed.