Cyanide Detoxification Research Articles

Detoxification of Cyanide in a Gold Processing Plant Tailings Water Using Calcium and Sodium Hypochlorite

Detoxification of Cyanide in a Gold Processing Plant Tailings Water Using Calcium and Sodium Hypochlorite

Detoxification of cyanide by plants and hormone action.

In higher plants cyanide is a co-product of ethylene synthesis. The increase in ethylene production that occurs during the senescence of certain flowers and the ripening of climacteric fruit is accompanied by a rise in beta-cyanoalanine synthase activity. Although these events correlate temporally and spatially, the potential for cyanide detoxification in these tissues is high compared with the expected rate of cyanide formation from the ethylene pathway. However, in stigmas and styles of Petunia flowers a semi-quantitative relationship exists between the activity of beta-cyanoalanine synthase and the activity of ACC (1-aminocyclopropane-1-carboxylic acid) oxidase, the last enzyme in the ethylene pathway. To account for these observations it is proposed that ACC oxidase can react with other amino acids by a general mechanism that liberates cyanide. This hypothesis could also account for the substrate stereospecificity of ACC oxidase, for the extreme lability of this enzyme and for the high accumulation of asparagine in some tissues.

Mammalian cyanide detoxification with sulphane sulphur.

It has been understood for nearly a century that most of the cyanide entering mammals by either ingestion or inhalation of sublethal quantities is detoxified by combination with cyanide-reactive sulphur. The detoxifying reaction is primarily enzymic, and the enzymes that catalyse it, as well as those that catalyse reactions which generate the cyanide-reactive sulphane sulphur, have been isolated and studied extensively. However, analytical methods suitable for quantitative characterization of the physiological pool of sulphur available for this detoxification have only recently come under development. A new method is based on the evaluation of cyanide depletion under controlled, mild reaction conditions in the presence of a sulphurtransferase and cofactor. The use of differential pulse polarography with nested internal standards makes this a practical procedure for samples containing as little as a few nanomoles of cyanide-reactive material. As measured by this method, the sulphane sulphur concentration in mammalian liver tissue is in the fractional millimolar range, with even less in the bloodstream. Nevertheless, the specific binding of sulphane sulphur by serum albumin, and the rapid cyanolysis of the albumin-bound sulphur, may yet indicate a significant role for the sulphurtransferase capability of serum albumin in cyanide detoxification.

Nutritional and biochemical factors influencing the biological effects of cyanide.

Investigation of the dietary interaction between cyanide and selenium in the chick, whereby cyanide alleviates selenium toxicity, suggests that cyanide alters metabolic reductive potential. Cyanide enhances the elimination of selenium as dimethyl selenide, the formation of which requires both reducing equivalents and methyl groups. Even when the methionine supply is adequate, meeting the need for the methyl groups, the interaction can be lost if there is a deficiency of certain micronutrients or an excess of vitamin K. Cyanide reduces liver glycogen, implying greater emphasis on anaerobic metabolism through inhibition of cytochrome oxidase. This may increase reductive potential but may also result in increased free radical production, processes that can be modified by levels of micronutrients. There is no evidence that an excess of sulphur amino acids can markedly enhance cyanide detoxification, although, for reasons that are not yet clear, cystine may be beneficial. However, the balance of dietary amino acids may be more critical than had been realized, because an excess of alanine appears to exacerbate cyanide toxicity.

Molecular cloning of Brassica rapa nitrilases and their expression during clubroot development

SUMMARY Three isoforms of nitrilase were cloned from turnip, Brassica rapa L., and their expression during clubroot development caused by Plasmodiophora brassicae was investigated. The isoforms were designated BrNIT-T1, BrNIT-T2 and BrNIT-T4 based on homology to known nitrilases. BrNIT-T1 and BrNIT-T2 have 80% homology to three nitrilases from Arabidopsis thaliana (AtNIT1, AtNIT2 and AtNIT3). BrNIT-T4 showed 90% homology to AtNIT4. To confirm their enzyme activity, the recombinant proteins were expressed in Escherichia coli. The recombinant BrNIT-T1 and BrNIT-T2 but not BrNIT-T4 converted indole-3-acetonitrile to indole-3-acetic acid, an endogenous plant auxin, although kinetic analysis showed that indole-3-acetonitrile is a poor substrate compared with various aliphatic and aromatic nitriles. By contrast, the recombinant BrNIT-T4 specifically converted beta-cyano-l-alanine to aspartic acid and asparagine and these findings agree with the idea that it is involved in the cyanide detoxification pathway. Real-time PCR analysis clearly showed that these isoforms were differentially expressed during clubroot development. BrNIT-T1 transcripts were very low in non-infected roots but were enhanced up to 100-fold in infected roots exhibiting club growth. By contrast, BrNIT-T2 transcripts remained at a very low level during clubroot formation. All these results clearly indicate the specific involvement of BrNIT-T1 in clubroot formation. The BrNIT-T4 transcripts were substantially reduced in the clubroot-growing phase, but thereafter they increased rapidly to a level found in non-infected roots as the clubroot growth reached a plateau. These findings suggest the specific involvement of BrNIT-T4 in clubroot maturation. In fully developed clubs, the BrNIT-T1 and BrNIT-T2 transcripts also increased. Free indole-3-acetic acid (IAA) content increased in the early and the latest phase of infected roots compared with non-infected roots, but decreased substantially at the middle phase. Thus, free IAA may play a role in the initiation and maturation of clubroot. Total IAA content was significantly higher in infected roots than in non-infected roots throughout clubroot development and IAA conjugation/conjugate hydrolysis system as well as BrNIT-Ts appear to be involved in clubroot development.

Expression of MdCAS1 and MdCAS2, encoding apple β-cyanoalanine synthase homologs, is concomitantly induced during ripening and implicates MdCASs in the possible role of the cyanide detoxification in Fuji apple (Malus domestica Borkh.) fruits

Fruit ripening involves complex biochemical and physiological changes. Ethylene is an essential hormone for the ripening of climacteric fruits. In the process of ethylene biosynthesis, cyanide (HCN), an extremely toxic compound, is produced as a co-product. Thus, most cyanide produced during fruit ripening should be detoxified rapidly by fruit cells. In higher plants, the key enzyme involved in the detoxification of HCN is beta-cyanoalanine synthase (beta-CAS). As little is known about the molecular function of beta-CAS genes in climacteric fruits, we identified two homologous genes, MdCAS1 and MdCAS2, encoding Fuji apple beta-CAS homologs. The structural features of the predicted polypeptides as well as an in vitro enzyme activity assay with bacterially expressed recombinant proteins indicated that MdCAS1 and MdCAS2 may indeed function as beta-CAS isozymes in apple fruits. RNA gel-blot studies revealed that both MdCAS1 and MdCAS2 mRNAs were coordinately induced during the ripening process of apple fruits in an expression pattern comparable with that of ACC oxidase and ethylene production. The MdCAS genes were also activated effectively by exogenous ethylene treatment and mechanical wounding. Thus, it seems like that, in ripening apple fruits, expression of MdCAS1 and MdCAS2 genes is intimately correlated with a climacteric ethylene production and ACC oxidase activity. In addition, beta-CAS enzyme activity was also enhanced as the fruit ripened, although this increase was not as dramatic as the mRNA induction pattern. Overall, these results suggest that MdCAS may play a role in cyanide detoxification in ripening apple fruits.

Cyanide-metabolizing enzyme rhodanese in human tissues: comparison with domestic animals

The enzyme rhodanese (thiosulfate:cyanide sulfurtransferase) is an ubiquitous enzyme with activity in all living organisms, from bacteria to man. It has been suggested that this enzyme plays a central role in cyanide detoxification. The purpose of this investigation was to determine rhodanese activities in different tissues of human and compare them with those found in domestic animals. Normal human tissues were obtained from consenting patients who had undergone surgery after confirmation of nonneoplastic condition. Tissue extracts were prepared from liquid nitrogen-frozen samples and rhodanese specific activity and activity per gram tissue was measured by determination of enzymatically formed thiocyanate. The highest activity of rhodanese was found in kidney, followed by liver, brain, lung, muscle, and stomach. Other tissues studied did not show significant rhodanese activity. The results obtained in this study were compared with previously reported information on domestic animals. Human liver contains lower rhodanese activity compared with ruminants and nonruminants, except for dog which has comparable hepatic activity with human. Human kidney contains significant activity and the value is higher than those of camels, pigs, dogs, and chickens and is comparable with those of goats. The results of this study might indicate the involvement of rhodanese in cyanide detoxification in tissues which might be more exposed to cyanide due to higher blood supply to these tissues. On the other hand, rhodanese might perform other functions in human organs which are specific to these tissues.

Differences in Michaelis–Menten kinetics for different cultivars of maize during cyanide removal

Knowledge of the kinetic parameters, the half-saturation constant ( K m) and the maximum metabolic capacity ( v max), is very useful for the characterization of enzymes and biochemical processes. Little is known about rates of which vegetation metabolizes environmental chemicals. It is known, however, that vascular plants possess an enzyme system that detoxifies cyanide by converting it into the amino acid asparagine. This study investigated the differences in Michaelis–Menten kinetics of cyanide removal by different cultivars of maize. Detached leaves (1.0 g fresh weight) of seven different cultivars of maize ( Zea mays L.) were kept in glass vessels with 100 mL of aqueous solution spiked with potassium cyanide at 25±0.5 °C for 28 h. Four treatment concentrations of cyanide were used, ranging from 0.43 to 7.67 mg CN L −1. The disappearance of cyanide from the aqueous solution was analyzed spectrophotometrically. Realistic values of K m and v max were estimated by a computer program using non-linear regression treatment. Lineweaver–Burk plots were also used to estimate the kinetic parameters for comparison. Using non-linear regression treatments, values of v max and K m were found to be between 10.80 and 22.80 mg CN kg −1 h −1, and 2.57 and 7.09 mg CN L −1, respectively. The highest v max was achieved by the cultivars HengFen 1, followed by NongDa 108. The lowest v max was demonstrated by JingKe 8. The highest K m was found in NongDa 108, followed by HengFen 1. The lowest K m was associated with JingKe 8. Results from this study indicated that significant removal of cyanide from an aqueous solution was observed in the presence of plant materials without apparent phytotoxicity, even at the high concentration of cyanide used in this study. All maize cultivars used in this study were able to metabolize cyanide efficiently, although with different metabolic capacities. Results also showed a small variation of metabolic rates between the different cultivars. This leads to the conclusion that plants can be used as a phytoremediation agent in the detoxification of cyanide, presenting a feasible option for cleaning up soils and water contaminated with cyanide.

Beta-Cyanoalanine Synthase as a Molecular Marker for Induced Resistance by Fungal Glycoprotein Elicitor and Commercial Plant Activators

ABSTRACT The biocontrol agent Pythium oligandrum produces glycoprotein elicitor in the cell wall fraction, designated CWP, and induces resistance to a broad range of pathogens. To understand the mechanism of CWP-induced resistance to pathogens, gene expression at the early stage of CWP treatment in tomato roots was analyzed using a cDNA array. At 4 h after CWP treatment, 144 genes were up-regulated and 99 genes were down-regulated. In the 144 up-regulated genes, nine genes exhibited about eightfold increased expression. Analysis of the response of these nine genes to three commercial plant activators indicated that a high level of one gene, beta-cyanoalanine synthase gene (LeCAS) encoding hydrogen cyanide (HCN) detoxification enzyme, was stably induced in tomato roots by such treatment. However, expression of LeCAS was not significantly induced in tomato roots at 4 h by abiotic stresses, whereas only a very low level of induction of such expression by cold stress was observed. This LeCAS expression was also induced after exogenous treatment with a low level of 1-amino-cyclopropane-1-carboxylate as the precursor of ethylene, but not with either salicylic acid or methyl jas-monate. The induction of LeCAS expression in CWP-treated and plant activator-treated roots is likely to be caused by the detoxification of HCN during ethylene production. Transient activation of LeCAS expression caused by ethylene production in tomato roots may be a general phenomenon in fungal elicitor-induced and synthetic plant activator-induced resistance. LeCAS seems to be useful for screening possible novel plant activators for plant protection against pathogens.

BIOCHEMICAL PROPERTIES AND BIOLOGICAL FUNCTIONS OF THE ENZYME RHODANESE IN DOMESTIC ANIMALS

*Summary The enzyme rhodanese (thiosulfate: cyanide sulfurtransferase) is a ubiquitous enzyme and its activity is present in all living organisms. Many functions including cyanide detoxification, formation of iron-sulfur centers and participation in energy metabolism have been attributed to this enzyme. The enzyme catalyzes the transfer of a sulfur atom from sulfane containing compounds (such as thiosulfate) to thiophilic anions (such as cyanide). The sulfhydryl group of cysteine-247 in the molecule of rhodanese participates in a double displacement of sulfur transfer mechanism. In this review attempt will be made to summarize the latest information available on the molecular properties and the pattern of distribution of rhodanese in different tissues of domestic animals and to combine these different lines of research to arrive at a plausible explanation regarding the biological function of this important enzyme in living organisms.

Evidence for a functional genetic polymorphism of the human thiosulfate sulfurtransferase (Rhodanese), a cyanide and H 2S detoxification enzyme

Rhodanese or thiosulfate sulfurtransferase (TST) is a mitochondrial matrix enzyme that plays roles in cyanide detoxification, the formation of iron–sulfur proteins and the modification of sulfur-containing enzymes. Transsulfuration reaction catalyzed by TST is also involved in H 2S detoxification. To date, no polymorphism of the human TST gene had been reported. We developed a screening strategy based on a PCR-SSCP method to search for mutations in the 3 exons of TST and their proximal flanking regions. This strategy has been applied to DNA samples from 50 unrelated French individuals of Caucasian origin. Eleven polymorphisms consisting in seven nucleotide substitutions in non-coding regions, two silent mutations and two missense mutations were characterized. The functional consequences of the identified mutations were assessed in vivo by measurement of erythrocyte TST activity and/or in vitro using heterologous expression in Saccharomyces cerevisiae or transient transfection assay in HT29 and Caco-2 cell lines. The P 285A variant appears to encode a protein with a 50% decrease of in vitro intrinsic clearance compared to the wild-type enzyme. Additionally, the six polymorphisms located upstream the ATG initiation codon are responsible for a significant decrease (ranging from 40% to 73%) in promoter activity of a reporter gene compared to the corresponding wild-type sequence. This work constitutes the first report of the existence of a functional genetic polymorphism affecting TST activity and should be of great help to investigate certain disorders for which impairment of CN - or H 2S detoxification have been suggested to be involved.

Cyanide Metabolism in Higher Plants: Cyanoalanine Hydratase is a NIT4 Homolog

Cyanoalanine hydratase (E.C. 4.2.1.65) is an enzyme involved in the cyanide detoxification pathway of higher plants and catalyzes the hydrolysis of beta-cyano-L-alanine to asparagine. We have isolated the enzyme from seedlings of blue lupine (Lupinus angustifolius) to obtain protein sequence information for molecular cloning. In contrast to earlier reports, extracts of blue lupine cotyledons were found also to contain cyanoalanine-nitrilase (E.C. 3.5.5.4) activity, resulting in aspartic acid production. Both activities co-elute during isolation of cyanoalanine hydratase and are co-precipitated by an antibody directed against Arabidopsis thaliana nitrilase 4 (NIT4). The isolated cyanoalanine hydratase was sequenced by nanospray-MS/MS and shown to be a homolog of Arabidopsis thaliana and Nicotiana tabacum NIT4. Full-length cDNA sequences for two NIT4 homologs from blue lupine were obtained by PCR using degenerate primers and RACE-experiments. The recombinant LaNIT4 enzymes, like Arabidopsis NIT4, hydrolyze cyanoalanine to asparagine and aspartic acid but show a much higher cyanoalanine-hydratase activity. The two nitrilase genes displayed differential but overlapping expression. Taken together these data show that the so-called 'cyanoalanine hydratase' of plants is not a bacterial type nitrile hydratase enzyme but a nitrilase enzyme which can have a remarkably high nitrile-hydratase activity.

The Mercaptopyruvate Pathway in Cysteine Catabolism: A Physiologic Role and Related Disease of the Multifunctional 3-Mercaptopyruvate Sulfurtransferase

The conversion of cysteine to 3-sulfino-alanine is a major pathway in cysteine catabolism. Cysteine dioxygenase catalyzes the reaction and dietary intake of the essential amino acid methionine and the semi-essential amino acid cysteine increases the level of this enzyme by suppressing enzyme degradation via polyubiquitination. The production of cellular antioxidants such as glutathione, thioredoxin, and their families is important in cysteine metabolism, and these cellular antioxidants have critical roles in the maintenance of the cellular redox status. The mercaptopyruvate pathway, in which cysteine or aspartate transaminase catalyzes the transamination from cysteine to 3-mercaptopyruvate and then 3-mercaptopyruvate sulfurtransferase catalyzes the transsulfuration from 3-mercaptopyruvate to pyruvate, also contributes to maintain the cellular redox. 3-Mercaptopyruvate sulfurtransferase serves as an antioxidant protein: when the enzyme is exposed to stoichiometric amounts of the oxidant hydrogen peroxide, it is inhibited via the formation of low redox sulfenate at the catalytic site cysteine. On the other hand, activity is restored by the reductant dithiothreitol or reduced thioredoxin. 3-Mercaptopyruvate sulfurtransferase also detoxifies cyanide via transsulfuration from a stable persulfide at the catalytic site cysteine, a reaction intermediate, suggesting that cyanide detoxification is not necessarily an enzymatic reaction. Furthermore, a congenital defect of the enzyme causes mercaptolactate-cysteine disulfiduria associated with or without mental retardation, although the pathogenesis remains unclear. These facts suggest that 3-mercaptopyruvate sulfurtransferase has physiologic roles as an antioxidant and a cyanide antidote; is essential for neural function, and participates in cysteine degradation.

Effects of salinity levels on proteome ofSuaeda aegyptiaca leaves

Saline soils are the major problem of cultivated lands of Iran. Suaeda aegyptiaca is a salt-tolerant plant (halophytes) that grow naturally in salt-affected areas of Iran. We have employed proteomics to identify the mechanisms of salt responsiveness in leaves of S. aegyptiaca grown under different salt concentrations. Ten-day-old plants were treated with 0, 150, 300, 450, and 600 mM NaCl. After 30 days of treatment, leaf samples were collected and analyzed using 2-D-PAGE. Out of 700 protein spots reproducible detected within replications, 102 spots showed significant response to salt treatment compared to 0 mM NaCl. We analyzed expression pattern of salt-responsive proteins using a hierarchical and two nonhierarchical (Fuzzy ART and SOM) statistical methods and concluded that Fuzzy ART is the superior method. Forty proteins of 12 different expression groups were analyzed using LC/MS/MS. Of these, 27 protein spots were identified including proteins involved in oxidative stress tolerance, glycinebetain synthesis, cytoskeleton remodeling, photosynthesis, ATP production, protein degradation, cyanide detoxification, and chaperone activities. The expression pattern of these proteins and their possible roles in the adaptation of S. aegyptiaca to salinity is discussed.

Evidence for a functional genetic polymorphism of the human mercaptopyruvate sulfurtransferase (MPST), a cyanide detoxification enzyme

Mercaptopyruvate sulfurtransferase (MPST) plays a central role in both cysteine degradation and cyanide detoxification. Moreover, deficiency in MPST activity has been suggested to be responsible for a rare inheritable disorder known as mercaptolactate-cysteine disulfiduria (MCDU). To date, no mutation of the human MPST gene has been reported. We developed a screening strategy to search for mutations in the MPST gene of 50 unrelated French individuals. Two intronic polymorphisms (IVS1 − 110C > G and IVS2 + 39C > T) and a nonsense mutation (Tyr 85Stop) were identified and their functional consequences were assessed in vivo by measurement of erythrocyte MPST activity and/or in vitro using heterologous expression or transient transfection assay. The nonsense mutation likely leads to the synthesis of a severely truncated protein without enzymatic activity, as supported by our in vitro data. This work constitutes the first report of the existence of a functional genetic polymorphism affecting MPST and should be of great help to investigate certain disorders such as MCDU.

Circadian and Ultradian (12 H) Rhythms of Hepatic Thiosulfate Sulfurtransferase (Rhodanese) Activity in Mice During the First Two Months of Life

Thiosulfate sulfurtransferase (TST) is an important ‘enzyme of protection,’ that accelerates the detoxification of cyanide, converting it into thiocyanate. The TST physiological rhythm was investigated at wks 2, 4, and 8 of post‐natal development (PND) in the mouse. The results revealed a statistically significant gender‐related difference, with the highest activity in females, at all the documented PND stages. In the second week of PND (pre‐weaning time), the circadian rhythm of the enzyme activity was associated with ultradian components. The prominent circadian rhythm (τ=24 h) peaked at the beginning of the light span, more precisely ∼3 HALO (Hours After Light Onset). A week after weaning (wk 4 of PND), an impairment of the rhythm, with the peak shifted toward the second half of photophase, was recorded. Four to 6 wks later, about wk 8 of PND, the circadian rhythm pattern was stabilized, with its peak then located at the beginning of the dark span (13 HALO). The obtained results showed a 12 h phase‐shift of the circadian TST peak time during PND, suggesting that the rhythm stabilization is age‐dependent.

Cyanide detoxification by recombinant bacterial rhodanese

Cyanide is a major environmental pollutant of the chemical and metallurgical industries. Although extremely toxic, cyanide can enzymatically be converted to the less toxic thiocyanate by rhodaneses (thiosulfate:cyanide sulfurtransferases, EC 2.8.1.1). We engineered a genetic system to express high levels of recombinant Pseudomonas aeruginosa rhodanese (r-RhdA) in Escherichia coli, and used this organism to test the role of r-RhdA in cyanide detoxification. Inducible expression of the rhdA gene under the control of the hybrid T 7- lacO promoter yielded active r-RhdA over a 4-h period, though r-RhdA-expressing E. coli showed decreased viability starting from 1 h post-induction. At this time, Western blot analysis and enzymatic assay showed r-RhdA partition between the cytoplasm (95%) and the periplasm (5%). The accessibility of thiosulfate to r-RhdA was a limiting step for the sulfur transfer reaction in the cellular system, but cyanide conversion to thiocyanate could be increased upon permeabilization of the bacterial membrane. Specific r-RhdA activity was higher in the whole-cell assay than in the in vitro assay with pure enzyme (2154 vs. 816 μmol min −1 mg −1 r-RhdA, respectively), likely reflecting enzyme stability. The r-RhdA-dependent cyanide detoxification resulted in increased resistance of r-RhdA overexpressing E. coli to 5 mM cyanide. Bacterial survival was paralleled by release of thiocyanate into the medium. Our results indicate that cyanide detoxification by engineered E. coli cells is feasible under laboratory conditions, and suggest that microbial rhodaneses may contribute to cyanide transformation in natural environments.

Detoxification of Cyanide by Woody Plants

Vascular plants possess an enzyme system that detoxifies cyanide by converting it to the amino acid asparagine. This paper examines the potential of three woody plants from the Salicaceae family to degrade cyanide. Pre-rooted trees were grown in carefully designed bioreactors with aqueous solution spiked with potassium cyanide at 23.0 +/- 1 degree C for a maximum of 144 h. Cyanide concentrations ranged from 0.95 to 1.15 CN mg/L. Cyanide in water and in plant tissues was analyzed spectrophotometrically. Results from the investigation indicated that significant reduction of aqueous cyanide was found during the presence of plants in all treatments. Little amounts of applied cyanide were detected in the tissues of plants, mainly in roots and bottom stem. Cyanide remaining in tissues varied with the species of plants, despite similar periods of exposure. The data also indicated that photolysis, hydrolysis, and microbial degradation were not occurring and that volatilization was minimal. In conclusion, transport and metabolism of cyanide in plants is most likely.

Cyanide Removal by Chinese Vegetation. Quantification of the Michaelis-Menten Kinetics (6 pp)

Little is known about metabolism rates of environmental chemicals by vegetation. A good model compound to study the variation of rates among plant species is cyanide. Vascular plants possess an enzyme system that detoxifies cyanide by converting it to the amino acid asparagine. Knowledge of the kinetic parameters, the half-saturation constant (Km) and the maximum metabolic capacity (vmax), is very useful for enzyme characterization and biochemical purposes. The goal of this study is to find the enzyme kinetics (K(M) and vmax) during cyanide metabolism in the presence of Chinese vegetation, to provide quantitative data for engineered phytoremediation, and to investigate the variation of metabolic rates of plants. Detached leaves (1.0 g fresh weight) from 12 species out of 9 families were kept in glass vessels with 100 mL of aqueous solution spiked with potassium cyanide at 23 degrees C for 28 h. Four different treatment concentrations of cyanide were used, ranging from 0.44 to 7.69 mg CN/L. The disappearance of cyanide from the aqueous solution was analyzed spectrophotometrically. Realistic values of the half-saturation constant (KM) and the maximum metabolic capacity (vmax) were estimated by a computer program using non-linear regression treatments. As a comparison, Lineweaver-Burk plots were also used to estimate the kinetic parameters. The values obtained for K(M) and vmax varied with plant species. Using non-linear regression treatments, values of vmax and K(M) were found in a range between 6.68 and 21.91 mg CN/kg/h and 0.90 to 3.15 mg CN/L, respectively. The highest vmax was by Chinese elder (Sambucus chinensis), followed by upright hedge-parsley (Torilis japonica). The lowest Vmax was demonstrated by the hybrid willow (Salix matssudana x alba). However, the highest K(M) was found in the water lily (Nymphea teragona), followed by the poplar (Populus deltoides Marsh). The lowest K(M) was demonstrated by corn (Zea mays L.). The values of vmax were normally distributed with a mean of 13 mg CN/kg/h. Significant removal of cyanide from aqueous solution was observed in the presence of plant materials without phytotoxicity, even at high doses of cyanide. This gives rise to the conclusion that the Chinese plant species used in this study are all able to efficiently metabolize cyanide, although with different maximum metabolic capacities. A second conclusion is that the variation of metabolism rates between species is small. All these plants had a similar K(M), indicating the same enzyme is active in all plants. Detoxification of cyanide with trees seems to be a feasible option for cleaning soils and water contaminated with cyanide. For phytoremediation projects, screening appropriate plant species adapted to local conditions should be seriously considered. More chemicals should be investigated to find common principles of the metabolism of environmental chemicals by plants.

English

Rhodanese is a multifunctional, mitochondrial, sulphur transferase that catalyses the detoxification of cyanide by sulphuration in a double displacement (ping pong) mechanistic reaction. It is widely distributed occurring in varieties of plants and animals, where it activity is modulated by a number of factors including differences in species, organs, sex, age and diet. The enzyme is a single polypeptide chain of 289 amino acids with molecular weight of up to 37,000. The active site of rhodanese contains a tryptophanyl residue in close proximity with an essential sulphahydryl group. Many methods for assaying rhodanese have been reported, the most prominent being the one based on the colorimetric estimation of thiocyanate formed from the reaction of cyanide and thiosulphate, catalysed by rhodanese.    Key words: Rhodanese, cyanide, sulphur transferase.