In vivo detoxification of cyanide by cystathionase gamma-lyase

The Effects of Different Garlic-Derived Allyl Sulfides on Anaerobic Sulfur Metabolism in the Mouse Kidney

Diallyl sulfide (DAS), diallyl disulfide (DADS) and diallyl trisulfide (DATS) are major oil-soluble organosulfur compounds of garlic responsible for most of its pharmacological effects. The present study investigated the influence of repeated intraperitoneally (ip) administration of DAS, DADS and DATS on the total level of sulfane sulfur, bound sulfur (S-sulfhydration) and hydrogen sulfide (H₂S) and on the activity of enzymes, which catalyze sulfane sulfur formation and transfer from a donor to an acceptor in the normal mouse kidney, i.e., γ-cystathionase (CSE) and rhodanese (TST). The activity of aldehyde dehydrogenase (ALDH), which is a redox-sensitive protein, containing an –SH group in its catalytic center, was also determined. The obtained results indicated that all tested compounds significantly increased the activity of TST. Moreover, DADS and DATS increased the total sulfane sulfur level and CSE activity in the normal mouse kidney. ALDH activity was inhibited in the kidney after DATS administration. The results indicated also that none of the studied allyl sulfides affected the level of bound sulfur or H₂S. Thus, it can be concluded that garlic-derived DADS and DATS can be a source of sulfane sulfur for renal cells but they are not connected with persulfide formation.

Enzyme therapeutics for systemic detoxification

Life relies on numerous biochemical processes working synergistically and correctly. Certain substances disrupt these processes, inducing living organism into an abnormal state termed intoxication. Managing intoxication usually requires interventions, which is referred as detoxification. Decades of development on detoxification reveals the potential of enzymes as ideal therapeutics and antidotes, because their high substrate specificity and catalytic efficiency are essential for clearing intoxicating substances without adverse effects. However, intrinsic shortcomings of enzymes including low stability and high immunogenicity are major hurdles, which could be overcome by delivering enzymes with specially designed nanocarriers. Extensive investigations on protein delivery indicate three types of enzyme-nanocarrier architectures that show more promise than others for systemic detoxification, including liposome-wrapped enzymes, polymer-enzyme conjugates, and polymer-encapsulated enzymes. This review highlights recent advances in these nano-architectures and discusses their applications in systemic detoxifications. Therapeutic potential of various enzymes as well as associated challenges in achieving effective delivery of therapeutic enzymes will also be discussed.

The effect of lipoate on anaerobic cysteine metabolism in erythrocytes of patients treated with peritoneal dialysis

BACKGROUND

The studies aimed to evaluate the changes in cysteine sulfur metabolism in erythrocytes of end-stage renal failure (ESRF) patients treated with continuous ambulatory peritoneal dialysis (CAPD) caused by a one-month lipoate (LA) supplementation at a daily dose of 600 mg.

METHODS

The level of sulfane sulfur and activity of sulfurtransferases were determined in erythrocytes of CAPD patients and in the control group.

RESULTS

The sulfane sulfur level in erythrocytes of CAPD patients did not differ compared with healthy volunteers but LA supplementation increased the reactive sulfur concentration. LA elevated also cystathionase activity.

CONCLUSIONS

LA supplementation in ESRF patients treated with CAPD increases the sulfane sulfur level which indicates the augmentation of its antioxidant and regulatory properties.

Magnetic tumor targeting of β-glucosidase immobilized iron oxide nanoparticles

Directed enzyme/prodrug therapy (DEPT) has promising application for cancer therapy. However, most current DEPT strategies face shortcomings such as the loss of enzyme activity during preparation, low delivery and transduction efficiency in vivo and difficultly of monitoring. In this study, a novel magnetic directed enzyme/prodrug therapy (MDEPT) was set up by conjugating β-glucosidase (β-Glu) to aminated, starch-coated, iron oxide magnetic iron oxide nanoparticles (MNPs), abbreviated as β-Glu-MNP, using glutaraldehyde as the crosslinker. This β-Glu-MNP was then characterized in detail by size distribution, zeta potential, FTIR spectra, TEM, SQUID and magnetophoretic mobility analysis. Compared to free enzyme, the conjugated β-Glu on MNPs retained 85.54% ± 6.9% relative activity and showed much better temperature stability. The animal study results showed that β-Glu-MNP displays preferable pharmacokinetics characteristics in relation to MNPs. With an adscititious magnetic field on the surface of a tumor, a significant quantity of β-Glu-MNP was selectively delivered into a subcutaneous tumor of a glioma-bearing mouse. Remarkably, the enzyme activity of the delivered β-Glu in tumor lesions showed as high as 20.123±5.022 mU g(-1) tissue with 2.14 of tumor/non-tumor β-Glu activity.

Dose and time-dependent effects of cyanide on thiosulfate sulfurtransferase, 3-mercaptopyruvate sulfurtransferase, and cystathionine λ-lyase activities

We assessed the dose-dependent effect of potassium cyanide (KCN) on thiosulfate sulfurtransferase (TST), 3-mercaptopyruvate sulfurtransferase (3-MPST), and cystathionine λ-lyase (CST) activities in mice. The time-dependent effect of 0.5 LD50 KCN on cyanide level and cytochrome c oxidase (CCO), TST, 3-MPST, and CST activities was also examined. Furthermore, TST, 3-MPST, and CST activities were measured in stored mice cadavers. Hepatic and renal TST activity increased by 0.5 LD50 KCN but diminished by ≥2.0 LD50. After 0.5 LD50 KCN, the elevated hepatic cyanide level was accompanied by increased TST, 3-MPST, and CST activities, and CCO inhibition. Elevated renal cyanide level was only accompanied by increased 3-MPST activity. No appreciable change in enzyme activities was observed in mice cadavers. The study concludes that high doses of cyanide exert saturating effects on its detoxification enzymes, indicating their exogenous use during cyanide poisoning. Also, these enzymes are not reliable markers of cyanide poisoning in autopsied samples.

Comparative proteomic identification of the hemocyte response to cold stress in white shrimp, Litopenaeus vannamei

To understand molecular responses of crustacean hemocytes to cold stress, flow cytometer analysis and two-dimensional electrophoresis proteomic approach were used to investigate altered proteins in hemocytes of Litopenaeus vannamei during cold stress treatment. Through flow cytometer analysis, 13°C for 24h post-cold stress was selected as the suitable temperature and condition for cold stress treatment. MALDI-TOF/TOF MS analysis revealed that 6 forms of 6 proteins were significantly up-regulated, including three enzymes (cystathionase, glyceraldehyde 3-phosphate dehydrogenase and glyoxalase 1) and one immune-related protein (oncoprotein nm23), whereas 24 forms of 3 proteins were significantly down-regulated in the treated shrimp (hemocyanin, hemocyte transglutaminase and transketolase). There were 20 spots identified as hemocyanin meaning that it may play important roles in environmental regulation in shrimp. Real-time fluorescence quantitative PCR confirmed that the levels of transcription of the hemocyanin, partial mRNA for hemocyanin, cystathionase, glyoxalase 1 and oncoprotein nm23 genes were found to relate well with that of their translation products after cold stress treated, while only the levels of hemocyte transglutaminase transcripts were not corresponded with that of their translation products. Further investigation of these data may lead to better understanding of the molecular responses of crustacean hemocytes to cold stress.

Emerging role of hydrogen sulfide in health and disease: critical appraisal of biomarkers and pharmacological tools

H2S (hydrogen sulfide) is a well known and pungent gas recently discovered to be synthesized enzymatically in mammalian and human tissues. In a relatively short period of time, H2S has attracted substantial interest as an endogenous gaseous mediator and potential target for pharmacological manipulation. Studies in animals and humans have shown H2S to be involved in diverse physiological and pathophysiological processes, such as learning and memory, neurodegeneration, regulation of inflammation and blood pressure, and metabolism. However, research is limited by the lack of specific analytical and pharmacological tools which has led to considerable controversy in the literature. Commonly used inhibitors of endogenous H2S synthesis have been well known for decades to interact with other metabolic pathways or even generate NO (nitric oxide). Similarly, commonly used H2S donors release H2S far too quickly to be physiologically relevant, but may have therapeutic applications. In the present review, we discuss the enzymatic synthesis of H2S and its emerging importance as a mediator in physiology and pathology. We also critically discuss the suitability of proposed 'biomarkers' of H2S synthesis and metabolism, and highlight the complexities of the currently used pharmacological H2S 'donor' molecules and 'specific' H2S synthesis inhibitors in their application to studying the role of H2S in human disease.

The expression and activity of cystathionine-γ-lyase and 3-mercaptopyruvate sulfurtransferase in human neoplastic cell lines

The expression and activity of cystathionine γ-lyase (CST) and 3-mercaptopyruvate sulfurtransferase (MPST) were investigated in the human neoplastic cells lines: astrocytoma U373, neuroblastoma SH-SY5Y, melanoma A375, and melanoma WM35. Gene expression analysis demonstrated that the investigated neoplastic cells showed the expression of MPST and what is particularly interesting, the expression of CST. The presence of CST in these cells was confirmed using RT-PCR and western blot analysis. However, in U373 cells, a very low activity of CST was detected. In all the investigated cell lines, the activity of MPST was higher than that of CST, which suggests that in these cells, the main pathway of sulfane sulfur formation is the MPST-catalyzed reaction. RP-HPLC analysis showed a large disparity between the level of cystathionine and GSH in the investigated neoplastic cells. In SH-SY5Y cells, the low level of GSH and low GSH/GSSG ratio corresponded with the highest CST activity. Further investigations could aim at verifying whether the stimulation of CST, at the level of protein or gene expression, could change the proliferation of neoplastic cells.

Conversion of Apricot Cyanogenic Glycosides to Thiocyanate by Liver and Colon Enzymes

Some of the edible plants like apricot kernel, flaxseed, and cassava generate hydrogen cyanide (HCN) when cyanogenic glycosides are hydrolyzed. Rhodanese (thiosulfate: cyanide sulfurtransferases of TSTs; EC: 2.8.1.1) is a sulfide-detoxifying enzymes that converts cyanides into thiocyanate and sulfite. This enzyme exists in a liver and kidneys in abundance. The present study is to evaluate the conversion of apricot cyanogenic glycosides into thiocyanate by human hepatic (HepG2) and colonal (HT-29) cells, and the induction of the enzymes in the rat. The effects of short term exposure of amygdalin to rats have also been investigated. Cytosolic, mitochondrial, and microsomal fractions from HepG2 and HT-29 cells and normal male Spraque-Dawley rats were used. When apricot kernel extract was used as substrate, the rhodanese activity in liver cells was higher than the activity in colon cells, both from established human cell line or animal tissue. The cytosolic fractions showed the highest rhodanese activity in all of the cells, exhibiting two to three times that of microsomal fractions. Moreover, the cell homogenates could metabolize apricot extract to thiocyanate suggesting cellular hydrolysis of cyanogenic glycoside to cyanide ion, followed by a sulfur transfer to thiocyanate. After the consumption of amygdalin for 14 days, growth of rats began to decrease relative to that of the control group though a significant change in thyroid has not been observed. The resulting data support the conversion to thiocyanate, which relate to the thyroid dysfunction caused by the chronic dietary intake of cyanide. Because Korean eats a lot of Brassicaceae vegetables such as Chinese cabbage and radish, the results of this study might indicate the involvement of rhodanese in prolonged exposure of cyanogenic glycosides.

The level of sulfane sulfur in the fungus Aspergillus nidulans wild type and mutant strains

The interdependence of the sulfane sulfur metabolism and sulfur amino acid metabolism was studied in the fungus Aspergillus nidulans wild type strain and in mutants impaired in genes encoding enzymes involved in the synthesis of cysteine (a precursor of sulfane sulfur) or in regulatory genes of the sulfur metabolite repression system. It was found that a low concentration of cellular cysteine leads to elevation of two sulfane sulfurtransferases, rhodanase and cystathionine gamma-lyase, while the level of 3-mercaptopyruvate sulfurtransferase remains largely unaffected. In spite of drastic differences in the levels of biosynthetic enzymes and of sulfur amino acids due to mutations or sulfur supplementation of cultures, the level of total sulfane sulfur is fairly stable. This stability confirms the crucial role of sulfane sulfur as a fine-tuning regulator of cellular metabolism.

Role of Hydrogen Sulfide in the Cardioprotection Caused by Ischemic Preconditioning in the Rat Heart and Cardiac Myocytes

Endogenous H(2)S is synthesized mainly by cystathionine gamma-lyase in the heart. The present study investigated the role of H(2)S in cardioprotection induced by ischemic preconditioning. We have examined the effect of endogenous H(2)S and exogenous application of NaHS (H(2)S donor) on cardiac rhythm in the isolated rat heart subjected to low-flow ischemia insults as well as cell viability and function in isolated myocytes exposed to simulated ischemia solution. Preconditioning with NaHS (SP) or ischemia (IP) for three cycles (3 min each cycle separated by 5 min of recovery) significantly decreased the duration and severity of ischemia/reperfusion-induced arrhythmias in the isolated heart while increasing cell viability and the amplitude of electrically induced calcium transients after ischemia/reperfusion in cardiac myocytes. Both IP and SP also significantly attenuated the decreased H(2)S production during ischemia. Moreover, decreasing endogenous H(2)S production significantly attenuated the protective effect of IP in both the isolated heart and isolated cardiac myocytes. Blockade of protein kinase C with chelerythrine or bisindolylmaleimide I as well as ATP-sensitive K(+) (K(ATP)) channel with glibenclamide (a nonselective K(ATP) blocker) and HMR-1098 (1-[[5-[2-(5-Chloro-o-anisamido)ethyl]-2-methoxyphenyl]sulfonyl]-3-methylthiourea) (a sarcolemmal K(ATP) channel blocker) reversed the cardioprotection induced by SP or IP. However, blockade of mitochondrial K(ATP) channels with 5-hydroxydecanoic acid had no effect on the cardioprotection of SP, suggesting that, unlike the mechanism involved in IP, mitochondrial K(ATP) channels most probably do not play a major role in the cardioprotection of SP. Our findings suggest that endogenous H(2)S contributes to cardioprotection induced by IP, which effect may involve protein kinase C and sarcolemmal K(ATP) channels.

Aminotransferase, l-amino acid oxidase and β-lyase reactions involving l-cysteine S-conjugates found in allium extracts

Several cysteine S-conjugates that occur in extracts of garlic and other plants of the allium family possess anti-oxidant properties, and many, including S-allyl-L-cysteine (SAC) and S-allylmercapto-L-cysteine (SAMC), are promising anti-cancer agents. To understand possible biochemical mechanisms contributing to the protective effects, the ability of selected allium-derived L-cysteine S-conjugates to undergo various enzyme-catalyzed transformations was investigated. SAC, SAMC, S-propylmercapto-L-cysteine and S-penta-1,3-dienylmercapto-L-cysteine were shown to be substrates of: (a) highly purified rat kidney glutamine transaminase K (GTK); (b) purified snake venom L-amino acid oxidase; and (c) a cysteine S-conjugate beta-lyase present in rat liver cytosol. S-Methylmercapto-L-cysteine was shown to be a substrate of GTK and L-amino acid oxidase, but not of the cysteine S-conjugate beta-lyase. Evidence is presented that a major enzyme responsible for the cysteine S-conjugate beta-lyase reactions in the rat liver cytosol is gamma-cystathionase. The possible role of gamma-cystathionase in generating sulfane sulfur from the disulfide-containing cysteine S-conjugates present in allium extracts, and the possible role of this sulfane sulfur in enzyme regulation, targeting of cancer cells and detoxification reactions is discussed. An interesting side finding of the present work is that rat liver mitochondria are more active than rat liver cytosol in catalyzing a cysteine S-conjugate beta-lyase reaction with the mitochondrial protoxicant S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC) at physiological pH and at low substrate concentration.

Differential metabolism of acrylonitrile to cyanide is responsible for the greater sensitivity of male vs female mice: role of CYP2E1 and epoxide hydrolases

Acrylonitrile (AN) is a potent toxicant and a known rodent carcinogen. AN epoxidation to cyanoethylene oxide (CEO) via CYP2E1 and its subsequent metabolism via epoxide hydrolases (EH) to yield cyanide is thought to be responsible for the acute toxicity and mortality of AN. Recent reports showed that male mice are more sensitive than females to the acute toxicity/mortality of AN. The present work was undertaken to assess the metabolic and enzymatic basis for the greater sensitivity of male vs female mice to AN toxicity. Male and female wild-type and CYP2E1-null mice received AN at 0, 2.5, 10, 20, or 40 mg/kg by gavage. Cyanide concentrations were measured at 1 or 3 h after dosing. Current data demonstrated that cyanide levels in blood and tissues of AN-treated wild-type mice of both sexes were significantly greater than in vehicle-treated controls and increased in a dose-dependent manner. In contrast, cyanide levels in AN-treated CYP2E1-null mice were not statistically different from those measured in vehicle-treated controls. Furthermore, higher levels of cyanide were detected in male wild-type mice vs females in association with greater sensitivity of males to the acute toxicity/mortality of this chemical. Using Western blot analysis, negligible difference in CYP2E1 expression with higher levels of soluble and microsomal EH (sEH and mEH) was detected in the liver of male vs female mice. In kidneys, male mice exhibited higher expression of both renal CYP2E1 and sEH than did female mice. In conclusion, higher blood and tissue cyanide levels are responsible for the greater sensitivity of male vs female mice to AN. Further, higher expression of CYP2E1 and EH in male mice may contribute to greater formation of CEO and its subsequent metabolism to yield cyanide, respectively.

The smooth muscle relaxant effect of hydrogen sulphide in vitro: evidence for a physiological role to control intestinal contractility

1. Sodium hydrogen sulphide (NaHS), a donor of hydrogen sulphide (H(2)S), produced dose-related relaxation of the rabbit isolated ileum (EC(50), 76.4+/-7.9 microM) and rat vas deferens (EC(50), 64.8+/-5.4 microM) and reduced ACh-mediated contraction of the guinea-pig isolated ileum. 2. NaHS also reduced the response of the guinea-pig (EC(50), 80.0+/-5.7 microM) and rat (EC(50), 108.2+/-11.2 microM) ileum preparations to electrical stimulation of the intramural nerves. In guinea-pig ileum this effect was spontaneously reversible and mimicked by sodium nitroprusside (SNP, EC(50), 2.1 microM). Combination of NaHS (20 microM) with SNP (0.5 microM) produced a greater than additive inhibition of the twitch response of the ileum to electrical stimulation. 3. The inhibitory effect of NaHS on the field-stimulated guinea-pig ileum was unaffected by pretreatment with L-NAME (100 microM), indomethacin (10 microM), naloxone (1 microM) or glibenclamide (100 microM). Furthermore, NaHS (200 microM) did not affect the contractile response of the ileum to KCl (10 to 60 mM). 4. Propargylglycine (PAG, 1 mM) and beta-cyanoalanine (BCA, 1 mM) (inhibitors of cystathionine-gamma-lyase) but not aminooxyacetic acid (AOAA, 1 mM) (inhibitor of cystathionine-beta-synthetase) caused a slowly developing increase in the contraction of the guinea-pig ileum to field stimulation. This effect was reversed by cysteine (1 mM). 5. These results show that NaHS relaxes gastrointestinal and urogenital smooth muscle and suggest that H(2)S is responsible for these effects. The possibility that endogenous H(2)S, formed as a consequence of activation of intramural nerves, plays a part in controlling the contractility of the guinea-pig ileum is discussed.

The smooth muscle relaxant effect of hydrogen sulphide in vitro: evidence for a physiological role to control intestinal contractility

1. Sodium hydrogen sulphide (NaHS), a donor of hydrogen sulphide (H(2)S), produced dose-related relaxation of the rabbit isolated ileum (EC(50), 76.4+/-7.9 microM) and rat vas deferens (EC(50), 64.8+/-5.4 microM) and reduced ACh-mediated contraction of the guinea-pig isolated ileum. 2. NaHS also reduced the response of the guinea-pig (EC(50), 80.0+/-5.7 microM) and rat (EC(50), 108.2+/-11.2 microM) ileum preparations to electrical stimulation of the intramural nerves. In guinea-pig ileum this effect was spontaneously reversible and mimicked by sodium nitroprusside (SNP, EC(50), 2.1 microM). Combination of NaHS (20 microM) with SNP (0.5 microM) produced a greater than additive inhibition of the twitch response of the ileum to electrical stimulation. 3. The inhibitory effect of NaHS on the field-stimulated guinea-pig ileum was unaffected by pretreatment with L-NAME (100 microM), indomethacin (10 microM), naloxone (1 microM) or glibenclamide (100 microM). Furthermore, NaHS (200 microM) did not affect the contractile response of the ileum to KCl (10 to 60 mM). 4. Propargylglycine (PAG, 1 mM) and beta-cyanoalanine (BCA, 1 mM) (inhibitors of cystathionine-gamma-lyase) but not aminooxyacetic acid (AOAA, 1 mM) (inhibitor of cystathionine-beta-synthetase) caused a slowly developing increase in the contraction of the guinea-pig ileum to field stimulation. This effect was reversed by cysteine (1 mM). 5. These results show that NaHS relaxes gastrointestinal and urogenital smooth muscle and suggest that H(2)S is responsible for these effects. The possibility that endogenous H(2)S, formed as a consequence of activation of intramural nerves, plays a part in controlling the contractility of the guinea-pig ileum is discussed.

Rhodanese distribution in porcine (Sus scrofa) tissues

The enzyme rhodanese (thiosulfate/cyanide sulfurtransferase) is an ubiquitous enzyme and its activity is present in all living organisms from bacteria to man. Evidence has been accumulated to indicate that this enzyme plays a central role in cyanide detoxification. A comparison was made of rhodanese activity in different tissues of young male and adult male and female pig (Sus scrofa). The highest activity of rhodanese was in liver and kidney cortex of all animals. Among the remaining tissues examined, the kidney medulla and the stomach epithelium tended to have higher levels than other tissues, although this was not significant (P>0.05). The rhodanese activity of heart ventricle tissue of 6-month-old male animals was higher than 7-week-old male animals (P<0.05), and 6-month-old male animals had higher rhodanese activity in lung tissue, compared to 6-month-old female pigs (P<0.05). Medulla and spleen of younger male animals exhibited higher levels of activity (P<0.10) compared to older male pigs. The results of this study may indicate the involvement of rhodanese in cyanide detoxification in pig tissues, which have greater potential to be exposed to higher levels of cyanide.

In vitro and in vivo comparison of sulfur donors as antidotes to acute cyanide intoxication

Antidotes for cyanide (CN) intoxication include the use of sulfane sulfur donors (SSDs), such as thiosulfate, which increase the conversion of CN to thiocyanate by the enzyme rhodanese. To develop pretreatments that might be useful against CN, SSDs with greater lipophilicity than thiosulfate were synthesized and assessed. The ability of SSDs to protect mice against 2LD50 of sodium cyanide (NaCN) administered either 15 or 60 min following administration of an SSD was assessed. To study the mechanism of action of the SSD, the candidate compounds were examined in vitro for their effect on rhodanese and 3-mercaptopyruvate sulfurtransferase (MST) activity under increasing SSD concentrations. Tests were conducted on nine candidate SSDs: ICD1021 (3-hydroxypyridin-2-yl N-[(N-methyl-3-aminopropyl)]-2-aminoethyl disulfide dihydrochloride), ICD1022, (3-hydroxypyridin-2-yl N-[(N-methyl-3-aminopropyl)]-2-aminoethyl disulfide trihydrochloride), ICD1584 (diethyl tetrasulfide), ICD1585 (diallyl tetrasulfide), ICD1587 (diisopropyl tetrasulfide); ICD1738 (N-(3-aminopropyl)-2-aminoethyl 2-oxopropyl disulfide dihydrochloride), ICD1816 (3,3'-tetrathiobis-N-acctyl-L-alanine), ICD2214 (2-aminoethyl 4-methoxyphenyl disulfide hydrochloride) and ICD2467 (bis(4-methoxyphenyl) disulfide). These tests demonstrated that altering the chemical substituent of the longer chain sulfide modified the ability of the candidate SSD to protect against CN toxicity. At least two of the SSDs at selected doses provided 100% protection against 2LD50 of NaCN, normally an LD99. All compounds were evaluated using locomotor activity as a measure of potential adverse behavioral effects. Positive hypoactivity relationships were found with several disulfides but none was found with ICD1584, a tetrasulfide. Separate studies suggest that the chemical reaction of potassium cyanide (KCN) and cystine forms the toxic metabolite 2-iminothiazolidine-4-carboxylic acid. An alternative detoxification pathway, one not primarily involving the sulfur transferases. may be important in pretreatment for CN intoxication. Although studies to elucidate the precise mechanisms are needed. it is clear that these newly synthesized compounds provide a new rationale for anti-CN drugs, with fewer side-effects than the methemoglobin formers.

Hepatocyte-catalysed detoxification of cyanide by L- and D-cysteine

The hepatocyte metabolic pathways involved in the detoxification of cyanide by cysteine have been investigated in vitro using hepatocytes isolated from Sprague-Dawley rats. Cyanide toxicity towards isolated hepatocytes could be prevented by the addition of L- or D-cysteine, cystine, or the cysteine metabolites thiosulfate and mercaptopyruvate, which markedly increased thiocyanate formation. Prior depletion of hepatocyte GSH markedly increased thiosulfate formation from L- or D-cysteine without affecting thiocyanate formation from L- or D-cysteine. This suggested that the major metabolic pathway for thiocyanate formation did not involve thiosulfate. Mercaptopyruvate was a more likely metabolic intermediate, as thiocyanate formation from L-cysteine but not thiosulfate was inhibited markedly by aminooxyacetate, a cysteine aminotransferase inhibitor, and propargylglycine, a gamma-cystathionase inhibitor. Furthermore, propargylglycine prevented L-cysteine cytoprotection against cyanide toxicity. Thiocyanate formation from D-cysteine likely also involved mercaptopyruvate, as thiocyanate formation from D-cysteine but not L-cysteine was inhibited by benzoate, an inhibitor of D-amino acid oxidase. Furthermore, benzoate prevented D-cysteine cytoprotection against cyanide toxicity. Cystine may also be an intermediate, as hepatocyte thiocyanate formation from added L-cysteine was inhibited when L-cysteine autoxidation was prevented with the copper chelator bathocuproine disulfonate. Furthermore, thiocyanate formation by rat liver homogenates with L-cystine was far more rapid than that with L-cysteine. Hepatocyte thiocyanate metabolic intermediates of beta-mercaptopyruvate and thiocystine were proposed for L-cysteine, and beta-mercaptopyruvate was proposed for D-cysteine.