Long-term decomposition of aqueous S-nitrosoglutathione and S-nitroso-N-acetylcysteine: Influence of concentration, temperature, pH and light

Primary S-nitrosothiols (RSNOs) have received significant attention for their ability to modulate NO signaling in many physiological and pathophysiological processes. Such actions and their potential pharmaceutical uses demand a better knowledge of their stability in aqueous solutions. Herein, we investigated the effects of concentration, temperature, pH, room light and metal ions on the long-term kinetic behavior of two representative primary RSNOs, S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylcysteine (SNAC). The thermal decomposition of GSNO and SNAC were shown to be affected by the auto-catalytic action of the thiyl radicals. At 25 °C in the dark and protected from the catalytic action of metal ions, GSNO and SNAC solutions 1 mM showed half-lives of 49 and 76 days, and apparent activation energies of 84 ± 14 and 90 ± 6 kJ mol^-1, respectively. Both GSNO and SNAC exhibited increased stability in the pH range 5-7. At high pH the decomposition pathway of GSNO involves the formation of an intermediate (GS-NO₂^2-), which decomposes generating GSH and nitrite. GSNO solutions displayed lower sensitivity to the catalytic action of metal ions than SNAC and the exposure to room light led to a 5-fold increase in the initial rates of decomposition of both RSNOs. In all comparisons, SNAC solutions showed higher stability than GSNO solutions. These findings provide strategic information about the stability of GSNO and SNAC and may open new perspectives for their use as experimental or therapeutic NO donors.

Investigation of the presence in human urine of mercapturic acids derived from phenanthrene, a representative polycyclic aromatic hydrocarbon

Polycyclic aromatic hydrocarbons (PAH) are environmental carcinogens implicated as causes of cancer in certain industrial settings and in cigarette smokers. PAH require metabolic activation to exert their carcinogenic effects. One widely accepted pathway of metabolic activation proceeds through formation of "bay region" diol epoxides which are highly reactive with DNA and can cause mutations. Phenanthrene (Phe) is the simplest PAH with a bay region and an excellent model for the study of PAH metabolism. In previous studies in which [D10]Phe was administered to smokers, we observed higher levels of [D10]Phe-tetraols derived from [D10]Phe-diol epoxides in subjects who were null for the glutathione-S-transferase M1 (GSTM1) gene. We hypothesized that Phe-epoxides, the primary metabolites of Phe, were detoxified by glutathione conjugate formation, which would result ultimately in the excretion of the corresponding mercapturic acids in urine. We synthesized the four stereoisomeric mercapturic acids that would result from attack of glutathione on Phe-epoxides followed by normal processing of the conjugates. We also synthesized the corresponding dehydrated metabolites and sulfoxides. These 12 standards were used in liquid chromatography-nanoelectrospray ionization-high resolution tandem mass spectrometry analysis of urine samples from smokers and creosote workers, the latter exposed to unusually high levels of PAH. Only the sulfoxide derivatives were consistently detected in the urine of creosote workers; none of the compounds was detected in the urine of smokers. These results demonstrate a new pathway of PAH-mercapturic acid formation, but do not provide an explanation for the role of GSTM1 null status on Phe-tetraol formation.

Regulation of STAT3 and NF-κB activations by S-nitrosylation in multiple myeloma

Numerous reports suggest that aberrant activations of STAT3 and NF-κB promote survival and proliferation of multiple myeloma (MM) cells. In the present report, we demonstrate that a synthetic S-nitrosothiol compound, S-nitroso-N-acetylcysteine (SNAC), inhibits proliferation and survival of multiple MM cells via S-nitrosylation-dependent inhibition of STAT3 and NF-κB. In human MM cells (e.g. U266, H929, and IM-9 cells), SNAC treatment increased S-nitrosylation of STAT3 and NF-κB and inhibited their activities. Consequently, SNAC treatment resulted in MM cell cycle arrest at G1/S check point and inhibited their proliferation. SNAC also decreased the expression of cell survival factors and increased the activities of caspases, thus increased sensitivity of MM cells to melphalan, a chemotherapeutic agent for MM. In U266 xenografted mice, SNAC treatment decreased the activity of STAT3 and reduced the growth of human CD138 positive cells (U266 cells) in the bone marrow and also reduced their production of human IgE into the serum. Taken together, these data document the S-nitrosylation mediated inhibition of MM cell proliferation and cell survival via inhibition of STAT3 and NF-κB pathways and its efficacy in animal model of MM.

S-allylmercapto-N-acetylcysteine up-regulates cellular glutathione and protects vascular endothelial cells from oxidative stress

Oxidative stress and/or low cellular glutathione (GSH) levels are associated with the development and progression of numerous pathological conditions. Cells possess various antioxidant protection mechanisms, including GSH and phase II detoxifying enzymes. N-acetylcysteine (NAC) supplies cells with cysteine to increase GSH level but its efficacy is relatively low because of its limited tissue penetration. Allicin (diallyl thiosulfinate), a reactive sulfaorganic compound, increases cellular GSH and phase II detoxifying enzymes in vascular endothelial cells (EC). A novel compound was designed: S-allylmercapto-N-acetylcysteine (ASSNAC), a conjugate of S-allyl mercaptan (a component of allicin) and NAC. Both ASSNAC and NAC increased cellular GSH of ECs, reaching a maximum of up to four- and threefold increase after exposure for 24 or 6 h at a concentration of 0.2 or 1 mM, respectively. ASSNAC induced nuclear translocation of the activated transcription factor Nrf2 and expression of phase II detoxifying enzymes. EC exposure to tBuOOH resulted in 75% cytotoxicity, and pretreatment of cultures with 0.2 mM ASSNAC or 2mM NAC reduced cytotoxicity to 20 and 42%, respectively. In conclusion, ASSNAC is superior to NAC in protecting cells from oxidative stress because of its ability to up-regulate both GSH and the expression of phase II detoxifying enzymes.

[Classification and synthesis of ubiquitin-proteasome inhibitor]

The inhibition of protein degradation through the ubiquitin-proteasome pathway is a recently developed approach to cancer treatment which extends the range of cellular target for chemotherapy. This therapeutic strategy is very interesting since the proteasomes carry out the regulated degradation of unnecessary or damaged cellular proteins, a process that is dysregulated in many cancer cells. Based on this hypothesis, the proteasome complex inhibitor Bortezomib was approved for use in multiple myeloma patients by FDA in 2003. Drug discovery programs in academy and the pharmaceutical industry have developed a range of synthetic and natural inhibitors of the 20S proteasome core particle that have entered human clinical trials as significant anti-cancer leads. The main results from the use of proteasome inhibition in cancer chemotherapy, the structure of several proteasome inhibitors and their synthesis is going to be reviewed in this paper.

Synthesis and characterization of mercapturic acid (N-acetyl-L-cysteine)-aflatoxin B1 adduct and its quantitation in rat urine by an enzyme immunoassay

A simple and sensitive indirect noncompetitive enzyme immunoassay to quantitate mercapturic acid-aflatoxin B1 (AFB1) adduct in rat urine is reported. A novel procedure was developed for in vitro synthesis of an immunogen, bovine serum albumen-glutathione-aflatoxin B1 (BSA-GSH-AFB1) using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride. Sulphydryl group's analysis confirmed the conjugation of-SH groups to AFB1. Thin-layer chromatography and spectral analysis (absorption, fluorescence, and Fourier transform infrared) of the conjugates further confirmed the formation of the adducts. Polyclonal antibodies specific to mercapturic acid-AFB1 adduct were produced against BSA-GSH-AFB1. The assay was found to be linear in the range of 100 pg-100 ng of the analyte (y = a + bx). A 50% displacement of BSA-GSH-AFB1 antibodies was achieved at an inhibitory concentration (IC50) of 11.9 ng GSH-AFB1 (r2 = 0.98) and 1.22 ng N-acetyl-L-cysteine (NAC)-AFB1 (r2 = 0.98). Spiking 5 microg/mL of reference standard to the control rat urine showed a recovery of 98 +/- 2%. The immunoassay was validated in a rodent model exposed to a single oral dose of 1 mg/kg body mass of pure AFB1. The excretion of NAC-AFB1 adduct was quantitated at the end of 24 h. The concentration of the NAC-AFB1 adduct excreted in urine as determined by the immunoassay was found to be in the range of 3.22-5.97 microg/mg creatinine. The present method may find wide application as a biochemical tool in molecular epidemiological and intervention studies with respect to human exposure to dietary aflatoxins.

S-nitrosothiol-modified dendrimers as nitric oxide delivery vehicles

The synthesis and characterization of two generation-4 polyamidoamine (PAMAM) dendrimers with S-nitrosothiol exteriors are reported. The hyperbranched macromolecules were modified with either N-acetyl-D, L-penicillamine (NAP) or N-acetyl-L-cysteine (NACys) and analyzed via 1H and 13C NMR, UV absorption spectroscopy, MALDI-TOF mass spectrometry, and size exclusion chromatography. Treatment of the dendritic thiols with nitrite solutions yielded the corresponding S-nitrosothiol nitric oxide (NO) donors (G4-SNAP, G4-NACysNO). Chemiluminescent NO detection demonstrated that the dendrimers were capable of storing approximately 2 micromol NO x mg (-1) when exposed to triggers of S-nitrosothiol decomposition (e.g., light and copper). The kinetics of NO release were found to be highly dependent on the structure of the nitrosothiol (i.e., tertiary vs primary) and exhibited similar NO release characteristics to classical small molecule nitrosothiols reported in the literature. As a demonstration of utility, the ability of G4-SNAP to inhibit thrombin-mediated platelet aggregation was assayed. At equivalent nitrosothiol concentrations (25 microM), the G4-SNAP dendrimer resulted in a 62% inhibition of platelet aggregation, compared to only 17% for the small molecule NO donor. The multivalent NO storage, the dendritic effects exerted on nitrosothiol stability and reactivity, and the utility of dendrimers as drug delivery vehicles highlight the potential of these constructs as clinically useful S-nitrosothiol-based therapeutics.

Alpha,beta-unsaturated beta-silyl imide substrates for catalytic, enantioselective conjugate additions: a total synthesis of (+)-lactacystin and the discovery of a new proteasome inhibitor

Chiral (salen)Al mu-oxo dimer 1 catalyzes the highly enantioselective conjugate addition of carbon-centered nucleophiles to alpha,beta-unsaturated silyl imides. Allyldimethylsilane-substituted imide 4 was identified as an optimal substrate, undergoing addition reactions with a variety of nitrile nucleophiles in high yield and enantiomeric excess. The silicon-containing products are synthetically useful chiral building blocks, as demonstrated by their application to an enantioselective total synthesis of the potent proteasome inhibitor (+)-lactacystin (2). Elaboration of lactam 5a to the natural product was effected in 12 steps and in 11% overall yield and proceeded through an unusual spiro beta-lactone intermediate (11). This compound was found to inhibit the chymotrypsin-like site of the 26S proteasome at similar levels to known inhibitor clasto-lactacystin beta-lactone (omuralide).

Total Synthesis of Lactacystin and Salinosporamide A

Lactacystin and salinosporamide A are fascinating molecules with regard to both their chemical structures and biological activities. These naturally occurring compounds are potent and selective proteasome inhibitors. The molecular structures are characterized by their densely functionalized gamma-lactam cores. The structure and biological properties of these two compounds are attracting the attention of many chemists as challenging synthetic targets. We discuss their synthetic strategies in this review.

Investigation of bioactivation and toxicity of styrene in CYP2E1 transgenic cells

Styrene has been found to be toxic to the respiratory system, and the toxicity of styrene is metabolism-dependent. CYP2E1 is suggested to be one of the cytochrome P450 enzymes responsible for the bioactivation of styrene. Our work focused on the roles of CYP2E1 and epoxide, a metabolite of styrene epoxidation, in the cytotoxicity of styrene. Styrene was found to be more toxic to h2E1 cells than to the wild type, while there was no difference found when styrene oxide was administered. Both soluble and microsomal epoxide hydrolase inhibitors dramatically enhanced styrene toxicity. Glutathione and glutathione ethyl ester showed protection against styrene cytotoxicity. Cytotoxicity of a selection of styrene analogues, such as ethylbenzene, vinylcyclohexane, and ethylcyclohexane, was assessed to determine if unsaturation is required for styrene toxicity. Ethylbenzene and vinylcyclohexane were found to be as toxic as styrene to h2E1 cells, whereas little toxicity of ethylcyclohexane to h2E1 cells was observed. This indicates the importance of vinyl group of styrene in its cytotoxicity, but saturation of the vinyl group does not necessarily eliminate styrene toxicity. An N-acetylcysteine conjugate derived from styrene oxide was identified by LC/MS/MS in the sample obtained from the incubation of h2E1 cell lysate with styrene in the presence of N-acetylcysteine. Formation of the N-acetylcysteine conjugate was found to be NADPH-dependent. These studies provided strong evidence in support of toxic role of styrene epoxide metabolite in styrene toxicity.

Catalytic Asymmetric Total Synthesis of (+)-Lactacystin

Total synthesis of (+)-lactacystin, a potent and selective proteasome inhibitor, was accomplished using a catalytic enantioselective Strecker reaction of a ketoimine as the initial key step. An enone-derived N-phosphinoyl ketoimine 7 was selected as a stable masked alpha-hydroxy ketoimine analogue. Excellent enantioselectivity (98% ee) and practical catalyst activity were produced under the optimized catalyst preparation method using 2.5 mol % Gd{N(SiMe3)2}3 as a metal source and 3.8 mol % D-glucose-derived ligand 8. This reaction was conducted on a 5 g scale. The chiral tetrasubstituted C-5 carbon efficiently controlled the stereochemistry of the other three chiral centers of lactacystin. Chelation-controlled Meerwein-type reduction of ketone 5 using i-PrMgBr (originally reported by Kang in a related substrate) selectively produced the desired secondary alcohol at the C-9 position. The C-6 hydroxy and C-7 methyl groups were introduced via a silyl conjugate addition followed by the Tamao oxidation and Donohoe methylation, respectively, in a highly stereoselective manner. A practical amount of enantiomerically pure clasto-lactacystin beta-lactone (2), the biologically active form of (+)-lactacystin, can be synthesized using this route. clasto-Lactacystin beta-lactone (2) was converted to (+)-lactacystin following the reported procedure.

Development of Monoclonal Antibodies to 4-Hydroxyestrogen-2-N-Acetylcysteine Conjugates: Immunoaffinity and Spectroscopic Studies

Catechol estrogen quinones (CEQ) derived from oxidation of the catechol estrogens 4-hydroxyestrone (4-OHE1) and 4-hydroxyestradiol (4-OHE2) can conjugate with glutathione (GSH), a reaction that prevents damage to DNA and can provide biomarkers of exposure to CEQs. Monoclonal antibodies (MAb) to 4-OHE1(E2)-2-N-acetylcysteine [4-OHE1(E2)-2-NAcCys] were developed and characterized by immunological and spectroscopic studies. The NAcCys conjugate is the hydrolytic product of the corresponding conjugate with GSH, followed by N-acetylation of cysteine. MAbs were produced by immunizing mice with 4-OHE1(E2)-2-NAcCys attached to an appropriate linker that was conjugated to keyhole limpet hemocyanin (KLH). Hybridoma cell lines were screened using 4-OHE1(E2)-2-NAcCys conjugated to ovalbumin (OA). There is no immunological cross-reactivity between KLH and OA. Hence, positive hybridoma cell lines secreting antibody against 4-OHE1(E2)-2-NAcCys could be rapidly identified using OA-4-OHE1(E2)-2-NAcCys. An affinity column was developed and used to purify MAb against 4-OHE1(E2)-2-NAcCys. The purified MAb was immobilized on an agarose bead column. This column was used to capture and preconcentrate the hapten of interest out of urine samples. A number of structurally related standards were used to estimate the selectivity and specificity of the chosen MAb. Capillary electrophoresis (CE) with field-amplified sample stacking in absorbance detection mode and laser-induced low temperature luminescence measurements were used to identify and quantitate the 4-OHE1(E2)-2-NAcCys conjugates and related compounds released from the affinity column. Femtomole detection limits have been demonstrated. Future prospects in clinical diagnostics for testing human exposure to CEQ by urine analysis are briefly addressed.

A formal synthesis of (+)-lactacystin

A synthetic route to the neurotrophic agent (+)-lactacystin has been developed utilizing a base-promoted intramolecular alkylidenecarbene C-H insertion as the key transformation.

A Concise Route to (+)-Lactacystin

A facile chromatography-free route to Kang's intermediate for the synthesis of (+)-lactacystin, a potent proteasome inhibitor, has been developed starting with Brown's asymmetric crotylation of tert-butyl 5-formyl-2,2-dimethyl-1,3-dioxan-5-ylcarbamate, easily available from 2-amino-2-(hydroxymethyl)propane-1,3-diol (Tris).

Synthesis and biological evaluation in human monocyte-derived macrophages of N-(N-acetyl-L-cysteinyl)-S-acetylcysteamine analogues with potent antioxidant and anti-HIV activities

We synthesized a series of N-(N-acetyl-L-cysteinyl)-S-acetylcysteamine (10) analogues bearing various acyl groups on thiol cysteine or cysteamine residues, to investigate the structure-activity relationship for pro-GSH and anti-HIV properties in human macrophages. The S-substituents were ranked in the following order of efficacy: H > or = acetyl > isobutyryl > pivaloyl > benzoyl. We found that none of these derivatives had pro-GSH or antiviral activities in vitro higher than that of 10, but several displayed similar levels of anti-HIV activity, making them possible candidates for use as adjuvant therapies in conjunction with HAART, for treating neurological aspects of HIV infection.

Synthesis of isothiocyanate-derived mercapturic acids

Twelve mercapturic acids derived from saturated and unsaturated aliphatic and aromatic isothiocyanates were synthesised, by adding isothiocyanate to a solution of N-acetyl-L-cysteine and sodium bicarbonate, in a typical yield of 77%. Isothiocyanates were synthesised first by adding the corresponding alkyl bromide to phthalimide potassium salt. The obtained N-alkyl-phthalimide was hydrazinolysed yielding the alkyl amine, which subsequently was reacted with thiophosgene yielding the isothiocyanate with an overall yield of 16%. Mercapturic acids in urine can serve as a biomarker of intake to determine the health promoting potential of isothiocyanates present in cruciferous vegetables.

Short and versatile route to a key intermediate for lactacystin synthesis

[reaction: see text] A key intermediate 14 for the synthesis of lactacystin 1 has been constructed in four steps and 33% overall yield. The key steps involve cyclization of a suitably functionalized glutamic acid derivative and concomitant alkylation of the resulting beta,beta-diketoester system, C-acylation of the cyclic alpha-amidoketone 9, and decarboxylbenzylation of 12. Alkylation of a related beta,beta-diketoester 5 was additionally achieved with several electrophiles.

Identification of cytosolic leucyl aminopeptidase (EC 3.4.11.1) as the major cysteinylglycine-hydrolysing activity in rat liver

Cysteinylglycine hydrolysis is a step in the metabolism of glutathione and glutathione S-conjugates. We had previously observed that in rat liver the enzymatic activity is predominantly located in the cytosol. Here we demonstrate that cytosolic leucyl aminopeptidase (EC 3.4.11.1) is the major cysteinylglycine hydrolysing activity in rat liver. Evidence was obtained from the use of peptidase inhibitors and from immunoprecipitation studies using Pansorbin-coupled antibodies raised against hog kidney cytosolic leucyl aminopeptidase. Both isolated cytosolic leucyl aminopeptidase and the cysteinylglycine-hydrolysing activity in rat liver cytosol are bound with equal efficiency to the affinity matrix. We demonstrate that cytosolic leucyl aminopeptidase exhibits leucinamidase and cysteinylglycinase activity. Cysteinylglycine, cystinyl-bis-glycine, S-nitrosocysteinylglycine, and bimane-S-cysteinylglycine are hydrolysed at high rates; low activity is seen with leukotriene D4. Our findings establish a previously unrecognised physiological function of cytosolic leucyl aminopeptidase, participating in glutathione metabolism and in the degradation of glutathione S-conjugates via the mercapturic acid pathway.

Synthesis of 9,9,9-trideutero-1,4-dihydroxynonane mercapturic acid (d3-DHN-MA), a useful internal standard for DHN-MA urinalysis

Racemic 1,4-dihydroxynonane mercapturic acid (DHN-MA) and 9,9,9-trideutero-1,4-dihydroxynonane mercapturic acid (d3-DHN-MA) are synthesized on a 400-mg scale (overall yield approximately 40%) by a two-step sequence involving Michael addition of N-acetyl-L-cysteine to methyl 4-hydroxynon-2(E)-enoate or methyl 9,9,9-trideutero-4-hydroxynon-2 (E)-enoate, followed by reduction of the intermediate adducts with lithium borohydride. The requisite starting methyl esters are obtained, respectively, from heptanal or 7,7,7-trideuteroheptanal and methyl 4-chlorophenylsulfinylacetate via a sulfoxide piperidine and carbonyl reaction described in the literature. The 7,7,7-trideuteroheptanal is easily prepared by classical methods in four steps from 6-bromo-1-hexanol. 13C NMR data indicate that DHN-MA as well as d3-DHN-MA are obtained as mixtures of four diastereomers. Preliminary results show that d3-DHN-MA could be used as an internal standard for mass spectrometric quantification of DHN-MA in human urine.

A convenient method for 3-pyrroline synthesis

[reaction: see text]. The synthesis of a range of 3-pyrrolines has been achieved from primary amine starting materials using a two-step alkylation/alkylidene carbene CH-insertion reaction sequence. We have shown that insertion into a range of CH-bond types is possible, and the formation of nitrogen-bearing quaternary stereocenters is a relatively facile process. The insertion reaction occurs with >95% retention of stereochemistry, but the presence of protecting groups on nitrogen is generally deleterious to the cyclization process.

NAC/MEA conjugate: a new potent antioxidant which increases the GSH level in various cell lines

I-152 is a prodrug of NAC and MEA with potent pro-GSH effects in human macrophages, astrocytes and lymphocytes. This molecule could be of interest in HIV infection in respect to its antioxidant and anti-HIV activities, but also in other diseases to counteract oxidative stress, that is, inflammation, cardiovascular diseases, and neurodegenerative diseases.

Prebiotic formation of 'energy-rich' thioesters from glyceraldehyde and N-acetylcysteine

The 'energy-rich' thioester, N-acetyl-S-lactoylcysteine, is formed from low concentrations of glyceraldehyde and N-acetylcysteine under anaerobic conditions at ambient temperature in aqueous solutions of sodium phosphate (pH 7.0). Reactions with 2 mM glyceraldehyde, 2 mM N-acetylcysteine, and 500 mM sodium phosphate (pH 7.0) convert about 3%/day of the glyceraldehyde to lactoyl thioester. The formation of lactoyl thioester in similar reactions with 500 mM imidazole hydrochloride (pH 7.0) is supported by the thiol-dependence of lactate formation, which is 3-fold greater in the presence of thiol (0.11%/day) than in the absence of thiol (0.04%/day). The formation of lactoyl thioester is thought to proceed by the phosphate (or imidazole)-catalyzed dehydration of glyceraldehyde to give pyruvaldehyde, which adds to the thiol to form a hemithioacetal that rearranges to the thioester. A limited amount of a second thioester, N-acetyl-S-glyceroyl-cycsteine, is also formed at the beginning of these reactions. The significance of these reactions to the origin of life is discussed.

Reversal of regioselection in the sharpless asymmetric aminohydroxylation of aryl ester substrates

[formula: see text] The asymmetric synthesis of beta-hydroxy-alpha-amino acids is reported which relies on the use of alpha,beta-unsaturated aryl ester substrates and the dihydroquinyl alkaloid ligand system (DHQ)2-AQN to control the regio- and enantioselectivity of the asymmetric aminohydroxylation (AA) process. alpha,beta-Unsaturated ester substrates of type 1 have a significant effect on the substrate-ligand recognition event which results in a reversal of regioselectivity in the AA reaction.

Capillary electrophoretic separation of N-acetylcysteine and its impurities as a method for quality control of pharmaceuticals

Capillary electrophoresis has been applied to separate and determine N-acetylcysteine (NAC) and related impurities. Determination conditions were found to be optimum with 100 mmol/l borate as the buffer, pH 8.40. The limit of detection was established for each substance examined. The method has been validated by examining linearity ranges, precision and repeatability. The method was used to determine the content of NAC in, and purity of, pharmaceutical preparations. The major impurities (N,N-diacetylcystine, N,S-diacetylcysteine and cystine) were determined at levels of 0.1%.

Total synthesis and biological activity of lactacystin, omuralide and analogs

Lactacystin (1) and the related beta-lactone omuralide (2) are remarkably selective and potent irreversible inhibitors of the 20 S proteasome, a large polymolecular protein machine which is responsible for the degradation of ubiquitin-labeled proteins. Because of this fact 1 and 2 have emerged as important tools in biochemistry and cell biology. The challenge of synthesis has been accepted by several research groups with the result that 1 and 2 and their analogs can now be synthesized by a variety of synthetic approaches. This review summarizes the synthetic processes which have been developed to date for the production of such compounds. The study of biological activity of analogs of 1 and 2 has clearly defined the structural features which are responsible for the potency of 1 and 2, as described in the closing section of this account. It is concluded that 1 and 2 are nearly optimal for the irreversible inactivation of the 20 S proteasome.

Acylase I-catalyzed deacetylation of N-acetyl-L-cysteine and S-alkyl-N-acetyl-L-cysteines

The aminoacylase that catalyzes the hydrolysis of N-acetyl-L-cysteine (NAC) was identified as acylase I after purification by column chromatography and electrophoretic analysis. Rat kidney cytosol was fractionated by ammonium sulfate precipitation, and the proteins were separated by ion-exchange column chromatography, gel-filtration column chromatography, and hydrophobic interaction column chromatography. Acylase activity with NAC and N-acetyl-L-methionine (NAM), a known substrate for acylase I, as substrates coeluted during all chromatographic steps. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the protein was purified to near homogeneity and had a subunit Mr of 43 000, which is identical with the Mr of acylase I from porcine kidney and bovine liver. n-Butylmalonic acid was a slow-binding inhibitor of acylase I and inhibited the deacetylation of NAC with a Ki of 192 +/- 27 microM. These results show that acylase I catalyzes the deacetylation of NAC. The acylase I-catalyzed deacetylation of a range of S-alkyl-N-acetyl-L-cysteines, their carbon and oxygen analogues, and the selenium analogue of NAM was also studied with porcine kidney acylase I. The specific activity of the acylase I-catalyzed deacetylation of these substrates was related to their calculated molar volumes and log P values. The S-alkyl-N-acetyl-L-cysteines with short (C0-C3) and unbranched S-alkyl substituents were good acylase I substrates, whereas the S-alkyl-N-acetyl-L-cysteines with long (>C3) and branched S-alkyl substituents were poLr acylase I substrates. The carbon and oxygen analogues of S-methyl-N-acetyl-L-cysteine and the carbon analogue of S-ethyl-N-acetyl-L-cysteine were poor acylase I substrates, whereas the selenium analogue of NAM was a good acylase I substrate.

A rapid, sensitive method for the quantitation of N-acetyl-S-(2-hydroxyethyl)-L-cysteine in human urine using isotope-dilution HPLC-MS-MS

Because of increasing concern about exposure to carcinogens and other toxicants, reliable methods for biological monitoring of potentially exposed populations must be developed. For biological monitoring to be useful, appropriate biomarkers of exposures to xenobiotics must be identified, and sensitive, specific methods for quantifying the targeted biomarker must be developed. We have developed a sensitive and selective method for the analysis of N-acetyl-S-(2-hydroxyethyl)-L-cysteine (HEMA), urinary metabolite of at least three different known human carcinogens (vinyl chloride, ethylene oxide, and ethylene dibromide). The method uses strong anion-exchange solid-phase extraction and isotope-dilution high-performance liquid chromatography-tandem mass spectrometry. Our method is simple and is not labor intensive; the preparation time per sample is less than 10 min. Because urine samples vary in both their concentration and ion strength, intersample variability in HEMA recovery during the extraction is large. To overcome this inherent limitation, we use the isotope-dilution technique, which allows a complete correction for the extraction recovery for each sample. The limit of detection of the method is 0.68 microgram/L in a 1-mL urine sample with a coefficient of variation of 22% (determined by replicate analyses at both 4 and 11 micrograms/L) and an accuracy indistinguishable from 100%. Preliminary analyses of urine from a population with no known overt exposure to the parent toxicants show a frequency of detection of approximately 75%, which indicates that this method has the sensitivity to detect urinary HEMA derived from environmental exposure. We are currently using this method to establish a reference range of background exposure to these toxicants in the U.S. population.

Synthesis and characterization of m- and p-methylbenzyl-mercapturic acids derived from m- and p-xylenes

Chemical synthesis and physical properties of two mercapturic acids suggested as urinary metabolites of m- and p-xylenes are described. These compounds may be used for the identification and quantitative determination by high-performance liquid chromatography of the corresponding mercapturic acids in urine.

Isolation and characterization of N-acetyl-S-[2-carboxy-1-(1 H-imidazol-4-yl) ethyl]-L-cysteine, a new metabolite of histidine, from normal human urine and its formation from S-[2-carboxy-1-(1 H-imidazol-4-yl) ethyl]-L-cysteine

N-Acetyl-S-[2-carboxy-1-(1 H-imidazol-4-yl)ethyl]-L-cysteine (I), a new imidazole compound with a sulfur-containing side chain, was isolated from normal human urine by ion-exchange column chromatography, and characterized by physicochemical analyses involving 1H-NMR spectrometry, mass spectrometry and high-voltage paper electrophoresis as well as chemical synthesis. Approximately five milligrams of crystals of the compound were obtained from 450 litres of the urine. Compound I was synthesized by the addition of N-acetyl-L-cysteine to urocanic acid. The compound was also formed by incubation of S-[2-carboxy-1-(1 H-imidazol-4-yl)ethyl]-L-cysteine (II) with acetyl-CoA in the use of rat kidney or liver homogenate as an enzyme source in a Tris buffer at pH 7.4. Rat brain and spleen homogenates were the less or no effective preparations as the enzyme source. On the other hand, little N-acetylation of a diastereomer of compound II occurred in enzymatic reactions with rat tissue homogenates. Compound I was degraded to compound II by rat kidney or liver homogenate. These results suggest that compound I is a new N-acetylated metabolite of compound II, a compound previously found in human urine, and that the acetylating enzyme recognizes stereoisomerism of asymmetric carbon atoms on the molecule of compound II. These findings support an alternative pathway of L-histidine catabolism initiated by the adduction of glutathione and/or cysteine to urocanic acid, the first catabolite of histidine.

Synthesis of N-acetylcysteine conjugates of catechol estrogens

The synthesis of N-acetylcysteine conjugates of 2-hydroxyestrone (2-OHE1) and 4-hydroxyestrone (4-OHE1) is described. The reaction of estrone 2,3-quinone with N-acetylcysteine provided 2-OHE1 and its C-4 and C-1 thioether conjugates in a ratio of 1:1, while estrone 3,4-quinone with N-acetylcysteine gave 4-OHE1 and its C-2 thioether conjugate as a sole product. Their structures were characterized by inspection of NMR spectra, chemical derivatization (methylation and acetylation), and comparison with the reactivity of 4-bromoestrone 2,3-quinone or 2-bromoestrone 3,4-quinone toward N-acetylcysteine.

Stimulation of erythrocyte membrane Mg(2+)-ATPase activity by glutathione S-conjugates

Pig erythrocyte membrane Mg2+-ATPase activity was stimulated by various glutathione S-conjugates. For alkyl S-conjugates, the Km for the stimulation was lower, the more hydrophobic was the conjugate. 2,4-Dinitrophenyl-S-(N-acetyl)cysteine also stimulated the Mg2+-ATPase activity, suggesting a low specificity of the ¿glutathione S-conjugate pump¿. The Km values for the stimulation by 2,4-dinitrophenyl conjugates were lower than predictable on the basis of hydrophobicity which indicates a high affinity of the transporter for these conjugates.

Acrolein mercapturates: synthesis, characterization, and assessment of their role in the bladder toxicity of cyclophosphamide

Acrolein is the metabolite of cyclophosphamide (CP) believed to be involved in the bladder toxicity associated with this anticancer drug. The mechanism by which this extremely reactive intermediate is delivered to the bladder is not known. Glutathione (GSH) readily conjugates with acrolein, and the acrolein mercapturate S-(3-hydroxypropyl)-N-acetylcysteine (3-hydroxy-PrMCA) has been found in the urine of animals and man given CP. The objectives of this study were to prepare and characterize synthetic standards of the GSH acrolein adduct (3-oxopropyl)glutathione (3-oxoPrGSH), the acrolein mercapturates S-(3-oxopropyl)-N-acetylcysteine (3-oxoPrMCA) and 3-hydroxyPrMCA, and the S-oxidation product of 3-oxoPrMCA (3-oxoPrMCA S-oxide). In addition, the release of acrolein from, and the bladder toxicity of, these conjugates was determined. 3-OxoPrGSH and 3-oxoPrMCA were prepared with a 99% yield by condensing acrolein with GSH and N-acetylcysteine, respectively. 3-HydroxyPrMCA was prepared with a 63% yield by refluxing 3-chloropropanol and N-acetylcysteine in a basic medium. Oxidation of 3-oxoPrMCA with H2O2 was used to prepare 3-oxoPrMCA S-oxide. By decreasing the reaction time to 1 h, and adjusting the ratio of 3-oxoPrMCA to H2O2, the yield of 3-oxoPrMCA S-oxide was increased to 96%. The anhydrous aldehyde, 3-oxoPrMCA, afforded characteristic aldehydic proton resonances (1H NMR) in deuterated dimethyl sulfoxide. New resonances were observed in deuterated water, indicating a 75% hydration of the aldehyde to the corresponding geminal diol. This phenomenon was enhanced with 3-oxoPrMCA S-oxide where approximately 100% hydration of the aldehyde to the corresponding geminal diol was observed. When incubated at 25 degrees C in 100 mM potassium phosphate buffer containing 1 M KCl, pH 8.0, 3-oxoPrMCA released approximately 6% and 3-oxoPrMCA S-oxide released approximately 16-18% of the theoretical maximum yield of acrolein after 30 min, as indicated by an increase in absorbance at 210 nm and confirmed by trapping this aldehyde as a semicarbazone. There was less than a 2% yield of acrolein from 3-hydroxyPrMCA or 3-oxoPrGSH under similar conditions. At pH 7.4 the release of acrolein from 3-oxoPrMCA and 3-oxoPrMCA S-oxide was decreased by 50%. An assay where aldehydes are reacted with m-aminophenol in acid media produced fluorescence consistent with 72%, 46%, 23%, and 1% yields of acrolein from 3-oxoPrMCA S-oxide, 3-oxoPrMCA, 3-oxoPrGSH, and 3-hydroxyPrMCA, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Matlystatins, new inhibitors of typeIV collagenases from Actinomadura atramentaria. I. Taxonomy, fermentation, isolation, and physico-chemical properties of matlystatin-group compounds

During the course of a screening for inhibitors of typeIV collagenases, new metabolites, designated matlystatins, have been isolated from an actinomycete strain, which was identified as a strain of Actinomadura atramentaria. Matlystatins were composed of five congeners, which were separated and purified by n-butanol extraction and chromatography.

Reactions of nitrosonitrobenzenes with biological thiols: identification and reactivity of glutathion-S-yl conjugates

1,3-Dinitrobenzene (1,3-DNB) but not 1,2-dinitrobenzene (1,2-DNB) or 1,4-dinitrobenzene (1,4-DNB) is a potent testicular toxicant in rats. In vitro metabolism studies have established that 1,3-DNB is reduced to 3-nitroso-nitrobenzene (3-NNB), 3-nitrophenylhydroxylamine (3-NP) and 3-nitroaniline (3-NA) in testicular cytosol and Sertoli cell cultures. To establish a potential role for endogenous glutathione (GSH) in the detoxification of the electrophilic metabolite 3-NNB, we examined the chemical reaction of this compound with biological thiols, including GSH. The effect of pH and thiol concentration upon the reaction were studied. The reaction of GSH with 3-NNB was complex and gave three distinct products. These were identified as 3-NP, 3-NA and a glutathionyl derivative containing a covalently linked S-N bond. The hydroxyl amine and the amine were isolated and fully characterised. The glutathion-S-yl derivative was characterised in solution by proton NMR (400 MHz), infra-red and mass spectroscopy to establish its structure as the semimercaptal, N-(glutathion-S-yl)-N-hydroxy-3-nitroaniline (GSNOH-3NA). Similar reactions were performed with 4-nitrosonitrobenzene (4-NNB) to ascertain the reactivity of this chemical towards thiols. The addition of GSH to 4-NNB resulted in the rapid formation of 4-nitrophenylhydroxylamine (4-NP) and an adduct that was identified as the semimercaptal N-(glutathion-S-yl)-N-hydroxy-4-nitroaniline (GSNOH-4NA).

Differences in the localization and extent of the renal proximal tubular necrosis caused by mercapturic acid and glutathione conjugates of 1,4-naphthoquinone and menadione

We have previously demonstrated that administration of various benzoquinol-glutathione (GSH) conjugates to rats causes renal proximal tubular necrosis and the initial lesion appears to lie within that portion of the S3 segment within the outer stripe of the outer medulla (OSOM). The toxicity may be a consequence of oxidation of the quinol conjugate to the quinone followed by covalent binding to tissue macromolecules. We have therefore synthesized the GSH and N-acetylcysteine conjugates of 2-methyl-1,4-naphthoquinone (menadione) and 1,4-naphthoquinone. The resulting conjugates have certain similarities to the benzoquinol-GSH conjugates, but the main difference is that reaction with the thiol yields a conjugate which remains in the quinone form. 2-Methyl-3-(N-acetylcystein-S-yl)-1,4-naphthoquinone caused a dose-dependent (50-200 mumol/kg) necrosis of the proximal tubular epithelium. The lesion involved the terminal portion of the S2 segment and the S3 segment within the medullary ray. At the lower doses, that portion of the S3 segment in the outer stripe of the outer medulla displayed no evidence of necrosis. In contrast, 2-methyl-3-(glutathion-S-yl)-1,4-naphthoquinone (200 mumol/kg) caused no apparent histological alterations to the kidney. 2-(Glutathion-S-yl)-1,4-naphthoquinone and 2,3-(diglutathion-S-yl)-1,4-naphthoquinone (200 mumol/kg) were relatively weak proximal tubular toxicants and the lesion involved the S3 segment at the junction of the medullary ray and the OSOM. A possible reason(s) for the striking difference in the toxicity of the N-acetylcysteine conjugate of menadione, as opposed to the lack of toxicity of the GSH conjugate of menadione, is discussed. The basis for the localization of the lesion caused by 2-methyl-3-(N-acetylcystein-S-yl)-1,4-naphthoquinone requires further study.

Identification of N-acetyl(2,2-dichlorovinyl)- and N-acetyl(1,2-dichlorovinyl)-L-cysteine as two regioisomeric mercapturic acids of trichloroethylene in the rat

The regioselectivity of glutathione conjugation to trichloroethylene (TRI) and the metabolism of its cysteine and N-acetyl-L-cysteine conjugates were investigated in the rat. Intraperitoneal (ip) administration of TRI to rats at a dose of 400 mg/kg resulted in excretion in urine of small amounts of the two distinct regioisomers N-acetyl-S-(1,2-dichlorovinyl)-L-cysteine (1,2-DCV-NAC) and N-acetyl-S-(2,2-dichlorovinyl)-L-cysteine (2,2-DCV-NAC). The vicinal (vic) isomer was excreted in a 2 times higher amount (16 nmol) than the geminal (gem) isomer (8 nmol). Intraperitoneal administration of a 1:1 mixture (2.5 mumol/kg each) of the two regioisomers of S-(dichlorovinyl)-L-cysteine (DCVC) to the rat resulted in excretion of the corresponding mercapturic acids in urine, the main fractions being excreted within 8 h after administration. The gem-dichlorovinyl isomer appeared to be acetylated to a higher extend than the 1,2-dichlorovinyl isomer; 73% vs 50% of the dose administered. Intraperitoneal administration of a 1:1 mixture (12.5 mumol/kg each) of the two regioisomers of N-(trideuterioacetyl)-S-(dichlorovinyl)-L-cysteine (DCV-NAC-d3) resulted in excretion of both deuterium-labeled and unlabeled mercapturic acids in urine. The vic isomer was excreted unchanged at a significantly higher percentage, 34% of dose (i.e., still deuterium labeled), than the gem isomer, 17% of the dose. This suggests less efficient metabolism of the vic isomer when compared to the gem isomer. Both regioisomers of DCV-NAC-d3 were excreted in urine unlabeled at 40% of the dose, which indicates that for both isomers deacetylation, followed by reacetylation (resulting in unlabeled DCV-NAC), is an important metabolic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics of S-[3-chloroprop-2-enyl]glutathione in rats following acute inhalation exposure to 1,3-dichloropropene

1. An h.p.l.c. method has been developed to quantify the GSH conjugate of 1,3-dichloropropene (DCP). 2. The GSH conjugate of DCP (GSCP) was detected in the blood of rats exposed to DCP by inhalation, and elimination of GSCP from rat blood fitted a one-compartment model. 3. Exposure of rats to 78, 155, or 404 ppm DCP gave an elimination t 1/2 of 17 h, independent of exposure concentration.

Nonenzymatic formation of "energy-rich" lactoyl and glyceroyl thioesters from glyceraldehyde and a thiol

The "energy-rich" thioester, N-acetyl-S-lactoylcysteine, is formed under anaerobic conditions from glyceraldehyde and N-acetylcysteine at ambient temperature in aqueous solutions of sodium phosphate (pH 7.0). The conversion of glyceraldehyde to lactoyl thioester occurs at a rate of about 0.4%/day in reactions with 10 mM glyceraldehyde, 10 mM thiol, and 500 mM sodium phosphate (pH 7.0). Thioester formation proceeds at an estimated efficiency of 76%, since a similar reaction with 12.5 mM thiol yields 50.7% lactate at 6 months from only 66.5% of the glyceraldehyde (or its isomer, dihydroxyacetone). The formation of lactoyl thioester most likely occurs by the phosphate-catalyzed dehydration of glyceraldehyde to give pyruvaldehyde, which combines with thiol to form a hemithioacetal that rearranges to the thioester. A second energy-rich thioester, N-acetyl-S-glyceroylcysteine, is also produced from glyceraldehyde when these reactions are carried out in the presence of oxygen and to a limited extent in the absence of oxygen. In the presence of oxygen the formation of glyceroyl thioester continues until the thiol disappears completely by oxidation. The significance of these reactions to the energetics of the origin of life is discussed.

Metabolic activation of acetylenes. Covalent binding of [1,2-14C]octyne to protein, DNA and haem in vitro and the protective effects of certain thiol compounds

[1,2-14C]Oct-l-yne was used to investigate metabolic activation of the ethynyl substituent in vitro. Activation of octyne by liver microsomal cytochrome P-450-dependent enzymes gave intermediate(s) that bound covalently to protein, DNA and to haem. The time course and extent of covalent binding of octyne to haem and to protein were similar. However, two different activating mechanisms are probably involved. Whereas covalent binding to protein or to DNA was inhibited by nucleophiles such as N-acetylcysteine, that to haem was little affected. When N-acetylcysteine was included in the reaction mixtures, two major octyne-N-acetylcysteine adducts were isolated and purified by high-pressure liquid chromatography. G.l.c.-mass spectrometry and n.m.r. suggest that these are the cis-trans isomers of S-3-oxo-oct-1-enyl-N-acetylcysteine. Oct-1-yn-3-one reacted non-enzymically with N-acetylcysteine at pH 7.4 and 37 degrees C with a t1/2 of about 6 s also to yield S-3-oxo-oct-l-enyl-N-acetylcysteine. The same product was formed when microsomal fractions were incubated with oct-1-yn-3-ol, N-acetylcysteine and NAD(P)+. Octyn-3-one did not appear to react with haem or protoporphyrin IX. 5. A mechanism for the metabolic activation of oct-1-yne is proposed, consisting in (a) microsomal hydroxylation of the carbon atom alpha to the acetylenic bond and (b) oxidation to yield octyn-3-one as the reactive species.

Synthesis and relative stereochemistry of the four mercapturic acids derived from styrene oxide and N-acetylcysteine

The chemical reaction between (+/-)-styrene oxide and N-acetylcysteine produces both positional isomers (1 and 2) as a mixture of diastereoisomers with a preference for the benzylic thioether isomer 1 (2 : 1). Synthesis of the mercapturic acid conjugates from either (+)- or (-)-styrene oxide produces only two of the four possible stereoisomers. The single diastereoisomers of 1 and 2 were separated by high pressure liquid chromatography (HPLC) and identified by 1H- and 13C-nuclear magnetic resonance (NMR). The relative stereochemistry at the benzylic carbon center of the mercapturic acid conjugates was assigned on the basis of the established chemical correlation between optically pure styrene oxide and its precursor mandelic acid, and considerations on the mechanism of ring opening of epoxides by sulfur nucleophiles. The stereochemical definition of the isomers 3-6 should prove useful in investigations of the biotransformation of the glutathione (GSH) conjugates of styrene oxide.

Synthesis of N-acetylcysteine compounds

The preparation of some anilides, esters and amides of N-acetyl and N,S-diacetyl-cysteine is described. Of the compounds synthesized, particular interest was aroused by the p-hydroxyanilide derivatives of N-acetyl and N,S-diacetyl-L-cysteine owing to their analgesic, antiinflammatory and antipyretic activity.

Oxidizing action of purine N-oxide esters

A technique involving O-acetylation of purine N-oxide derivatives in buffered aqueous solutions has permitted studies of the reactivity of many compounds for which the O-acetyl derivatives are not otherwise available. The oxidizing properties of a variety of N-acetoxypurines have been measured through their ability to oxidize iodide ion ot iodine, a reaction which is representative of a more general oxidizing ability. Those esters that oxidize iodide ion also catalyze the autoxidation of sulfite, a property characteristic of radicals. The same esters also oxidize cysteine to cysteic acid and tryptophan, tyrosine, and uric acid to yet uncharacterized products. Their oxidizing reactivity was compared with the ability of the same esters to react as electrophiles in another assay that measured the rate of formation of pyridine substitution products. The sulfate ester of 3-hydroxyxanthine has been synthesized. Its reactivity is qualitatively the same as that of 3-acetoxyxanthine but proceeds at a higher rate. Syntheses of S-(8-xanthyl)-N-acetylcysteine, 8-(2-hydroxyethylthio)xanthine, and 1-methyl-8-mehtylmercaptoguanine are also described.