A spectrophotometric method for the direct determination of cysteine in the presence of other naturally occurring amino acids

1. An acid ninhydrin reagent was found to react specifically in forming a pink product (E(max.) 560mmu) with cysteine. 2. The method was highly sensitive for the determination of cysteine (in 28.0x10(3)). Homocysteine, glutathione, proline, ornithine and other naturally occurring amino acids tested did not give a similar reaction. 3. The reaction product was stable for at least 3-4hr. at room temperature and the extinction was proportional to the concentration in the range 0.05-0.5mumole of cysteine. 4. The acid ninhydrin reagent also gave yellow products (E(max.) 370-404mmu) with tryptophan, 5-hydroxytryptophan, 5-hydroxytryptamine and indol-3-ylacetic acid. 5. The method was applied for the determination of cysteine in perchloric acid extracts of rat brain, liver and blood.     

Potential Production and Detoxification of Penicillic Acid in Mold-Fermented Sausage (Salami)

About 10% of 346 Penicillium cultures isolated from mold-fermented sausage synthesized the toxic metabolite penicillic acid on liquid media. Five of these producing cultures inoculated onto sausage failed to produce this toxin in up to 70 days of ripening. Several amino acids normally occurring in meat (cysteine, glutathione, arginine, histidine, and lysine) were found capable of readily reacting with penicillic acid. The adducts formed by the reaction between cysteine or glutathione with penicillic acid were identified and found to be non-toxic to mice, quails, and in the rabbit skin test but exhibited toxicity to the chick embryo. Hypotheses accounting for this residual toxicity are advanced.     

Screening of Thiol Compounds: Depolarization-Induced Release of Glutathione and Cysteine from Rat Brain Slices

Superfusates from rat brain slices were screened for thiol compounds after derivatization with monobromobimane by reversed-phase HPLC. Only glutathione and cysteine were detected. The Ca(2+)-dependent release of these compounds from slices of different regions of rat brain was investigated, applying a highly sensitive and reproducible quantification method, based on reduction of superfusates with dithiothreitol, reaction of thiols with iodoacetic acid, precolumn derivatization with o-phthalaldehyde reagent solution, and analysis with reversed-phase HPLC. This methodology allowed determination of reduced and total thiols in aliquots of the same superfusates. Mostly reduced glutathione and cysteine were released upon K+ depolarization and the Ca2+ dependency suggests that they originate from a neuronal compartment. The GSH release was most prominent in the mesodiencephalon, cortex, hippocampus, and striatum and lowest in the pons-medulla and cerebellum. This underscores a physiologically significant role for glutathione in CNS neurotransmission.     

Astrocytes provide cysteine to neurons by releasing glutathione

Cysteine is the rate-limiting precursor of glutathione synthesis. Evidence suggests that astrocytes can provide cysteine and/or glutathione to neurons. However, it is still unclear how cysteine is released and what the mechanisms of cysteine maintenance by astrocytes entail. In this report, we analyzed cysteine, glutathione, and related compounds in astrocyte conditioned medium using HPLC methods. In addition to cysteine and glutathione, cysteine-glutathione disulfide was found in the conditioned medium. In cystine-free conditioned medium, however, only glutathione was detected. These results suggest that glutathione is released by astrocytes directly and that cysteine is generated from the extracellular thiol/disulfide exchange reaction of cystine and glutathione: glutathione + cystine<-->cysteine + cysteine-glutathione disulfide. Conditioned medium from neuron-enriched cultures was also assayed in the same way as astrocyte conditioned medium, and no cysteine or glutathione was detected. This shows that neurons cannot themselves provide thiols but instead rely on astrocytes. We analyzed cysteine and related compounds in rat CSF and in plasma of the carotid artery and internal jugular vein. Our results indicate that cystine is transported from blood to the CNS and that the thiol/disulfide exchange reaction occurs in the brain in vivo. Cysteine and glutathione are unstable and oxidized to their disulfide forms under aerobic conditions. Therefore, constant release of glutathione by astrocytes is essential to maintain stable levels of thiols in the CNS.     

Inhibition of tubulin polymerization by hypochlorous acid and chloramines

Protein thiol oxidation and modification by nitric oxide and glutathione are emerging as common mechanisms to regulate protein function and to modify protein structure. Also, thiol oxidation is a probable outcome of cellular oxidative stress and is linked to degenerative disease progression. We assessed the effect of the oxidants hypochlorous acid and chloramines on the cytoskeletal protein tubulin. Total cysteine oxidation by the oxidants was monitored by labeling tubulin with the thiol-selective reagent 5-iodoacetamidofluorescein; by reaction with Ellman's reagent, 5,5'-dithiobis(2-nitrobenzoic acid); and by detecting interchain tubulin disulfides by Western blot under nonreducing conditions. Whereas HOCl induced both cysteine and methionine oxidation of tubulin, chloramines were predominantly cysteine oxidants. Cysteine oxidation of tubulin, rather than methionine oxidation, was associated with loss of microtubule polymerization activity, and treatment of oxidized tubulin with disulfide reducing agents restored a considerable portion of the polymerization activity that was lost after oxidation. By comparing the reactivity of hypochlorous acid and chloramines with the previously characterized oxidants, peroxynitrite and the nitroxyl donor Angeli's salt, we have identified tubulin thiol oxidation, not methionine oxidation or tyrosine nitration, as a common outcome responsible for decreased polymerization activity.     

A new colorimetric technique for the estimation of vitamin C using Folin phenol reagent

A new colorimetric technique for the estimation of ascorbic acid by using Folin phenol reagent has been developed. The absorption maximum of the color developed by the interaction of ascorbic acid with Folin reagent is 760 nm. The technique obeys the Beer-Lambert law up to a concentration of 45 μg ascorbic acid as shown by the standard curve. The color developed has been found to be stable up to 18 h. Recovery experiments showed that the technique is almost 100% efficient. The development of the color is not obstructed by glucose, glutathione, bovine serum albumin, urea, cysteine, adenine, guanine, cytosine, uracil, sulfosalicylic acid, thymol, or oxyhemoglobin, which are compounds suspected of interfering in routine analysis. The technique is simple, quick, and efficient and can be employed for the estimation of ascorbic acid in a wide variety of biological materials.     

Inhibition of production of macrophage-derived angiogenic activity by the anti-rheumatic agents gold sodium thiomalate and auranofin

We have investigated the effect of gold sodium thiomalate and auranofin, gold compounds employed in the treatment of rheumatoid arthritis, on production of macrophage-derived angiogenic activity. Elicited mouse peritoneal macrophages were cultured in the presence or absence of gold compounds or thiomalic acid, and the macrophages or their conditioned media were then assayed for their angiogenic activity in rat corneas. Control macrophage conditioned medium was potently angiogenic. In contrast, conditioned medium from gold or thiomalic acid treated macrophages was not. Addition of gold compounds or thiomalic acid to control macrophage conditioned medium did not inhibit its angiogenic activity. Drug treatments did not significantly affect macrophage lactate dehydrogenase release, lysozyme release, or protein synthesis. We conclude that gold sodium thiomalate and auranofin potently reduce the detectable angiogenic activity produced by macrophages.     

Synthesis of N-(9-Acridinyl)maleimide,a Fluorometrical Reagent for Thiol Compounds

The authors have reported¹⁾ a new maleimide type fluorescent thiol reagent, N-(9-acridinyl)- maleimide (NAM). In this paper the syntheses of NAM and its coupling products with thiol compounds are presented. NAM was synthesized from 9-aminoacridine and maleic anhydride through dehydratic cyclization in polyphosphoric acid. NAM showed no substantial fluorescence but its coupling products with thiol compounds exhibited strong blue fluorescence. Application of NAM for the fluorometrical analysis of cysteine and glutathione are suggested.     

Thiols and the diazo group in photoaffinity labels

The photoactivable carbene precursor, 2-diazo-3,3,3-tri-fluoro-propionyloxy group, has been introduced recently for light-induced covalent crosslinking in studies of protein-phospholipid interactions in biomem-branes. The diazo group in this reagent has now been shown to undergo reduction in the dark by a number of thiols (dithiothreitol, 2-mercaptoethanol, cysteine and reduced glutathione) used commonly as protective agents for proteins. In contrast, thioglycolate did not cause significant reduction and, therefore, can be safely used as a protective agent. In vesicles formed from phospholipids containing the above photolabel in the fatty acyl chain, dithiothreitol and 2-mercaptoethanol caused reduction. However, cysteine and reduced glutathione caused insignificant reduction of the diazo group, presumably because of their non-permeant nature.     

Determination of intracellular glutathione in human skeletal muscle by reversed-phase high-performance liquid chromatography

A chromatographic method for the specific determination of cellular low molecular mass thiols has been applied to human muscle tissue. The method is based on the derivatisation of thiols using monobromobimane, which is a specific reagent for the sulphydryl group. The glutathione and cysteine bimane adducts were separated by reversed-phase HPLC, whilst quantitation of the cysteine and glutathione adducts was achieved by fluorescence spectroscopy. The method was found to yield a quantitative recovery of glutathione (ca. 96%), to be sensitive (down to 20 pmol glutathione/per injection) and reveal a low intra-individual coefficient of variation (C.V. < 5%) of the glutathione concentrations in human skeletal muscle. The concentrations of reduced and total glutathione were 1320 ± 37 μmol/kg wet weight (mean ± S.E.M.) and 1525 ± 66 μmol/kg wet weight, respectively. The method was also applied to tissues from nine healthy volunteers to determine if fluctuations in glutathione level occurred over a 24-h period. No diurnal variation of glutathione level in human skeletal muscle was observed.     

The biosynthesis of glutathione in human erythrocytes (author's transl)]

The concentrations of glutathione precursors in human erythrocytes were investigated. 300muM glutamate, 375 muM glycine, and 10muM cysteine were found by automated amino acid analysis. The concentration of 2-aminobutyrate, the precursor of ophthalmic acid, was 15muM. The influence of the activities of endogenous or added glutamyl-cysteine synthetase and glutathione synthetase on the rate of glutathione biosynthesis was measured in membrane-free hemolysates under physiological conditions. The results show that the rate of the overall biosynthesis mainly depends on the formation of the dipeptide glutamyl-cysteine. The effect of glutathione precursor concentrations on the synthesis of the tripeptide was investigated at constant (endogenous) activities of the synthesizing enzymes. The rate was not enhanced by addition of glutamate and/or glycine unless cysteine or glutamyl-cysteine was also added. It is concluded that the concentration of cysteine limits the actual rate of the glutamyl-cysteine-synthetase reaction in vivo. No cysteine or bis(glutamyl)cystine was detected in human hemolysate; however, these disulfides were converted to glutathione. This indicates that erythrocytes have an appropriate system for their reduction, since the disulfides themselves are not substrates for the glutathione-synthesizing enzymes. Studies with intact human red cells indicate that the uptake of cysteine is the rate-determining step in the biosynthesis of glutathione.     

Identification and reactivity of the catalytic site of pig liver thioltransferase

The active site cysteine of pig liver thioltransferase was identified as Cys22. The kinetics of the reaction between Cys22 of the reduced enzyme and iodoacetic acid as a function of pH revealed that the active site sulfhydryl group had a pKa of 2.5. Incubation of reduced enzyme with [1-14C]cysteine prevented the inactivation of the enzyme by iodoacetic acid at pH 6.5, and no stable protein-cysteine disulfide was found when the enzyme was separated from excess [1-14C]cysteine, suggesting an intramolecular disulfide formation. The results suggested a reaction mechanism for thioltransferase. The thiolated Cys22 first initiates a nucleophilic attack on a disulfide substrate, resulting in the formation of an unstable mixed disulfide between Cys22 and the substrate. Subsequently, the sulfhydryl group at Cys25 is deprotonated as a result of micro-environmental changes within the active site domain, releasing the mixed disulfide and forming an intramolecular disulfide bond. Reduced glutathione, the second substrate, reduces the intramolecular disulfide forming a transient mixed disulfide which is then further reduced by glutathione to regenerate the reduced enzyme and form oxidized glutathione. The rate-limiting step for a typical reaction between a disulfide and reduced glutathione is proposed to be the reduction of the intramolecular disulfide form of the enzyme by reduced glutathione.     

Inhibition of anthocyanin synthesis in the first internode of Sorghum bicolor by cobaltous ions. The site of action of cobalt

Cobaltous ions (Co²⁺) inhibited light-mediated anthocyanin synthesis and phenylalanine ammonia-lyase (PAL; EC 4. 3. 1. 5) activity in excised first internodes of Sorghum bicolor (L.) Moench. Ethephon (an exogenous source of ethylene) restored anthocyanin synthesis and PAL activity. Sulfhydryl-containing compounds like cysteine and glutathione completely restored anthocyanin synthesis and PAL activity. N-ethylmaleimide (NEM), a sulfhydryl reagent, inhibited anthocyanin synthesis and PAL activity; cysteine or glutathione reversed the effect of NEM. EDTA and other organic acids like citric, malic, oxalic and tartaric acid were also effective in bringing about recovery. It is suggested that the site of action of Co²⁺ might be at sulfhydryl groups or two adjacent carboxylic groups of a macromolecule that is involved either in anthocyanin or ethylene biosynthesis     

Characterization of mevalonate-activating enzymes in the neonatal chick liver.

1. The 5-phosphomevalonate (MVAP) and 5-pyrophosphomevalonate (MVAPP) formation by cell-free extracts from 7-10 days chick liver shows an absolute nucleotide requirement, ATP being the most effective phosphate donor, though ITP and UTP can be used less effectively. 2. Mn2+ is a better activator than Mg2+ at low concentrations (0.1-5.0 mM). At higher concentrations (10.0 mM) Mn2+ produces a clear decrease in the MVAP formation, whereas the maximum MVAPP formation occurs in the presence of 10.0 mM Mg2+. 3. Mevalonate-activating enzymes maintain their activities for 48 hr at 4 degrees C and 24 hr at 37 degrees C. No MVAP is formed when the extracts are heated to 65 degrees C for 10 min. 4. Unlike other vertebrate mevalonate and phosphomevalonate kinases, these enzymes from chick liver are not activated by -SH group protectors as dithiothreitol, reduced glutathione, cysteine or beta-mercaptoethanol. However, the enzymes are found to be sensitive to thiol binding reagents p-hydroxymercuribenzoate and 5,5'-dithiobis-(2-nitrobenzoic acid).     

Donor substrate specificity and thiol reduction of glutathione disulfide peroxidase.

By isolation of a mixed disulfide product of glutathione and cysteine, glutathione peroxidase was shown to be highly specific for only one donor substrate. Using the coupled assay of NADPH and yeast glutatione reductase, which is highly specific for flutathione disulfide, it was shown that the apparent inhibition of glutathione peroxidase by mercaptoethanol can be described kinetically and that it is competitive with glutathione. Also, when limiting amounts of hydroperoxide were present in the reaction mixture with mercaptoethanol or cysteine, the total amount of glutathione disulfide produced decreased as compared with that in a reaction mixture without mercaptoethanol or cysteine. This finding is consistent with enzymatic formation of mixed disulfides. Data presented suggest that the selenium in glutathione peroxidase was oxidized to a seleninic acid in the absence of glutathione. These results can be explained by a mechanism for glutathione peroxidase wherein the selenium atom is the only atom in the enzyme that undergoes oxidation reduction.     

Some complexes of 2.3-Dimercaptopropanol (BAL) with gold(I) and gold(III)

2, 3-Dimercaptopropanol (BAL) reacted with the tetrachloroaurate ion in solution to form a series of insoluble polymers of definite stoichiometry. Both gold(I) and gold(III) have been identified in these compounds. Reaction of BAL with the tetrabromoaurate ion and with thiomalic acid and D-penicillamine complexes of gold also produced insoluble precipitates. However, with an L-cysteine complex of gold no precipitate was isolated, although there was evidence of replacement of cysteine with BAL. The implications of these results for the use of BAL in cases of gold toxicity are discussed     

A residue critical for flavin binding in flavocytochrome b2 from Baker's yeast. Inactivation and labeling of flavin-free enzyme by 2-keto-3-butynoate

The reagent 2-keto-3-butynoic acid is the product formed in the reaction between the suicide reagent 2-hydroxy-3-butynoate and a number of flavoproteins. We describe in this paper the inactivation of flavin-free flavocytochrome b2 by 2-keto-3-butynoate, in a rapid reaction which introduces 0.9 mol reagent for total inactivation. The modification results in loss of affinity for flavin and affects a cysteine residue. We also describe in this paper a simple enzymatic method for preparing 2-keto-3-butynoate, as well as some properties of the reagent, in particular its stability and susceptibility to nucleophilic attack. We show that at neutral pH it is highly specific for thiol compounds. Some properties of the adduct formed with glutathione are described. These experiments should pave the way for the use of 2-keto-3-butynoate with other proteins.     

Modulation of the redox state of tubulin by the glutathione/glutaredoxin reductase system

Alterations in the redox status of proteins have been implicated in the pathology of several neurodegenerative diseases. We report that peroxynitrite-induced disulfides in porcine brain tubulin are repaired by the glutaredoxin reductase system composed of glutathione reductase, human or Escherichia coli glutaredoxin, reduced glutathione, and NADPH. Reduction of disulfide bonds between the alpha- and beta-tubulin subunits by the glutathione reductase system was assessed by Western blot. Tubulin cysteine oxidation and reduction was quantitated by monitoring the incorporation of 5-iodoacetamido-fluorescein, a thiol-specific labeling reagent. Tubulin disulfide bond reduction by the glutaredoxin reductase system restored tubulin polymerization activity that was lost following peroxynitrite addition. In support of redox modulations of tubulin by glutathione, thiol-disulfide exchange between tubulin and oxidized glutathione was detected and quantitated by HPLC. In addition, glutathionylation of tubulin was detected by dot blot using an anti-GSH antibody.     

The formation and biotransformation of cysteine conjugates of halogenated ethylenes by rabbit renal tubules

The nephrotoxicity of chlorotrifluoroethylene (CTFE) was examined using isolated rabbit renal tubules suspensions. Exposure of the tubules to CTFE resulted in consumption of CTFE, formation of a glutathione conjugate and inhibition of active organic acid transport. Synthetic cysteine, N-acetylcysteine or glutathione conjugates of CTFE inhibited transport indicating S-conjugation as a possible toxic pathway. 1,2-dichlorovinyl glutathione (DCVG), a model synthetic glutathione conjugate, was used to examine the degradation and toxicity of these conjugates. DCVG inhibited rabbit renal tubule transport in vivo and in vitro. The DCVG was found to be degraded with the evolution of glutamine and glycine to produce the ultimate nephrotoxicant, dichlorovinyl cysteine. Dichlorovinyl cysteine is then bioactivated with the release of ammonia. This sequential degradation explains the latency of DCVG-induced renal transport inhibition relative to dichlorovinyl cysteine. It is now evident that certain halogenated ethylenes are capable of being biotransformed to glutathione conjugates in the kidney with their subsequent hydrolysis to nephrotoxic cysteine conjugates.     

The identification of a structurally important cysteine residue in the glycerol dehydrogenase from Bacillus stearothermophilus

Evidence is presented to demonstrate that the Zn2+ metallo-enzyme glycerol dehydrogenase from the thermophile Bacillus stearothermophilus has one cysteine residue per subunit which is only available for reaction with thiol reagents in the metal-depleted form of the enzyme. Modification of the metal-depleted enzyme by methyl methanethiosulphonate prevents the reactivation of the enzyme by Zn2+ ions and induces dissociation of the oligomer into subunits. The rate of reaction of the cysteine residue with the thiol reagent DTNB is limited by a factor other than reagent concentration and it is proposed that the reagent only reacts with the cysteine residue in dissociated monomers. The enzyme has been labelled at the single cysteine residue by radioactive iodo[2-3H]acetic acid. Two radiolabelled peptides have been isolated and sequenced; one peptide is a component of the other. Spectroscopic evidence suggests that the cysteine residue is not involved in ligation of the essential metal ion. Chemical modification studies using the reagent diethylpyrocarbonate have suggested that two histidines are involved in the ligation of the metal.