A quantitating reagent for methanethiolation of protein sulfhydryl groups

p-Nitrophenoxycarbonyl methyl disulfide has been synthesized for use as a quantitating agent for methanethiolation of protein sulfhydryl groups. This reagent reacts specifically and quantitatively with cysteine residues of proteins to yield an unsymmetrical disulfide containing a CH₃S group and concomitantly releases the chromophore, p-nitrophenol. Titration of the sulfhydryl groups of glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12) with this reagent has been studied. Incorporation of CH₃S as measured by the release of p-nitrophenol paralleled the loss of sulfhydryl group dependent activity of the enzyme. The enzyme was found inactive on modification of four of the eight sulfhydryl groups present in the enzyme. Stability of p-nitrophenoxycarbonyl methyl disulfide has also been studied in different buffer systems. The rate of decomposition of the p-nitrophenyl ester due to hydrolysis was found negligible below a pH of 8.0 compared to its rate of reaction with free sulfhydryl groups.     

New reagents for the introduction of the thiomethyl group at sulfhydryl residues of proteins with concomitant spectrophotometric titration of the sulfhydryls: Methyl 3-nitro-2-pyridyl disulfide and methyl 2-pyridyl disulfide

Two new reagents for the titration of sulfhydryl groups in peptides and proteins and for their temporary blocking with the thiomethyl group have been developed. The sulfhydryl groups in cysteine, glutathione, and papain react quantiatively under mild conditions with these reagents, methyl 3-nitro-2-pyridyl disulfide and methyl 2-pyridyl disulfide, with concomitant methanethiolation and without the need to employ a large excess of reagent. Because of the chromophoric properties of the 3-nitro-2-thiopyridone and 2-thiopyridone products, spectrophotometric titration of the sulfhydryl groups can be carried out, accompanying their methanethiolation. The modification of the sulfhydryl groups in peptides and proteins with thiomethyl is rapidly and completely reversible upon addition of thiols such as l-cysteine.     

A general strategy for site-specific double labeling of globular proteins for kinetic FRET studies

Site-directed mutagenesis provides a straightforward means of creating specific targets for chemical modifications of proteins. This capability enhanced the applications of spectroscopic methods adapted for addressing specific structural questions such as the characterization of partially folded and transient intermediate structures of globular proteins. Some applications such as the steady state or time-resolved fluorescence resonance energy transfer (FRET) detection of the kinetics of protein folding require relatively large quantities (approximately 10-100 mg) of site-specific doubly labeled protein samples. Engineered cysteine residues are common targets for labeling of proteins. The challenge here is to develop methods for selective modification of one of two reactive sulfhydryl groups in a protein molecule. A general systematic procedure for selective labeling of each of two cysteine residues in a protein molecule was developed, using Escherichia coli adenylate kinase (AKe) as a model protein. Potential sites for insertion of cysteine residues were selected by examination of the crystal structure of the protein. A series of single-cysteine mutants was prepared, and the rates of the reaction of each engineered cysteine residue with a reference reagent [5,5'-dithiobis(2-nitrobenzoic acid) (DTNB)] were determined. Two-cysteine mutants were prepared by selection of pairs of sites for which the ratio of this reaction rate constant was high (>80). The conditions for the selective labeling reaction were optimized. In a first cycle of labeling, the more reactive cysteine residue was labeled with a fluorescent probe (donor). The second probe was attached to the less reactive site under unfolding conditions in the second cycle of labeling. The doubly and singly labeled mutants retained full enzymatic activity and the capacity for a reversible folding-unfolding transition. High yields (70-90%) of the preparation of the pure, site-specific doubly labeled AK mutant were obtained. The procedure described herein is a general outline of procedures, which can meet the double challenge of both site specificity and large-scale preparation of doubly labeled proteins.     

Determination of the Cysteine and Cystine Content of Proteins by Amino Acid Analysis: Application to the Characterization of Disulfide-Coupled Folding Intermediates

A method has been developed for the simultaneous detection of cysteine and cystine in proteins by amino acid analysis. In this method, the sulfhydryl groups of the cysteine residues are first blocked with 2-aminoethyl methanethiosulfonate (AEMTS). This reagent converts all free sulfhydryl groups to mixed disulfides with 2-aminoethanethiol (AET). The isolated blocked protein is subjected to oxidation with performic acid prior to hydrolysis and amino acid analysis. This procedure quantitatively converts the 2-aminoethanethiol blocking groups into taurine, and all cysteine residues (including those involved in disulfide bonds) into cysteic acid. Both of these derivatives are stable and can be recovered quantitatively by amino acid analysis. The speed and specificity with which AEMTS reacts with thiols make this method particularly effective for the characterization of disulfide-coupled folding intermediates.     

2,4-Dinitrophenyl [14C]cysteinyl disulfide allows selective radioactive labeling of protein thiols under spectrophotometric control

2,4-Dinitrophenyl [1-14C]cysteinyl disulfide readily introduces by disulfide exchange [14C]cysteine as a label into proteins with exposed thiols. The release of an equivalent amount of colored 2,4-dinitrothiophenolate allows the labeling reaction to be followed spectrophotometrically. In reaction with two cysteine residues of rabbit skeletal muscle actin, the thiol selectivity of the reagent corresponded to that of 5,5'-dithiobis(2-nitrobenzoic acid) (Ellman's reagent) and was superior to that of N-[14C]ethylmaleimide. Labeling of single SH groups of actin and papain proceeded faster than titration with Ellman's reagent under the same conditions. The [14C]cysteine label could be removed under mild conditions, e.g., with dithiothreitol, but proved to be stable during cyanogen bromide degradation of the protein and peptide purification. 2,4-Dinitrophenyl cysteinyl disulfide can be easily prepared within a few hours.     

Reactivities of sulfhydryl groups in native and metal-free aminoacylase I

Aminoacylase I from porcine kidney (EC 3.5.1.14) contains seven cysteine residues per subunit. Three sulfhydryl groups are accessible to modification by 4-hydroxymercuribenzoate (p-MB). The kinetics of the reaction suggest that only one of these groups affects acylase activity when modified by p-MB. Its reaction rate increases 2-3-fold when the essential metal ion of aminoacylase is removed. Modification of metal-free apoenzyme by N-ethylmaleimide (NEM) abolishes its activity without impairing Zn2+ binding. This indicates that the sulfhydryl group reacting with NEM is not directly coordinated to the metal. DTNB (5,5'-Dithio-bis(2-nitrobenzoate), Ellman's reagent) also modifies three sulfhydryl groups per subunit. In this case, the reactivities of native aminoacylase and apoenzyme are not significantly different. N-Hydroxy-2-aminobutyrate, a strong aminoacylase inhibitor, substantially increases the reactivity of the slowest reacting sulfhydryl in both native enzyme and metal-free aminoacylase. It appears that binding of the inhibitor or removal of the metal ion induces conformational changes of the amino-acylase active site that render a buried sulfhydryl group more accessible to modification.     

A water-soluble, monitorable peptide and protein crosslinking agent

A novel, freely water-soluble, heterobifunctional crosslinking reagent, N-maleimido-6-aminocaproyl ester of 1-hydroxy-2-nitro-4-benzenesulfonic acid (mal-sac-HNSA), was synthesized and used for conjugation of sulfhydryl (cysteine)-containing peptides to carrier proteins. Reaction with amino groups releases the dianion phenolate, HNSA, which allows convenient spectrophotometric quantitation of the reaction in progress. Since mal-sac-HNSA is completely water soluble, its concentration can be adjusted to maximize the rate of amine reaction and to minimize hydrolysis. Conjugates of peptides to appropriate carriers have elicited peptide-specific antibody and did not elicit detectable antibody specific to the crosslink.     

3-(N-Maleimido-propionyl)biocytin: a versatile thiol-specific biotinylating reagent

A biotin-containing, thiol-specific reagent, 3-(N-maleimido-propionyl) biocytin (MPB), was synthesized and used to biotinylate various proteins via native or artificially induced sulfhydryl groups. In combination with appropriate avidin- or streptavidin-conjugated markers (i.e., fluorescent, enzyme-conjugated, electron-dense, etc.), MPB essentially constitutes a universal, multipurpose, thiol-specific probe. The reagent could be used to detect protein SH groups on dot blots with sensitivities in the femtomole range. The labeling was very specific for sulfhydryl groups or reduced S-S bonds; proteins lacking free SH groups were unlabeled by this method. Due to the long spacer between the biotinyl group and the reactive maleimide, improved adsorption of biotinylated proteins to avidin columns was achieved. An SH-containing enzyme (beta-galactosidase) was biotinylated with MPB, and the resultant biotinylated enzyme could be used as an efficient histochemical probe. The use of this reagent is recommended to biotinylate proteins which contain nonessential SH groups or which can be easily thiolylated prior to reaction with MPB.     

Formation of S-(carboxymethyl)-cysteine in rat liver mitochondrial proteins: effects of caloric and methionine restriction

Maillard reaction contributes to the chemical modification and cross-linking of proteins. This process plays a significant role in the aging process and determination of animal longevity. Oxidative conditions promote the Maillard reaction. Mitochondria are the primary site of oxidants due to the reactive molecular species production. Mitochondrial proteome cysteine residues are targets of oxidative attack due to their specific chemistry and localization. Their chemical, non-enzymatic modification leads to dysfunctional proteins, which entail cellular senescence and organismal aging. Previous studies have consistently shown that caloric and methionine restrictions, nutritional interventions that increase longevity, decrease the rate of mitochondrial oxidant production and the physiological steady-state levels of markers of oxidative damage to macromolecules. In this scenario, we have detected S-(carboxymethyl)-cysteine (CMC) as a new irreversible chemical modification in mitochondrial proteins. CMC content in mitochondrial proteins significantly correlated with that of the lysine-derived analog N ^ε-(carboxymethyl)-lysine. The concentration of CMC is, however, one order of magnitude lower compared with CML likely due in part to the lower content of cysteine with respect to lysine of the mitochondrial proteome. CMC concentrations decreases in liver mitochondrial proteins of rats subjected to 8.5 and 25 % caloric restriction, as well as in 40 and 80 % methionine restriction. This is associated with a concomitant and significant increase in the protein content of sulfhydryl groups. Data presented here evidence that CMC, a marker of Cys-AGE formation, could be candidate as a biomarker of mitochondrial damage during aging.     

Differential response of cysteine residues in pig muscle phosphoglucose isomerase to seven sulfhydryl-modifying reagents

The three cysteine residues per subunit of pig muscle phosphoglucose isomerase show different reactivities toward various sulfhydryl reagents. The organomercurial, p-mercuribenzoate, can titrate two of the sulfhydryl groups under nondenaturing conditions. 2,2'-Dithiodipyridine, 5,5'-dithiobis(2-nitrobenzoic acid), iodoacetamide, methyl 2-pyridyl disulfide, and 2-(2'-pyridylmercapto)mercuri-4-nitrophenol all label only one sulfhydryl group under the same conditions, whereas iodoacetic acid does not react with any of the sulfhydryl groups except when the enzyme is fully denatured. It is concluded, therefore, that charge, rather than steric restraint, is the determining factor for the differences seen in the modification patterns of the enzyme by these reagents. When enzyme was first labeled with 2,2'-dithiodipyridine and subsequently with p-mercuribenzoate, it was found that the latter, in a secondary process, will stoichiometrically react with the anion released by the former after the initial reaction with cysteine. The differences in reactivity of the cysteine residues toward the referred-to reagents have been exploited to specifically modify each of the three individual cysteine residues of pig muscle phosphoglucose isomerase.     

Assignment of catalytically essential cysteine residues in aspartase by selective chemical modification with N-(7-dimethylamino-4-methylcoumarynyl)maleimide

N-(7-Dimethylamino-4-methylcoumarynyl)maleimide (DACM), a fluorescent reagent for sulfhydryl groups, was employed to determine the functionally essential cysteine residues in aspartase from Escherichia coli. Analysis of the tryptic peptides containing DACM-labeled residues by reverse phase HPLC revealed that Cys-140 and Cys-430 were selectively modified, among 11 residues whose loci were recently determined by a DNA sequencing study (Takagi, J.S., et al. (1985) Nucl. Acids Res. 13, 2063-2074). When the modification was carried out in the presence of Mg2+ and L-aspartate, the enzyme activity remained unchanged and no cysteine residue was modified. This suggests that two cysteine residues are located at the L-aspartate binding site and that at least one of them is involved in the catalytic reaction.     

Effects of aminoethylation of cysteine residues on the structural and functional activities of the Escherichia coli 30 S ribosomal proteins

The purified 30 S ribosomal proteins from Escherichia coli strain Q13 were chemically modified by reaction with ethyleneimine, specifically converting cysteine residues to S-2-aminoethylcysteine residues. Proteins S1, S2, S4, S8, S11, S12, S13, S14, S17, S18 and S21 were found to contain aminoethylcysteine residues after modification, whereas proteins S3, S5, S6, S7, S9, S10, S15, S16, S19 and S20 did not. Aminoethylated proteins S4, S13, S17 and S18 were active in the reconstitution of 30 S ribosomes and did not have altered functional activities in poly(U)-dependent polyphenylalanine synthesis, R17-dependent protein synthesis, fMet-tRNA binding and Phe-tRNA binding. Aminoethylated proteins S2, S11, S12, S14 and S21 were not active in the reconstitution of complete 30 S ribosomes, either because the aminoethylated protein did not bind stably to the ribosome (S2, S11, S12 and S21) or because the aminoethylated protein did not stabilize the binding of other ribosomal proteins (S14). The functional activities of 30 S ribosomes reconstituted from a mixture of proteins containing one sensitive aminoethylated protein (S2, S11, S12, S14 or S21) were similar to ribosomes reconstituted from mixtures lacking that protein. These results imply that the sulfhydryl groups of the proteins S4, S13, S17 and S18 are not necessary for the structural or functional activities of these proteins, and that aminoethylation of the sulfhydryl groups of S2, S11, S12, S14 and S21 forms either a kinetic or thermodynamic barrier to the assembly of active 30 S ribosomes in vitro.     

N-ethyl maleimide as a probe for the study of functional sites and conformations of 30 S ribosomal subunits

Escherichia coli 30 S ribosomal subunits are inactive in a number of specific functions when Mg²⁺ concentration is reduced to 1 mM, and activity is recovered on heating under appropriate ionic conditions. When active and inactive forms were treated with N-ethyl maleimide, both forms reacted to a similar extent, but the reagent attached mostly to different proteins. Moreover, it caused irreversible inactivation only when reacting with the inactive form of the subunit. Though the activating treatment failed to restore activity to these subunits it did expose the same sulfhydryl groups as are available in the active state for reaction with the maleimide.Different ribosomal activities were eliminated at different maleimide concentrations, permitting the assignment of specific functions to sulfhydryl groups of specific ribosomal proteins. Protein S18 appears to be involved in subunit association, binding of fMet-tRNA and of aminoacyl-tRNA to the P-site. Proteins S1, S14 and S21 are all or in part involved in the binding of aminoacyl-tRNA to the A-site and in the binding of the antibiotic dihydrostreptomycin.The reaction with N-ethyl maleimide thus provides a criterion other than biological activity for characterizing different ribosomal forms and a tool for mapping the 30 S subunit for specific functional sites.     

Sulfhydryl Localization and Tetrazolium Reduction. 1. Reversible Inhibition of its Reduction by N-Ethyl Maleimide

The sulfhydryl inhibitor N-ethyl maleimide completely inhibited the reduction of 2,3,5-triphenyltetrazolium chloride in meristematic and embryonic vascular tissues of Coleus sp. stems, Ricinus communis root tips, ungerminated Tea mays embryos, and epicotyls and coleoptiles of germinated Tea mays embryos, in a concentration of 200 mg/lit. Inhibition was reversed by the addition of cysteine or reduced glutathione (200 mg/lit) to the inhibitor medium. N-ethyl maleimide was effective also in blocking the nitro-prusside and 1-(4-chIoromercuriphenylazo)-naphthol-2 sulfhydryl staining reactions, but other substituted maleimides were ineffective in inhibiting the tetrazolium reaction in these tissues. Experiments were conducted to determine the histological pattern of sulfhydryl groups as indicated by a modification of the Bennett 1-(4-chloro-mercuriphenylazo)-naphthol-2 test and a modification of the Rap-kine nitroprusside test in certain plant tissues. A positive correlation was observed between tissues reducing the tetrazolium indicator and tissues exhibiting sulfhydryl localization as indicated by the nitroprusside reagent (trichloroacetic acid pretreated) and 1—(4— chloromercuriphenylazo)—naphthol—2.     

Reactive cysteine in proteins: protein folding, antioxidant defense, redox signaling and more

Cysteine plays structural roles in proteins and can also participate in electron transfer reactions, when some structural folds provide appropriated environments for stabilization of its sulfhydryl group in the anionic form, called thiolate (RS(-)). In contrast, sulfhydryl group of free cysteine has a relatively high pK(a) (8,5) and as a consequence is relatively inert for redox reaction in physiological conditions. Thiolate is considerable more powerful as nucleophilic agent than its protonated form, therefore, reactive cysteine are present mainly in its anionic form in proteins. In this review, we describe several processes in which reactive cysteine in proteins take part, showing a high degree of redox chemistry versatility.     

Demonstration of the feasibility of observing nuclear magnetic resonance signals of 77Se covalently attached to proteins

Previous 77Se NMR relaxation time studies established the utility of 77Se NMR spectroscopy in studying low molecular weight (less than 500) selenium-containing molecules. Since the spin rotation and chemical shift anisotrophy mechanisms contributed significantly to the 77Se spin-lattice relaxation in these compounds, it was questionable as to whether the latter mechanism would be efficient enough to enable 77Se resonances to be observed in a reasonable period in high molecular weight selenobiomolecules. Thus, to address this problem, disulfide bonds of ribonuclease-A and lysozyme were reductively cleaved under denaturing conditions, and the resulting 7-8 sulfhydryl groups were treated with a new sulfhydryl group reagent containing selenium, 6,6'-diselenobis(3-nitrobenzoic acid), to give proteins containing covalently attached selenium in the form of selenenyl sulfides. The observation of high resolution 77Se NMR spectra of these proteins under denaturing conditions was accomplished. Five to six 77Se NMR resonances, which fell in a chemical shift range of 14-15 ppm, were observed for each protein and are compared to the chemical shifts of several model selenenyl sulfides derived from cysteine.     

Sulfhydryls of tubulin. A probe to detect conformational changes of tubulin.

The 20 cysteine residues of tubulin are heterogeneously distributed throughout its three-dimensional structure. In the present work, we have used the reactivity of these cysteine residues with 5, 5'-dithiobis(2-nitrobenzoic acid) (DTNB) as a probe to detect the global conformational changes of tubulin under different experimental conditions. The 20 sulfhydryl groups can be classified into two categories: fast and slow reacting. Colchicine binding causes a dramatic decrease in the reactivity of the cysteine residues and causes complete protection of 1.4 cysteine residues. Similarly, other colchicine analogs that bind reversibly initially decrease the rate of reaction; but unlike colchicine they do not cause complete protection of any sulfhydryl groups. Interestingly, in all cases we find that all the slow reacting sulfhydryl groups are affected to the same extent, that is, have a single rate constant. Glycerol has a major inhibitory effect on all these slow reacting sulfhydryls, suggesting that the reaction of slow reacting cysteines takes place from an open state at equilibrium with the native. Ageing of tubulin at 37 degrees C leads to loss of self-assembly and colchicine binding activity. Using DTNB kinetics, we have shown that ageing leads to complete protection of some of the sulfhydryl groups and increased reaction rate for other slow reacting sulfhydryl groups. Ageing at 37 degrees C also causes aggregation of tubulin as indicated by HPLC analysis. The protection of some sulfhydryl groups may be a consequence of aggregation, whereas the increased rate of reaction of other slow reacting sulfhydryls may be a result of changes in global dynamics. CD spectra and acrylamide quenching support such a notion. Binding of 8-anilino-1-naphthalenesulfonate (ANS) and bis-ANS by tubulin cause complete protection of some cysteine residues as indicated by the DTNB reaction, but has little effect on the other slow reacting cysteines, suggesting local effects.     

Rapid inactivation of 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), a potent mutagen in chlorinated drinking water,by sulfhydryl compounds

The mutagenic activity of 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX), which is formed during chlorination of drinking water, was effectively inhibited by sulfhydryl compounds such as cysteine, cysteamine, glutathione, dithiothreitol and 2-mercaptoethanol. Preincubation of 0.5 μg MX with 15 μg cysteine (molar ratio 1:37) in a phosphate buffer (pH 6.0–8.0) at 37°C for 15 min prior to exposure of bacterial cells depleted the mutagenic activity of MX. Together with the result showing a change in the UV spectra, it is suggested that sulfhydryl compounds inactivate MX by direct chemical interaction before MX induces DNA damage. On the other hand, a variety of antioxidants other than the sulfhydryl compounds showed no inhibitory effects. Investigation using structural analogs of cysteine revealed that the thiol moiety was indispensable for antimutagenic activity and the amino moiety appeared to enhance the MX-inactivating reaction of the SH group.     

A sensitive gel filtration method for determination of protein sulfhydryl groups with 14C-chloromercuribenzoate

A gel filtration method employing ¹⁴C-chloromercuribenzoic acid is described for the quantitative determination of sulfhydryl groups in microgram quantities of protein. The method has been applied to several native proteins, hemoglobin, monoamine oxidase, and yeast cytochrome c. In all cases values in close agreement with known literature values were obtained. Horse heart cytochrome c and lysozyme, which have no sulfhydryl groups, did not bind the mercurial reagent. Modifications of the method are described for determining the sulfhydryl content of denatured proteins in the presence of sodium lauryl sulfate. The precision of the method was found to be compatible with known methods for determining the sulfhydryl composition of proteins.     

Effect of ligand-induced conformational changes on the reactivity of specific sulfhydryl residues in rat brain hexokinase

Rat brain hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) contains 21 cysteine residues. On the basis of the amino acid sequence of the enzyme, these are predicted to be distributed among 14 peptides produced by tryptic digestion. Ten of these peptides, containing cysteine residues derivatized by reaction with the specific sulfhydryl reagent 2-bromoacetamido-4-nitrophenol have been identified in HPLC peptide maps; the four missing peptides are predicted to be relatively large and hydrophobic in character, properties that may have prevented their detection under the present conditions. The sequences encompassed by the 10 identified peptides include 12 of the 21 cysteine residues in the enzyme. The relative reactivity of these sulfhydryl groups with 2-bromoacetamido-4-nitrophenol has been assessed, and is in general accord with what might be predicted on the basis of their accessibility in the previously proposed structure for this enzyme. The effect of various ligands on reactivity of identified sulfhydryl groups has been determined; unique patterns of altered reactivity, resulting from ligand-induced conformational changes, have been observed. Biphasic effects were observed with increasing concentrations of either glucose 6-phosphate (Glc-6-P) or Pi. In both cases, decreased reactivity of sulfhydryls in the N-terminal half of the molecule was observed at low concentrations of the ligand, while further increase in ligand concentration resulted in decreased reactivity of sulfhydryl groups in the C-terminal half. In contrast, sulfhydryls in both N- and C-terminal halves were protected concomitantly by increasing concentrations of Glc. These results are consistent with previous studies that indicated (a) the existence of two sites for binding of Glc-6-P or Pi, a high affinity site in the N-terminal half and a site with lower affinity in the C-terminal half of the brain hexokinase molecule, and (b) binding of Glc to a single site located in the C-terminal half but evoking conformational effects throughout the molecule; the glucose analog, N-acetylglucosamine, previously shown to have more limited effects on conformation, affected reactivity of sulfhydryl groups only in the C-terminal half of the molecule. As reflected by effects on reactivity of sulfhydryl groups, conformational changes induced by binding of nucleotides depends markedly on the specific nature of the purine or pyrimidine base as well as the length and chelation status of the polyphosphate side chain. These results focus attention on specific regions of the molecule (the immediate environment of the sulfhydryl groups) that are affected by the binding of these ligands.