Reflections on the role of the thiol group in biology

Dynamics of dissociative electron attachment to aliphatic thiols

Dissociative electron attachment (DEA) shows functional group-dependent site selectivity in H- ion channels. In this context, thiol functional groups have yet to be studied in great detail, although they carry importance in radiation damage studies where low-energy secondary electrons are known to induce damage through the DEA process. In this context, we report detailed measurements of absolute cross-sections and momentum images of various anion fragments formed in the DEA process in simple aliphatic thiols. We also compare the observed dynamics with that reported earlier in hydrogen sulphide, the precursor molecule for this functional group, and with that in aliphatic alcohols. Our findings show substantial resemblance in the underlying dynamics in these compounds and point to a possible generalisation of these features in the DEA to thiols. In addition, we identify various pathways that contribute to the S- and SH- channels.

Near-Infrared Probes for Biothiols (Cysteine, Homocysteine, and Glutathione): A Comprehensive Review

Biothiols (cysteine, homocysteine, and glutathione) are an important class of compounds with a free thiol group. These biothiols plays an important role in several metabolic processes in living bodies when present in optimum concentration. Researchers have developed several probes for the detection and quantification of biothiols that can absorb in UV, visible, and near-infrared (NIR) regions of the electromagnetic spectrum. Among them, NIR organic probes have attracted significant attention due to their application in in vivo and in vitro imaging. In this review, we have summarized probes for these biothiols, which could work in the NIR region, and discussed their sensing mechanism and potential applications. Along with focusing on the pros and cons of the reported probes we have classified them according to the fluorophore used and summarized their photophysical and sensing properties (emission, response time, limit of detection).

Thiol, volatile and semi-volatile compounds alleviate the stress of zinc oxide nanoparticles of the pomegranate callus

Pomegranate trees are tropical and subtropical shrubs with nutritional benefits and pharmaceutical and therapeutic uses. Antioxidative systems protect the structure and function of cellular membranes. This study demonstrated the connection between oxidative stress generated by excess nanoparticles ZnO (ZnO-NPs) accumulation in pomegranate calli and the involvement of thiol groups and volatile and semi-volatile compounds in alleviating this stress. The effect of the non-enzymatic antioxidant system was studied using callus treated with three levels of ZnO-NPs or bulk particles (ZnO-BPs). With rising ZnO levels in the media, callus growth was gradually decreased by ZnO in both forms (NPs and BPs). Malondialdehyde (MDA) measurements revealed that different concentrations of both forms promoted lipid peroxidation. The supply of both forms had a considerable stimulatory influence on the cysteine (Cys) content in calli. Raised ZnO-NP concentrations increased glutathione (GSH) and non-protein thiols (NPTs) content in calli, but higher ZnO-BP concentrations lowered their content. Conversely, ZnO-NP levels reduced the protein thiols (PTs) content in calli, but ZnO-BP concentrations increased their content. GC-MS analysis was employed to investigate the volatile and semi-volatile chemical profiles within calli following exposure to 0 and 150 μg mL^-1 of ZnO in both forms. GC-MS analysis detected 77, 67, and 83 compounds in ZnO-treated calli, of which 14, 16, and 20 with a similarity value greater than 70%, based on a NIST library, were recognized as metabolites for ZnO untreated and NPs- and BPs-treated calli, respectively. Six substances, including five alkanes and one morphinan, showed similarities in metabolite composition between control and NPs- or BPs-treated calli. ZnO-NPs-treated calli contained two alkane compounds only similar to the control, but ZnO-BPs-treated calli had six metabolites, including four alkanes, one carboxylic acid, and one ester. However, eight alkanes were similar within the callus treated with NPs and BPs.

Determination of low molecular thiols and protein sulfhydryl groups using heterocyclic disulfides

A promising area in the analytical chemistry of thiol-containing compounds is the use of heterocyclic disulfides as analytical agents, but now only a few of them are widely used. In this paper, we evaluate the possibility of using three different heterocyclic disulfides 2,2'-dithiobis[5-phenyl-1,3,4-oxadiazole] (I), 2,2'-dithiobis[benzoxazole] (II) and 8,8'-dithiobis-quinoline (III) as analytical reagents for the low-mass aminothiols cysteine and glutathione determination. The optimal analysis conditions were found. Spectrophotometric, kinetic, CE, and HPLC methods using I, II, III for the determination of cysteine and glutathione were developed. The obtained methods are characterized by accuracy and sensitivity (detection limits in the range of 10^-5-10^-6 M) sufficient to quantify cysteine and glutathione in their physiological concentrations. Finally, the proposed disulfides were used to determine the SH-content in the bovine serum albumin (BSA). Considering a number of criteria (applicable pH range, absorption properties, susceptibility to hydrolysis) it was concluded that the proposed reagents have advantages over the commonly used ones (such as the Ellman reagent).

Methoxy-Substituted Nitrodibenzofuran-Based Protecting Group with an Improved Two-Photon Action Cross-Section for Thiol Protection in Solid Phase Peptide Synthesis

Photoremovable caging groups are useful for biological applications because the deprotection process can be initiated by illumination with light without the necessity of adding additional reagents such as acids or bases that can perturb biological activity. In solid phase peptide synthesis (SPPS), the most common photoremovable group used for thiol protection is the o-nitrobenzyl group and related analogues. In earlier work, we explored the use of the nitrodibenzofuran (NDBF) group for thiol protection and found it to exhibit a faster rate toward UV photolysis relative to simple nitroveratryl-based protecting groups and a useful two-photon cross-section. Here, we describe the synthesis of a new NDBF-based protecting group bearing a methoxy substituent and use it to prepare a protected form of cysteine suitable for SPPS. This reagent was then used to assemble two biologically relevant peptides and characterize their photolysis kinetics in both UV- and two-photon-mediated reactions; a two-photon action cross-section of 0.71-1.4 GM for the new protecting group was particularly notable. Finally, uncaging of these protected peptides by either UV or two-photon activation was used to initiate their subsequent enzymatic processing by the enzyme farnesyltransferase. These experiments highlight the utility of this new protecting group for SPPS and biological experiments.

A programmable chemical switch based on triggerable Michael acceptors

Developing an engineerable chemical reaction that is triggerable for simultaneous chemical bond formation and cleavage by external cues offers tunability and orthogonality which is highly desired in many biological and materials applications. Here, we present a chemical switch that concurrently captures these features in response to chemically and biologically abundant and important cues, viz., thiols and amines. This thiol/amine-triggerable chemical switch is based on a Triggerable Michael Acceptor (TMAc) which bears good leaving groups at its β-position. The acceptor undergoes a "trigger-to-release" process where thiol/amine addition triggers cascaded release of leaving groups and generates a less activated acceptor. The newly generated TMAc can be further reversed to liberate the original thiol/amine by a second nucleophile trigger through a "trigger-to-reverse" process. Within the small molecular volume of the switch, we have shown five locations that can be engineered to achieve tunable "trigger-to-release" kinetics and tailored reversibility. The potential of the engineerable bonding/debonding capability of the chemical switch is demonstrated by applications in cysteine-selective and reversible protein modification, universal self-immolative linkers, and orthogonally addressable hydrogels.

Thioredoxin-1 improves the immunometabolic phenotype of antitumor T cells

Adoptive transfer of tumor epitope-reactive T cells has emerged as a promising strategy to control tumor growth. However, chronically-stimulated T cells expanded for adoptive cell transfer are susceptible to cell death in an oxidative tumor microenvironment. Because oxidation of cell-surface thiols also alters protein functionality, we hypothesized that increasing the levels of thioredoxin (Trx), an antioxidant molecule facilitating reduction of proteins through cysteine thiol-disulfide exchange, in T cells will promote their sustained antitumor function. Using pre-melanosome protein (Pmel)-Trx1 transgenic mouse-derived splenic T cells, flow cytometry, and gene expression analysis, we observed here that higher Trx expression inversely correlated with reactive oxygen species and susceptibility to T-cell receptor restimulation or oxidation-mediated cell death. These Trx1-overexpressing T cells exhibited a cluster of differentiation 62L^hi (CD62L^hi) central memory-like phenotype with reduced glucose uptake (2-NBDG^lo) and decreased effector function (interferon γ^lo). Furthermore, culturing tumor-reactive T cells in the presence of recombinant Trx increased the dependence of T cells on mitochondrial metabolism and improved tumor control. We conclude that strategies for increasing the antioxidant capacity of antitumor T cells modulate their immunometabolic phenotype leading to improved immunotherapeutic control of established tumors.

A peptide-based multifunctional fluorescent probe for Cu2+, Hg2+ and biothiols

A peptide-based fluorescent probe (Dansyl-His-Pro-Gly-Trp-NH₂, D-P4) bearing the dansyl fluorophore and tryptophan residue has been developed for the detection of Hg²⁺, Cu²⁺ and biothiols (–SH).

Non-protein thiol imaging and quantification in live cells with a novel benzofurazan sulfide triphenylphosphonium fluorogenic compound

Thiols (-SH) play various roles in biological systems. They are divided into protein thiols (PSH) and non-protein thiols (NPSH). Due to the significant roles thiols play in various physiological/pathological functions, numerous analytical methods have been developed for thiol assays. Most of these methods are developed for glutathione, the major form of NPSH. Majority of these methods require tissue/cell homogenization before analysis. Due to a lack of effective thiol-specific fluorescent/fluorogenic reagents, methods for imaging and quantifying thiols in live cells are limited. Determination of an analyte in live cells can reveal information that cannot be revealed by analysis of cell homogenates. Previously, we reported a thiol-specific thiol-sulfide exchange reaction. Based on this reaction, a benzofurazan sulfide thiol-specific fluorogenic reagent was developed. The reagent was able to effectively image and quantify total thiols (PSH+NPSH) in live cells through fluorescence microscopy. The reagent was later named as GUALY's reagent. Here we would like to report an extension of the work by synthesizing a novel benzofurazan sulfide triphenylphosphonium derivative [(((7,7'-thiobis(benzo[c][1,2,5]oxadiazole-4,4'-sulfonyl))bis(methylazanediyl))bis(butane-4,1-diyl))bis(triphenylphosphonium) (TBOP)]. Like GUALY's reagent, TBOP is a thiol-specific fluorogenic agent that is non-fluorescent but forms fluorescent thiol adducts in a thiol-specific fashion. Different than GUALY's reagent, TBOP reacts only with NPSH but not with PSH. TBOP was effectively used to image and quantify NPSH in live cells using fluorescence microscopy. TBOP is a complementary reagent to GUALY's reagent in determining the roles of PSH, NPSH, and total thiols in thiol-related physiological/pathological functions in live cells through fluorescence microscopy. Graphical Abstract Live cell imaging and quantification of non-protein thiols by TBOP.

Variable-temperature X-ray diffraction study of structural parameters of NH---S hydrogen bonds in triethylammonium and pyridinium silanethiolates

Two hydrogen-bonded, well defined compounds were synthesized from tris(2,6-diisopropyl)phenoxysilanethiol (TDST) and triethylamine (TDST-TEA) or pyridine (TDST-py). The crystalline compounds were characterized in the solid state by variable-temperature X-ray diffraction measurements and ATR FT-IR spectroscopy. The toluene solutions of TDST-TEA and TDST-py were studied by NMR spectroscopy. The total hydrogen-bond energies and FT-IR spectra were calculated with the use of BLYP-D/TZP and B3LYP/6-31G(d,p)/GD3BJ methods. Thermochemical parameters and potential energy scans were calculated at the B3LYP/6-31G(d,p)/GD3BJ level. All results point to the higher energy of bonding in TDST-TEA both in the solid state and in solution. At the same time the potential energy scan reveals a very broad double-well hydrogen bond in TDST-py, indicating good stabilization of the system for a wide range of D-H...A distances.

Colorimetric Sensor Array for Thiols Discrimination Based on Urease-Metal Ion Pairs

Thiols play a crucial role in various physiological functions, and the discrimination of thiols is a significant but difficult issue. Herein, we presented a new strategy for strengthening the discrimination of thiols by a facile colorimetric sensor array composed of a series of urease-metal ion pairs. The proposed sensor array was fabricated based on the interactions between thiols and metal ions and the effective activation of urease by thiols. Different thiols exhibited different affinities toward the metal ions, producing differential retentions of urease activity and generating distinct colorimetric response patterns. These response patterns are characteristic for each thiol and can be quantitatively differentiated by linear discriminant analysis (LDA). Cysteine (Cys), glutathione (GSH), and four other kinds of thiols have been well distinguished on the basis of this sensor array at a low concentration (1.0 μM). Remarkably, the practicability of the proposed sensor array was further validated by high accuracy (96.67%) identification of 30 unknown thiol samples. In this strategy, urease and its metal ion inhibitors were adapted to fabricate the sensor array, offering a facile way to develop sensitive array sensing systems based on inexpensive and commercially available enzymes and their inhibitors.

6-Bromo-7-hydroxy-3-methylcoumarin (mBhc) is an efficient multi-photon labile protecting group for thiol caging and three-dimensional chemical patterning

The photochemical release of chemical reagents and bioactive molecules provides a useful tool for spatio-temporal control of biological processes. However, achieving this goal requires the development of highly efficient one- and two-photon sensitive photo-cleavable protecting groups. Thiol-containing compounds play critical roles in biological systems and bioengineering applications. While potentially useful for sulfhydryl protection, the 6-bromo-7-hydroxy coumarin-4-ylmethyl (Bhc) group can undergo an undesired photoisomerization reaction upon irradiation that limits its uncaging efficiency. To address this issue, here we describe the development of 6-bromo-7-hydroxy-3-methylcoumarin-4-ylmethyl (mBhc) as an improved group for thiol-protection. One- and two-photon photolysis reactions demonstrate that a peptide containing a mBhc-caged thiol undergoes clean and efficient photo-cleavage upon irradiation without detectable photoisomer production. To test its utility for biological studies, a K-Ras-derived peptide containing an mBhc-protected thiol was prepared by solid phase peptide synthesis using Fmoc-Cys(mBhc)-OH for the introduction of the caged thiol. Irradiation of that peptide using either UV or near IR light in presence of protein farnesyltransferase (PFTase), resulted in generation of the free peptide which was then recognized by the enzyme and became farnesylated. To show the utility of this caging group in biomaterial applications, we covalently modified hydrogels with mBhc-protected cysteamine. Using multi-photon confocal microscopy, highly defined volumes of free thiols were generated inside the hydrogels and visualized via reaction with a sulfhydryl-reactive fluorophore. The simple synthesis of mBhc and its efficient removal by one- and two-photon processes make it an attractive protecting group for thiol caging in a variety of applications.

Rapid and thiol-specific high-throughput assay for simultaneous relative quantification of total thiols, protein thiols, and nonprotein thiols in cells

Thiol groups in biological molecules play a significant role in various physiological functions and pathological conditions. Thiols are divided into two major groups: protein thiols and nonprotein thiols. Numerous methods have been reported for thiol assays. Most of these methods have been developed for glutathione, the principal nonprotein thiol, despite the fact that cellular protein thiols are more abundant than glutathione. Further, these methods usually involve a process of biological sample preparation followed by a separation method, and they are time-consuming. We reported previously a series of thiol-specific fluorogenic benzofurazan sulfides. These nonfluorescent benzofurazan sulfides react rapidly and specifically with a thiol to form a strong fluorescent thiol adduct. The rapid reaction, thiol-specific and fluorogenic nature of the sulfides successfully yielded an application of one of the sulfides for relative quantitation of total thiols in live cells through fluorescence microscopy. In this work, we employed the same compound to develop the first high-throughput method for simultaneous monitoring of protein thiols, nonprotein thiols, and total thiols in cells in a 96-well plate on a fluorescence microplate reader at λ_ex = 430 nm and λ_em = 520 nm, respectively. The method is rapid and sensitive, and has been validated by an HPLC thiol assay method. The method can detect thiols with cell concentrations as low as 500 cells/well. We also demonstrated that the method can readily monitor changes in cellular thiol levels. Although the method cannot provide an absolute quantification for thiols because fluorescence intensity of different thiol adducts varies, it provides an accurate measurement of relative quantification, relative to the control. The method will be a valuable tool in thiol-related biomedical/pharmaceutical research.

Convenient detection of the thiol functional group using H/D isotope sensitive Raman spectroscopy

Convenient detection of thiol groups using Raman spectroscopy.

Benzofurazan sulfides for thiol imaging and quantification in live cells through fluorescence microscopy

Thiol groups play a significant role in various cellular functions. Cellular thiol concentrations can be affected by various physiological or pathological factors. A fluorescence imaging agent that can effectively and specifically image thiols in live cells through fluorescence microscopy is desirable for live cell thiol monitoring. Benzofurazan sulfides 1a-1e were synthesized and found to be thiol specific fluorogenic agents except 1d. They are not fluorescent but form strong fluorescent thiol adducts after reacting with thiols through a sulfide-thiol exchange reaction. On the other hand, they exhibit no reaction with other biologically relevant nucleophilic functional groups such as -NH(2), -OH, or -COOH revealing the specificity for the detection of thiols. Sulfide 1a was selected to confirm its ability to image cellular thiols through fluorescence microscopy. The compound was demonstrated to effectively image and quantify thiol changes in live cells through fluorescence microscopy using 430 and 520 nm as the excitation and emission wavelengths, respectively. The quantification results of total thiol in live cells obtained from fluorescence microscopy were validated by an high-pressure liquid chromatography/ultraviolet (HPLC/UV) total thiol assay method. The reagents and method will be of a great value to thiol redox-related research.

Separation and quantification of the cellular thiol pool of pea plants treated with heat, salt and atrazine

A novel procedure for the separation of the cellular thiol pool according to the molecular weight and localization of compounds with sulphydryl groups is presented. This simple and rapid method allows the differentiation of thiols into three major fractions-low molecular weight (LMT, primarily glutathione and free cysteine), protein-bound (TPT) and pellet-bound (PBT, associated with cell walls and broken organelles). Moreover, determination of the ratio between surface (readily reactive) thiols (ATG) and those that are more or less buried in the protein structure (BTG) can be achieved. In intact pea leaves, the amounts of the total thiols (LMT+PBT+TPT) varies from 2.5 to 4.8 micromol/g of fresh material. The data for LMT, PBT and TPT were related to each other in the approximate ratio 1:2:7. Treatments of pea plants with high temperature, salinity and low amounts of atrazine affect these sulphydryl types differently. For a greater understanding of the applicability of this method to physiological research, the main mechanisms leading to alterations in the cellular thiol pool are discussed. Furthermore, it is suggested that the proportion of available to buried thiols (ATG/BTG) in proteins could be used as a convenient marker for stress impacts.

A transverse tubule NADPH oxidase activity stimulates calcium release from isolated triads via ryanodine receptor type 1 S -glutathionylation

We report here the presence of an NADPH oxidase (NOX) activity both in intact and in isolated transverse tubules and in triads isolated from mammalian skeletal muscle, as established by immunochemical, enzymatic, and pharmacological criteria. Immunohistochemical determinations with NOX antibodies showed that the gp91(phox) membrane subunit and the cytoplasmic regulatory p47(phox) subunit co-localized in transverse tubules of adult mice fibers with the alpha1s subunit of dihydropyridine receptors. Western blot analysis revealed that isolated triads contained the integral membrane subunits gp91(phox) and p22(phox), which were markedly enriched in isolated transverse tubules but absent from junctional sarcoplasmic reticulum vesicles. Isolated triads and transverse tubules, but not junctional sarcoplasmic reticulum, also contained varying amounts of the cytoplasmic NOX regulatory subunits p47(phox) and p67(phox). NADPH or NADH elicited superoxide anion and hydrogen peroxide generation by isolated triads; both activities were inhibited by NOX inhibitors but not by rotenone. NADH diminished the total thiol content of triads by one-third; catalase or apocynin, a NOX inhibitor, prevented this effect. NADPH enhanced the activity of ryanodine receptor type 1 (RyR1) in triads, measured through [3H]ryanodine binding and calcium release kinetics, and increased significantly RyR1 S-glutathionylation over basal levels. Preincubation with reducing agents or NOX inhibitors abolished the enhancement of RyR1 activity produced by NADPH and prevented NADPH-induced RyR1 S-glutathionylation. We propose that reactive oxygen species generated by the transverse tubule NOX activate via redox modification the neighboring RyR1 Ca2+ release channels. Possible implications of this putative mechanism for skeletal muscle function are discussed.

Activation of choline acetyl transferase by dihydrolipoic acid

In a previous article we present results that demonstrated that the enzyme, choline acetyl transferase, was strongly activated by dihydrolipoic acid and that the oxidized form of this substance, lipoic acid itself, was an inhibitor of the enzyme and overcame the stimulatory effects of reduced lipoic acid. The experiments presented in this article show that dialysis of a partially purified preparation of choline acetyl transferase causes complete disappearance of enzyme activity and that addition of dihydrolipoic acid restores activity towards normal. In addition we present experiments with extracts of rat brain and heart as well as rabbit bladder tissue. In these extracts dihydrolipoic acid strongly activates the enzyme. Dialysis of brain and heart extracts causes loss of activity with partial restoration of activity by addition of dihydrolipoic acid. Reduced glutathione has no ability to stimulate activity of the enzyme. We conclude that the results of these experiments strongly support the view that dihydrolipoic acid acts as a coenzyme in the choline acetyl transferase reaction.

Regulation of the activity of choline acetyl transferase by lipoic acid

The observations reported in this article demonstrate that lipoic acid strongly influences the activity of a purified preparation of choline acetyl transferase. The reduced form, dihydrolipoic acid, is a powerful activator of the enzyme while lipoic acid itself has an inhibitory effect and counteracts the stimulatory effect of dihydrolipoic acid. It is proposed that dihydrolipoic acid serves an essential function in the action of this enzyme and that the ratio of reduced to oxidized lipoic acid in the cell may play an important role in the regulation of the activity of the enzyme. The implications of these findings for cell function and acetyl choline formation are discussed.