The Sulfur Chemistry of Proteins

Cyclic Disulfide-Bridged Dinitroxide Biradical for Measuring Thiol Redox Status by Electron Paramagnetic Resonance

Among low-molecular-weight thiols, glutathione (GSH) is the main antioxidant in the cell, and its concentration is an indicator of the redox status. A cyclic disulfide-linked dinitroxide was designed for monitoring GSH by electron-paramagnetic resonance (EPR) spectroscopy. Reaction of the disulfide with GSH and three other thiols was measured at 9.6 GHz (X-band) and shown to be of first order in thiols. It is proposed that the reaction of the disulfide with 1 equiv of thiolate produced a short-lived intermediate that reacts with 1 equiv of thiolate to produce the cleavage product. The equilibrium ratio of the cleaved and intact disulfide is a measure of the redox state. Since the long-term goal is to use the disulfide to probe physiology in vivo, the feasibility of EPR spectroscopy and imaging of the disulfide and its cleavage product was demonstrated at 1 GHz (L-band).

Thioesters provide a plausible prebiotic path to proto-peptides

It is widely assumed that the condensation of building blocks into oligomers and polymers was important in the origins of life. High activation energies, unfavorable thermodynamics and side reactions are bottlenecks for abiotic peptide formation. All abiotic reactions reported thus far for peptide bond formation via thioester intermediates have relied on high energy molecules, which usually suffer from short half-life in aqueous conditions and therefore require constant replenishment. Here we report plausible prebiotic reactions of mercaptoacids with amino acids that result in the formation of thiodepsipeptides, which contain both peptide and thioester bonds. Thiodepsipeptide formation was achieved under a wide range of pH and temperature by simply drying and heating mercaptoacids with amino acids. Our results offer a robust one-pot prebiotically-plausible pathway for proto-peptide formation. These results support the hypothesis that thiodepsipeptides and thiol-terminated peptides formed readily on prebiotic Earth and were possible contributors to early chemical evolution.

One of the early processes enabling the origins of life is thought to be the condensation of building blocks into oligomers and polymers. In this article, the authors report the synthesis of thiodepsipeptides and HS-peptides under mild temperatures and various pH, suggesting they could have formed on early prebiotic Earth.

DBN-based ionic liquids with high capability for the dissolution of wool keratin

Effects of ILs structures on the dissolution capability were systematically studied, and the optimal IL [DBNE]DEP with high dissolution capability for goat wool was finally obtained.

Chemical aspects of hydrogen sulfide measurements in physiological samples

BACKGROUND

Owing to recent discoveries of many hydrogen sulfide-mediated physiological processes, sulfide biology is in the focus of scientific research. However, the promiscuous chemical properties of sulfide pose complications for biological studies, which led to accumulation of controversial observations in the literature.

SCOPE OF REVIEW

We intend to provide an overview of fundamental thermodynamic and kinetic features of sulfide redox- and coordination-chemical reactions and protonation equilibria in relation to its biological functions. In light of these chemical properties we review the strengths and limitations of the most commonly used sulfide detection methods and recently developed fluorescent probes. We also give a personal perspective on blood and tissue sulfide measurements based on proposed biomolecule-sulfide interactions and point out important chemical aspects of handling sulfide reagent solutions.

MAJOR CONCLUSIONS

The diverse chemistries of sulfide detection methods resulted in orders of magnitude differences in measured physiological sulfide levels. Investigations that were aimed to dissect the underlying molecular reasons responsible for these controversies made the important recognition that there are large sulfide reserves in biological systems. These sulfide pools are tightly regulated in a dynamic manner and they are likely to play a major role in regulation of endogenous-sulfide-mediated biological functions and avoiding toxic side effects.

GENERAL SIGNIFICANCE

Working with sulfide is challenging, because it requires considerable amounts of chemical knowledge to adequately handle reagent sulfide solutions and interpret biological observations. Therefore, we propose that a rigorous chemical approach could aid the reconciliation of the increasing number of controversies in sulfide biology. This article is part of a Special Issue entitled Current methods to study reactive oxygen species - pros and cons and biophysics of membrane proteins. Guest Editor: Christine Winterbourn.

Novel wound dressing based on nanofibrous PHBV-keratin mats

Keratin is an important protein used for wound healing and tissue recovery. In this study, keratin was first extracted from raw materials and chemically modified to obtain stable keratin (m-keratin). The raw and m-keratin were examined by Raman spectroscopy. The molecular weight of the m-keratin was analysed by SDS-PAGE. The m-keratin was then blended with poly(hydroxybutylate-co-hydroxyvalerate) (PHBV) and electrospun to afford nanofibrous mats. These mats were characterized by field emission scanning electron microscopy (FE-SEM), electron spectroscopy for chemical analysis (ESCA) and atomic force microscopy (AFM). From the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) data, it was found that introduction of keratin enhanced cell proliferation. From wound-healing test and histological examination results, it was shown that the composite mats accelerated wound recovery remarkably as compared to the PHBV control. It was concluded that PHBV-keratin may be a good candidate as a wound dressing.

Topology of the disulfide bonds in the antiviral lectin scytovirin

The antiviral lectin scytovirin (SVN) contains a total of five disulfide bonds in two structurally similar domains. Previous reports provided contradictory results on the disulfide pairing in each individual domain, and we have now re-examined the disulfide topology. N-terminal sequencing and mass spectrometry were used to analyze proteolytic fragments of native SVN obtained at acidic pH, yielding the assignment as Cys7-Cys55, Cys20-Cys32, Cys26-Cys38, Cys68-Cys80, and Cys74-Cys86. We also analyzed the N-terminal domain of SVN (SD1, residues 1-48) prepared by expression/oxidative folding of the recombinant protein and by chemical synthesis. The disulfide pairing in the chemically synthesized SD1 was forced into predetermined topologies: SD1A (Cys20-Cys26, Cys32-Cys38) or SD1B (Cys20-Cys32, Cys26-Cys38). The topology of native SVN was found to be in agreement with the SD1B and the one determined for the recombinant SD1 domain. Although the two synthetic forms of SD1 were distinct when subjected to chromatography, their antiviral properties were indistinguishable, having low nM activity against HIV. Tryptic fragments, the "cystine clusters" [Cys20-Cys32/Cys26-Cys38; SD1] and [Cys68-Cys80/Cys74-C-86; SD2], were found to undergo rapid disulfide interchange at pH 8. This interchange resulted in accumulation of artifactual fragments in alkaline pH digests that are structurally unrelated to the original topology, providing a rational explanation for the differences between the topology reported herein and the one reported earlier (Bokesh et al., Biochemistry 2003;42:2578-2584). Our observations emphasize the fact that proteins such as SVN, with disulfide bonds in close proximity, require considerable precautions when being fragmented for the purpose of disulfide assignment.

The cysteine content of caseion micelles

Following enzymic digestion with pronase, masked SH groups in micellar casein became available for titration in disaggregating media with mercurial reagents. The content of cystine and cysteine was also estimated after reduction with (a) sulphite, and (b) borohydride, and also by reaction in alkaline conditions with Cd(OH)2. The results show that the micelles contain mainly cysteine, and that it is likely that cystine is not present.

Is hydrogen sulfide a circulating "gasotransmitter" in vertebrate blood?

Hydrogen sulfide (H(2)S) is gaining acceptance as a signaling molecule and has been shown to elicit a variety of biological effects at concentrations between 10 and 1000 micromol/l. Dissolved H(2)S is a weak acid in equilibrium with HS(-) and S(2-) and under physiological conditions these species, collectively referred to as sulfide, exist in the approximate ratio of 20% H(2)S, 80% HS(-) and 0% S(2-). Numerous analyses over the past 8 years have reported plasma or blood sulfide concentrations also in this range, typically between 30 and 300 micromol/l, thus supporting the biological studies. However, there is some question whether or not these concentrations are physiological. First, many of these values have been obtained from indirect methods using relatively harsh chemical conditions. Second, most studies conducted prior to 2000 failed to find blood sulfide in micromolar concentrations while others showed that radiolabeled (35)S-sulfide is rapidly removed from blood and that mammals have a relatively high capacity to metabolize exogenously administered sulfide. Very recent studies using H(2)S gas-sensing electrodes to directly measure sulfide in plasma or blood, or HPLC analysis of head-space gas, have also indicated that sulfide does not circulate at micromolar levels and is rapidly consumed by blood or tissues. Third, micromolar concentrations of sulfide in blood or exhaled air should be, but are not, malodorous. Fourth, estimates of dietary sulfur necessary to sustain micromolar levels of plasma sulfide greatly exceed the daily intake. Collectively, these studies imply that many of the biological effects of sulfide are only achieved at supra-physiological concentrations and they question whether circulating sulfide is a physiologically relevant signaling molecule. This review examines the blood/plasma sulfide measurements that have been reported over the past 30 years from the perspective of the analytical methods used and the potential sources of error.

Evaluation of non-isothermal methods in stability studies of human insulin pharmaceutical preparations

The purpose of this research was to study the thermal stability of a human insulin pharmaceutical preparation using non-isothermal conditions and comparison with classical isothermal experiments. The isothermal studies were performed in the temperature range 20-60 degrees C, whereas non-isothermal stability studies were performed using a linear increasing temperature program, heating rate 0.25 degrees C per hour and temperature interval 30-70 degrees C. Under isothermal conditions, an apparent first-order degradation process was observed at all temperatures. The linear Arrhenius plot suggested that the insulin degradation mechanism was the same within the studied temperature range, with quite large uncertainties due to the small number of degrees of freedom based only on the scatter in the plot, giving an estimated shelf-life at 25 degrees C of 199.1 days. In non-isothermal conditions, the integral approach was used to estimate the activation parameters. It provides results in good agreement with those of the traditional method, but with the advantage that the uncertainty in the final result directly reflects the goodness of fit of the experimental data, since it takes into account the scatter in the original data. The estimated shelf-life in non-isothermal conditions was quite close to the value derived from isothermal data, 191.4 days, although the 95% confidence interval estimated were slightly higher. This is due to the differences in the estimation method and the nature of the experimental errors. The bootstrap technique is also applied to estimating confidence limits for the Arrhenius parameters and shelf-life. This method is very useful when the underlying distribution function of the parameters is unknown. The results obtained indicate that the Arrhenius parameters follow a normal distribution, whereas the shelf-life follows a log-normal distribution. In any case, the results obtained show that there is no difference between the asymptotic and bootstrap confidence intervals.

Identification of the new UV filter compound cysteine-l-3-hydroxykynurenineO-β-d-glucoside in human lenses

UV filters protect the human lens and retina from UV light-induced damage. Here, we report the identification of a new UV filter, cysteine-l-3-hydroxykynurenine O-beta-d-glucoside, which is present in older normal human lenses. Its structure was confirmed by independent synthesis. It is likely this novel UV filter is formed in the lens by nucleophilic attack of cysteine on the unsaturated ketone derived from deamination of 3-hydroxykynurenine O-beta-d-glucoside. Quantitation studies revealed considerable variation in normal lens levels that may be traced to the marked instability of the cysteine adduct. The novel UV filter was not detected in advanced nuclear cataract lenses.

The formation and mechanism of multimerization in a freeze-dried peptide

Atrial natriuretic peptide (ANP) is a peptide with 25 amino acid residues (hANP 4-28) and one intra-chain disulfide bond. We used the size-exclusion chromatography (SEC), sodium dodecyl sulfate in polyacrylamide gel electrophoresis (SDS-PAGE), and reversed-phase liquid chromatography with electrospray mass spectrometry (LC-MS) methods to examine the freeze-dried products of ANP to determine the types, sizes, and amounts of the multimer formation in different stability protocols (by varying the conditions with lyophilization cycles, excipients, storage temperatures, and times). Under the non-annealing lyophilization cycle or lyophilization with high concentration of bulking agent (mannitol), multimer formation increased with increasing storage times. Two kinds of multimers were observed; the major portion is reducible and the minor portion is non-reducible. The reducible multimers are disulfide-linked multimers as determined by LC-MS. The non-reducible multimer was mainly a dimer, possibly linked by a covalent bond between the side chain of tyrosine in one molecule and the dehydroalanine intermediate in another molecule, based on the evidences of the mass of the non-reducible dimer along with the elution position in SEC, and the change of the UV spectrum in the aromatic region. The analysis of degradants suggests that the mechanism start from an beta-elimination of disulfide linkage to form a free thiolate ion (HS-) and a dehydroalanine-type peptide intermediate. The HS- then catalyzed ANP to form the disulfide-linked multimers. The dehydroalanine-type ANP intermediate then reacted with another ANP molecule to form a non-disulfide-linked dimer through reaction with the side chain of tyrosine. These results suggest that the source of multimer formation be initiated by phase transition (from amorphous to crystalline phase) either in the freeze-dried process or during storage. That phase change may induce a drastic change in pH and moisture to damage the peptide. The detailed mechanism and the kinetics of ANP multimerization are discussed. The formation of the multimers was diminished by using the thermal treatment (the annealing step) with a proper ratio of mannitol to ANP peptide in the lyophilization, and/or increase of the acetate buffer concentration in the formulation.

Preparation of fcgamma for addition to sulfhydryl-expressing ligands with minimal disturbance of the hinge

We described previously a scheme for linking functionally intact human normal Fcgamma1, via a thioether linkage emerging from its hinge, to any molecule expressing a free sulfhydryl group (SH). The scheme entails reducing the Fc to release four SH from the two inter-gamma disulfide bonds (SS) in the hinge, blocking one SH by a stochastic alkylation, restoring by SS-interchange the inter-gamma SS whose two SH are still available, and attaching a bismaleimide linker to the one remaining SH. One thereby obtains Fc with a single maleimide group (Fc-maleimide) for attachment to the SH-displaying partner. Restoration of the inter-gamma SS is necessary if the final chimeric construct is to be able to activate the classical complement pathway. However, during this preparation of Fc-maleimide, there is apparently some SS-formation between non-homologous SH, so that not all hinges emerge with a reconstituted inter-gamma SS. To reduce this error we have modified the preparative procedure after investigating an initial partial reduction of the hinge, and reviewing the conditions for stochastic alkylation. During partial reduction by dithiothreitol, the two hinge SS were cleaved apparently randomly: there was no evidence for one bond being more susceptible to reduction than the other, and little indication that the reduction of one bond enhanced the susceptibility of the other. By limiting reduction to an average of one SS per molecule, and alkylation to 0.8 SH per molecule, a final Fc-maleimide product is obtained in which most of the molecules have passed through the entire preparation with one of their hinge SS, and by inference much of the hinge conformation, remaining intact.

A step toward understanding the folding mechanism of bovine adrenodoxin

The iron-sulfur clusters of iron-sulfur proteins are not only essential for the structure and function but they also seem to play an important role in the folding process of these proteins. So far, no data on reversible unfolding/refolding of iron-sulfur proteins under aerobic conditions have been reported. We found appropriate conditions, which might also be applicable for other iron-sulfur proteins, for reversible unfolding/refolding of bovine adrenodoxin (Adx) that prevent cluster decomposition during the unfolding process. The unfolding/refolding studies have been performed under aerobic conditions using fluorescence measurements (with mutant Y82W of Adx, providing a sensitive internal probe), absorption, and circular dichroism (CD) spectroscopy as well as activity measurements. Without protecting reagent, adrenodoxin becomes an apoprotein upon denaturation which is an irreversible process with respect to cluster rebinding. However, reversibility of unfolding/refolding can be observed after protein denaturation in the presence of dithiothreitol (DTT). Upon removal of the denaturant, we regained 65, 63, and 64% refolding from CD, fluorescence, and activity measurements, respectively. In the case of thermal denaturation, the percentage of refolding is about 60% according to CD measurements. DTT appears to stabilize the [2Fe-2S] cluster and prevents its decomposition during aerobic unfolding, providing thereby the means of correct refolding of the protein.

The inactivation of enterovirus infectivity by the sulfhydryl reagent p-chloromercuribenzoate

The infectivity of several enteroviruses was inactivated by the sulfhydryl reagent p-chloromercuribenzoate (PCMB). The rate of inactivation was dependent on the ionic environment in which the reaction was carried out. Inactivation of infectivity was reversed by the thiol compound, reduced glutathione. Under certain conditions, PCMB prevented the adsorption of some enteroviruses to monolayer cultures of monkey kidney cells. The results suggest that enterovirus sulfhydryl groups are involved in the establishment of infection, and that they play a role in the adsorption of virus to host cells.