The thiol of human serum albumin: Acidity, microenvironment and mechanistic insights on its oxidation to sulfenic acid

The Thiol Group Reactivity and the Antioxidant Property of Human Serum Albumin Are Controlled by the Joint Action of Fatty Acids and Glucose Binding

The binding of ubiquitous serum ligands (free fatty acids) to human serum albumin (HSA) or its glycation can affect thiol group reactivity, thus influencing its antioxidant activity. The effects of stearic acid (SA) and glucose binding on HSA structural changes and thiol group content and reactivity were monitored by fluoroscopy and the Ellman method during a 14-day incubation in molar ratios to HSA that mimic pathophysiological conditions. Upon incubation with 5 mM glucose, HSA glycation was the same as HSA without it, in three different HSA:SA molar ratios (HSA:SA-1:1-2-4). The protective effect of SA on the antioxidant property of HSA under different glucose regimes (5-10-20 mM) was significantly affected by molar ratios of HSA:SA. Thiol reactivity was fully restored with 5-20 mM glucose at a 1:1 HSA:SA ratio, while the highest thiol content recovery was in pathological glucose regimes at a 1:1 HSA:SA ratio. The SA affinity for HSA increased significantly (1.5- and 1.3-fold, p < 0.01) with 5 and 10 mM glucose compared to the control. These results deepen the knowledge about the possible regulation of the antioxidant role of HSA in diabetes and other pathophysiological conditions and enable the design of future HSA-drug studies which, in turn, is important for clinicians when designing information-based treatments.

Comparative studies of structure and dynamics of caprine, leporine, ovine, and equine serum albumins

Serum albumin (SA) is the most prevalent protein found in blood. Human albumin was used as an albumin substitute in hypoalbuminemia pets due to high sequence similarity. SAs from furry animals were also reported to be the major indoor allergens. Sensitizing to one of SAs coupled with high sequence identity can lead to cross-reactive antibodies in allergic individuals. Thus, understanding the structural and dynamic characters of SAs is crucial for not only albumin substitution but also allergen therapy. Herein, Molecular dynamics (MD) simulations were performed to elucidate the structural and dynamic dissimilarity and similarity of economic animals [equine (ESA), caprine (CASA), ovine (OSA), and leporine (LSA)] to albumins from human (HSA), bovine (BSA), porcine (PSA), and pets [cat (FSA) and dog (CSA)]. The aim is to explore the feasibility of HSA substitution and understand how albumins cause the cross-reactivity. Generally, all albumins studied here show the scissoring motion like other mammalian albumins. The uniqueness of each albumin is defined by different sequence identity of domain I. Also, the drug binding affinity of studied albumins differs from HSA, CSA, FSA, BSA, and PSA. Especially, LSA displays the most deviated behavior from the group. So, such albumin may not be suitable for albumin therapy for pets and humans. CASA, OSA, and ESA share similar characteristics, therefore it is possible to use them to monitor the osmotic pressure among their species, but the allergenic response must be seriously considered. An insight obtained here can be useful to develop albumin therapy and understand clinical allergy.Communicated by Ramaswamy H. Sarma.

Disulfides form persulfides at alkaline pH leading to potential overestimations in the cold cyanolysis method

It is well established that proteins and peptides can release sulfur under alkaline treatment, mainly through the β-elimination of disulfides with the concomitant formation of persulfides and dehydroalanine derivatives. In this study, we evaluated the formation of glutathione persulfide (GSSH/GSS-) by exposure of glutathione disulfide (GSSG) to alkaline conditions. The kinetics of the reaction between GSSG and HO- was investigated by UV-Vis absorbance, reaction with 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB), and cold cyanolysis, obtaining an apparent second-order rate constant of ∼10-3 M-1 s-1 at 25 °C. The formation of GSSH and the dehydroalanine derivative was confirmed by HPLC and/or mass spectrometry. However, the mixtures did not equilibrate in a timescale of hours, and additional species, including thiol and diverse sulfane sulfur compounds were also formed, probably through further reactions of the persulfide. Cold cyanolysis is frequently used to quantify persulfides, since it measures sulfane sulfur. This method involves a step in which the sample to be analyzed is incubated with cyanide at alkaline pH. When cold cyanolysis was applied to samples containing GSSG, sulfane sulfur products that were not present in the original sample were measured. Thus, our results reveal the risk of overestimating the amount of sulfane sulfur compounds in samples that contain disulfides due to their decay to persulfides and other sulfane sulfur compounds at alkaline pH. Overall, our study highlights that the β-elimination of disulfides is a potential source of persulfides, although we do not recommend the preparation of GSSH from incubation of GSSG in alkali. Our study also highlights the importance of being cautious when doing and interpreting cold cyanolysis experiments.

Novel insights about albumin in cardiovascular diseases: Focus on heart failure

The Human Plasma Proteome has always been the most investigated compartment in proteomics-based biomarker discovery, and is considered the largest and deepest version of the human proteome, reflecting the state of the body in health and disease. Even if efforts have been always dedicated to the refinement of proteomic approaches to investigate more deeply the plasma proteome, it should not be forgotten that also highly abundant plasma proteins, like human serum albumin (HSA), often neglected in these studies, might provide fundamental physiological functions in plasma, and should be better considered. This review summarizes the important roles of HSA in the context of cardiovascular diseases (CVD), and in particular in heart failure. Notwithstanding much attention has been historically directed toward the association of HSA levels and CVD risk, the advances in the field of mass spectrometry research allow also a better characterization of the effects of oxidative modifications that could alter not only the structure but also the function of HSA.

Structural and Dynamic Alteration of Glycated Human Serum Albumin in Schiff Base and Amadori Adducts: A Molecular Simulation Study

Human serum albumin (HSA) is a protein carrier in blood transporting metabolites and drugs. Glycated HSA (GHSA) acts as a potential biomarker for diabetes. Thus, many attempts have been made to detect GHSA. Glycation was reported to damage the structure and ligand binding capability, where no molecular detail is available. Recently, the crystal structure of GHSA has been solved, where two glucose isomers (pyranose/GLC and open-chain/GLO) are located at Sudlow's site I. GLO was found to covalently bind to K195, while GLC is trapped by noncontact interactions. GHSA exists in two forms (Schiff base (SCH) and Amadori (AMA) adducts), but how both disrupt albumin activity microscopically remains unknown. To this end, molecular dynamics simulations were performed here to explore the nature of SCH and AMA. Both forms are found to alter the main protein dynamics, resulting in (i) the widening of Sudlow's site I entrance, (ii) the size reduction of nine fatty acid-binding pockets, (iii) the enlargement of Sudlow's site I and the shrinking of Sudlow's site II, (iv) the enhancement of C34 reactivity, and (v) the change in the W214 microenvironment. These unique characteristics found here can be useful for understanding the effect of glycation on the albumin function in more detail and designing specific and selective GHSA detection strategies.

Direct Oral FXa Inhibitors Binding to Human Serum Albumin: Spectroscopic, Calorimetric, and Computational Studies

Direct FXa inhibitors are an important class of bioactive molecules (rivaroxaban, apixaban, edoxaban, and betrixaban) applied for thromboprophylaxis in diverse cardiovascular pathologies. The interaction of active compounds with human serum albumin (HSA), the most abundant protein in blood plasma, is a key research area and provides crucial information about drugs' pharmacokinetics and pharmacodynamic properties. This research focuses on the study of the interactions between HSA and four commercially available direct oral FXa inhibitors, applying methodologies including steady-state and time-resolved fluorescence, isothermal titration calorimetry (ITC), and molecular dynamics. The HSA complexation of FXa inhibitors was found to occur via static quenching, and the complex formation in the ground states affects the fluorescence of HSA, with a moderate binding constant of 10⁴ M^-1. However, the ITC studies reported significantly different binding constants (10³ M^-1) compared with the results obtained through spectrophotometric methods. The suspected binding mode is supported by molecular dynamics simulations, where the predominant interactions were hydrogen bonds and hydrophobic interactions (mainly π-π stacking interactions between the phenyl ring of FXa inhibitors and the indole moiety of Trp214). Finally, the possible implications of the obtained results regarding pathologies such as hypoalbuminemia are briefly discussed.

Capture of Electrophilic Quinones in the Extracellular Space: Evidence for a Phase Zero Reaction

Electrophilic quinones are produced during the combustion of gasoline in the atmosphere. Although these reactive species covalently bind to protein-based nucleophiles in cells, resulting in the formation of protein adducts involved in the modulation of redox signaling pathways and cytotoxicity, the extracellular regulation of quinones is not understood. In this study, incubation of 1,2-naphthoquinone (1,2-NQ) with the low-molecular-weight fraction of mouse plasma resulted in the consumption of cysteine (CysSH) in the plasma in a concentration-dependent manner. Covalent modification of albumin was markedly repressed by the addition of either the low-molecular-weight fraction of mouse plasma or CysSH, suggesting that CysSH protects by forming a conjugate with 1,2-NQ. Similar phenomena also occurred for other atmospheric quinones 1,4-NQ and 1,4-benzoquinone (1,4-BQ). The addition of cystine to a culture medium without amino acids enhanced the release of CysSH from A431 cells and blocked 1,2-NQ-mediated arylation of intracellular proteins, suggesting that 1,2-NQ interacts with extracellular CysSH. Liquid chromatography-tandem mass spectrometry analysis revealed that 1,2-NQ and 1,4-BQ undergoes nucleophilic attack by CysSH, yielding a 1,2-NQH₂-SCys adduct and 1,4-BQH₂-SCys adduct, respectively. Unlike 1,2-NQ and 1,4-BQ, the authentic 1,2-NQH₂-SCys adduct and 1,4-BQH₂-SCys adduct had little effect on the covalent modification of cellular proteins and viability of A431 cells. These results suggest that electrophilic quinones are readily trapped by CysSH released from A431 cells, forming less-toxic CysSH adducts and thereby repressing covalent modification of cellular proteins. These findings provide evidence for the existence of a "phase zero" reaction of electrophiles prior to their uptake by cells.

Development of Dietary Thiol Antioxidant via Reductive Modification of Whey Protein and Its Application in the Treatment of Ischemic Kidney Injury

Thiol antioxidants play important roles in cell and body defense against oxidative stress. In body fluid, albumin is the richest source of thiol antioxidants. One recent study showed that the reductive modification of thiol residues in albumin potentiated its antioxidative activity. Given that whey protein (WP) contains albumin and other thiol-active proteins, this property of WP could be exploited to develop novel thiol antioxidants. The aim of this study was to address this possibility. WP was reductively modified with dithiothreitol (DTT). The modified protein exhibited significantly elevated free sulfhydryl groups (-SH) and thiol antioxidative activity. It detoxified H2O2 and prevented H2O2-initiated protein oxidation and cell death in a -SH group-dependent way in vitro. In addition, it reacted with GSH/GSSG and altered the GSH/GSSG ratio via thiol-disulfide exchange. In vivo, oral administration of the reductively modified WP prevented oxidative stress and renal damage in a mouse model of renal injury caused by ischemia reperfusion. It significantly improved renal function, oxidation, inflammation, and cell injury. These protective effects were not observed in the WP control and were lost after blocking the -SH groups with maleimide. Furthermore, albumin, one of the ingredients of WP, also exhibited similar protective effects when reductively modified. In conclusion, the reductive modification of thiol residues in WP transformed it into a potent thiol antioxidant that protected kidneys from ischemia reperfusion injury. Given that oxidative stress underlies many life-threatening diseases, the reductively modified dietary protein could be used for the prevention and treatment of many oxidative-stress-related conditions, such as cardiovascular diseases, cancer, and aging.

N-Acetylcysteine Regenerates In Vivo Mercaptoalbumin

Human serum albumin (HSA) represents the most abundant plasma protein, with relevant antioxidant activity due to the presence of the sulfhydryl group on cysteine at position 34 (Cys34), the latter being one of the major target sites for redox-dependent modifications leading to the formation of mixed disulfide linkages with low molecular weight thiols. Thiolated forms of HSA (Thio-HSA) may be useful as markers of an unbalanced redox state and as a potential therapeutic target. Indeed, we have previously reported that albumin Cys34 can be regenerated in vitro by N-Acetylcysteine (NAC) through a thiol-disulfide breaking mechanism, with a full recovery of the HSA antioxidant and antiplatelet activities. With this case study, we aimed to assess the ability of NAC to regenerate native mercaptoalbumin (HSA-SH) and the plasma antioxidant capacity in subjects with redox unbalance, after oral and intravenous administration. A placebo-controlled crossover study, single-blinded, was performed on six hypertensive subjects, randomized into two groups, on a one-to-one basis with NAC (600 mg/die) or a placebo, orally and intravenously administered. Albumin isoforms, HSA-SH, Thio-HSA, and glutathione levels were evaluated by means of mass spectrometry. The plasma antioxidant activity was assessed by a fluorimetric assay. NAC, orally administered, significantly decreased the Thio-HSA levels in comparison with the pre-treatment conditions (T0), reaching the maximal effect after 60 min (−24.7 ± 8%). The Thio-HSA reduction was accompanied by a concomitant increase in the native HSA-SH levels (+6.4 ± 2%). After intravenous administration of NAC, a significant decrease of the Thio-HSA with respect to the pre-treatment conditions (T0) was observed, with a maximal effect after 30 min (−68.9 ± 10.6%) and remaining significant even after 6 h. Conversely, no effect on the albumin isoforms was detected with either the orally or the intravenously administered placebo treatments. Furthermore, the total antioxidant activity of the plasma significantly increased after NAC infusion with respect to the placebo (p = 0.0089). Interestingly, we did not observe any difference in terms of total glutathione corrected for hemoglobin, ruling out any effect of NAC on the intracellular glutathione and supporting its role as a disulfide-breaking agent. This case study confirms the in vitro experiments and demonstrates for the first time that NAC is able to regenerate mercaptoalbumin in vivo, allowing us to hypothesize that the recovery of Cys34 content can modulate in vivo oxidative stress and, hopefully, have an effect in oxidative-based diseases.

Mercaptoalbumin Is Associated with Graft Patency in Patients Undergoing Coronary Artery Bypass Grafting

Coronary artery bypass graft (CABG) surgery still represents the gold standard for patients with complex multivessel coronary artery disease. However, graft occlusion still occurs in a significant proportion of CABG conduits, and oxidative stress is currently considered to be a potential contributor. Human serum albumin (HSA) represents the main antioxidant in plasma through its reduced amino acid Cys34, which can efficiently scavenge several oxidants. In a nested case–control study including 36 patients with occluded grafts and 38 age- and sex-matched patients without occlusion, we assessed the levels of the native mercaptoalbumin (HSA-SH) and oxidized thiolated form of albumin (Thio-HSA) in relation with graft occlusion within 5 years after CABG. We found that the plasma level of preoperative HSA-SH was significantly lower in patients with occluded graft at 5 years follow-up than in patients with graft patency. Furthermore, low HSA-SH remained independently associated with graft occlusion even after adjusting for preoperative D-dimer, a well-known marker of activated coagulation recently found to be associated with graft occlusion. In conclusion, the preoperative level of HSA-SH is independently associated with graft occlusion in CABG and represents a measurable and potentially druggable predictor.

Blood Thiol Redox State in Chronic Kidney Disease

Thiols (sulfhydryl groups) are effective antioxidants that can preserve the correct structure of proteins, and can protect cells and tissues from damage induced by oxidative stress. Abnormal levels of thiols have been measured in the blood of patients with moderate-to-severe chronic kidney disease (CKD) compared to healthy subjects, as well as in end-stage renal disease (ESRD) patients on haemodialysis or peritoneal dialysis. The levels of protein thiols (a measure of the endogenous antioxidant capacity inversely related to protein oxidation) and S-thiolated proteins (mixed disulphides of protein thiols and low molecular mass thiols), and the protein thiolation index (the molar ratio of the S-thiolated proteins to free protein thiols in plasma) have been investigated in the plasma or red blood cells of CKD and ESRD patients as possible biomarkers of oxidative stress. This type of minimally invasive analysis provides valuable information on the redox status of the less-easily accessible tissues and organs, and of the whole organism. This review provides an overview of reversible modifications in protein thiols in the setting of CKD and renal replacement therapy. The evidence suggests that protein thiols, S-thiolated proteins, and the protein thiolation index are promising biomarkers of reversible oxidative stress that could be included in the routine monitoring of CKD and ESRD patients.

Kinetics of Azanone (HNO) Reactions with Thiols: Effect of pH

HNO (nitroxyl, IUPAC name azanone) is an electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present data on the pH-dependent kinetics of azanone reactions with the low molecular thiols glutathione and N-acetylcysteine, as well as with important serum proteins: bovine serum albumin and human serum albumin. The competition kinetics method used is based on two parallel HNO reactions: with RSH/RS- or with O2. The results provide evidence that the reaction of azanone with the anionic form of thiols (RS-) is favored over reactions with the protonated form (RSH). The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with thiolates is provided.

A Blood Biomarker for Duchenne Muscular Dystrophy Shows That Oxidation State of Albumin Correlates with Protein Oxidation and Damage in Mdx Muscle

Duchenne muscular dystrophy (DMD) is a severe X-linked muscle wasting disease with no cure. While the precise mechanisms of progressive dystropathology remain unclear, oxidative stress caused by excessive generation of oxidants is strongly implicated. Blood biomarkers that could track oxidant levels in tissues would be valuable to measure the effectiveness of clinical treatments for DMD; our research has focused on developing such biomarkers. One target of oxidants that has the potential to be harnessed as a clinical biomarker is the thiol side chain of cysteine 34 (Cys34) of the blood protein albumin. This study using the mdx mouse model of DMD shows that in plasma, albumin Cys34 undergoes thiol oxidation and these changes correlate with levels of protein thiol oxidation and damage of the dystrophic muscles. A comparison with the commonly used biomarker protein carbonylation, confirmed that albumin thiol oxidation is the more sensitive plasma biomarker of oxidative stress occurring in muscle tissue. We show that plasma albumin oxidation reflects muscle dystropathology, as increased after exercise and decreased after taurine treatment of mdx mice. These data support the use of albumin thiol oxidation as a blood biomarker of dystropathology to assist with advancing clinical development of therapies for DMD.

Roles of Ion Fluxes, Metabolism, and Redox Balance in Cancer Therapy

Recent Advances: The 2019 Nobel Prize awarded to the mechanisms for oxygen sensing and adaptation according to oxygen availability, highlighting the fundamental importance of gaseous molecules. Gaseous molecules, including reactive oxygen species (ROS), can interact with different cations generated during metabolic and redox dysregulation in cancer cells. Cross talk between calcium signaling and metabolic/redox pathways leads to network-based dyregulation in cancer. Significance: Recent discovery on using small molecules targeting the ion channels, redox signaling, and protein modification on metabolic enzymes can effectively inhibit cancer growth. Several FDA-approved drugs and clinical trials are ongoing to target the calcium channels, such as TRPV6 and TRPM8. Multiple small molecules from natural products target metablic and redox enzymes to exert an anticancer effect. Critical Issues: Small molecules targeting key ion channels, metabolic enzymes that control key aspects of metabolism, and redox proteins are promising, but their action mechanisms of the target are needed to be elucidated with advanced-omic technologies, which can give network-based and highly dimensioal data. In addition, small molecules that can directly modify the protein residues have emerged as a novel anticancer strategy. Future Directions: Advanced technology accelerates the detection of ions and metabolic and redox changes in clinical samples for diagnosis and informs the decision of cancer treatment. The improvement of ROS detection, ROS target identification, and computational-aid drug discovery also improves clincal outcome.Overall, network-based or holistic regulations of cancer via ion therapy and metabolic and redox intervention are promising as new anticancer strategies. Antioxid. Redox Signal. 34, 1108-1127.

Reductively modified albumin attenuates DSS-Induced mouse colitis through rebalancing systemic redox state

Albumin (Alb) is the most abundant plasma protein with multiple biological functions, including antioxidative property through its thiol activity. Given that inflammatory bowel disease is associated with a decreased level of Alb and an increased level of Alb oxidation, we asked whether Alb could have a therapeutic effect on colitis. Here we tested this possibility. Bovine serum albumin (BSA) was reductively modified with dithiothreitol (DTT) and administrated via gavage or intraperitoneal injection. Dextran sulfate sodium (DSS)-induced mice colitis was associated with massive oxidative stress, as indicated by the elevated sulfenic acid formation in blood, colon tissues, and feces. Treatment of mice with the reductively modified albumin (r-Alb) attenuated the oxidative stress and reduced local inflammation and tissue injury. These effects of r-Alb were only partially achieved by unmodified Alb and wholly lost after blocking the -SH groups with maleimide. In cultured colon epithelial cells, r-Alb prevented DSS- and H2O2-induced ROS elevation and barrier dysfunction, preceded by inhibition of sulfenic acid formation and P38 activation. Further analysis revealed that Alb was susceptible to H2O2-induced oxidation, and it detoxified H2O2 in a -SH group-dependent way. Moreover, Alb reacted with GSH/GSSG via thiol-disulfide exchange and reciprocally regulated the availability of -SH groups. Collectively, our study shows that r-Alb effectively attenuates DSS colitis via -SH group-mediated antioxidative action. Given that the oxidative stress underlies many life-threatening diseases, r-Alb, functioning as a potent antioxidant, could have a wide range of applications.

A pH-dependent protein stability switch coupled to the perturbed pKa of a single ionizable residue

The contribution of electrostatic interactions in protein stability has not been fully understood. Burial of an ionizable amino acid inside the hydrophobic protein core can affect its ionization equilibrium and shift its pKa differentially in the native (N) and unfolded (U) states of a protein and this coupling between the folding/unfolding cycle and the ionization equilibria of the ionizable residue can substantially influence the protein stability. Here, we studied the coupling of the folding/unfolding cycle with the ionization of a buried ionizable residue in a multi-domain protein, Human Serum Albumin (HSA) using fluorescence spectroscopy. A pH-dependent change in the stability of HSA was observed in the near native pH range (pH 6.0-9.0). The protonation-deprotonation equilibrium of a single thiol residue that is buried in the protein structure was identified to give rise to the pH-dependent protein stability. We quantified the pKa of the thiol residue in the N and the U states. The mean pKa of the thiol in the N state was upshifted by 0.5 units to 8.7 due to the burial of the thiol in the protein structure. Surprisingly, the mean pKa of the thiol in the U state was observed to be downshifted by 1.3 units to 6.9. These results indicate that some charged residues are spatially proximal to the thiol group in the U state. Our results suggest that, in addition to the N state, electrostatic interactions in the U state are important determinants of protein stability.

Sulfenic acid in human serum albumin: Reaction with thiols, oxidation and spontaneous decay

Human serum albumin (HSA) contains 17 disulfides and only one reduced cysteine, Cys34, which can be oxidized to a relatively stable sulfenic acid (HSA-SOH). This derivative has been previously detected and quantified. However, its properties are poorly understood. Herein, HSA-SOH formation from the exposure of HSA to hydrogen peroxide was confirmed using the sulfenic acid probe bicyclo [6.1.0]nonyne-biotin (BCN-Bio1), and by direct detection by whole protein mass spectrometry. The decay pathways of HSA-SOH were studied. HSA-SOH reacted with a thiol leading to the formation of a mixed disulfide. The reaction occurred through a concerted or direct displacement mechanism (S_N2) with the thiolate (RS^-) as nucleophile towards HSA-SOH. The net charge of the thiolate affected the value of the rate constant. In the presence of hydrogen peroxide, HSA-SOH was further oxidized to sulfinic acid (HSA-SO₂H) and sulfonic acid (HSA-SO₃H). The rate constants of these reactions were estimated. Lastly, HSA-SOH spontaneously decayed in solution. Mass spectrometry experiments suggested that the decay product is a sulfenylamide (HSA-SN(R')R″). Chromatofocusing analysis showed that the overoxidation with hydrogen peroxide predominates at alkaline pH whereas the spontaneous decay predominates at acidic pH. The present findings provide insights into the reactivity and fate of the sulfenic acid in albumin, which are also of relevance to numerous sulfenic acid-mediated processes in redox biology and catalysis.

Novel Chitosan-Riboflavin Conjugate with Visible Light-Enhanced Antifungal Properties against Penicillium digitatum

A novel chemical conjugate between chitosan (CH) and riboflavin (RF) has been synthesized and characterized via Fourier transform infrared, NMR, and other spectroscopic methods. Photophysical and photochemical properties such as absorption spectra, fluorescence emission, fluorescence anisotropy, and singlet oxygen generation were characterized as well. This new biopolymer-based conjugate was designed to have an antifungal effect enhanced through antimicrobial photodynamic therapy. The antifungal effect of this conjugate (CH-RF) was compared with CH and RF against Penicillium digitatum in vitro. The conjugate showed the highest fungal growth inhibition of all systems tested at a dose of 0.5% w/v. This new biopolymer-based compound could be a promising alternative to fungicides used in citrus fruits postharvest.

Direct Proteomic Mapping of Cysteine Persulfidation

Aims: Cysteine persulfidation (also called sulfhydration or sulfuration) has emerged as a potential redox mechanism to regulate protein functions and diverse biological processes in hydrogen sulfide (H₂S) signaling. Due to its intrinsically unstable nature, working with this modification has proven to be challenging. Although methodological progress has expanded the inventory of persulfidated proteins, there is a continued need to develop methods that can directly and unequivocally identify persulfidated cysteine residues in complex proteomes. Results: A quantitative chemoproteomic method termed as low-pH quantitative thiol reactivity profiling (QTRP) was developed to enable direct site-specific mapping and reactivity profiling of proteomic persulfides and thiols in parallel. The method was first applied to cell lysates treated with NaHS, resulting in the identification of overall 1547 persulfidated sites on 994 proteins. Structural analysis uncovered unique consensus motifs that might define this distinct type of modification. Moreover, the method was extended to profile endogenous protein persulfides in cells expressing H₂S-generating enzyme, mouse tissues, and human serum, which led to additional insights into mechanistic, structural, and functional features of persulfidation events, particularly on human serum albumin. Innovation and Conclusion: Low-pH QTRP represents the first method that enables direct and unbiased proteomic mapping of cysteine persulfidation. Our method allows to generate the most comprehensive inventory of persulfidated targets of NaHS so far and to perform the first analysis of in vivo persulfidation events, providing a valuable tool to dissect the biological functions of this important modification.

Adduct of the blistering warfare agent sesquimustard with human serum albumin and its mass spectrometric identification for biomedical verification of exposure

Apart from the well-known sulfur mustard (SM), additional sulfur-containing blistering chemical warfare agents exist. Sesquimustard (Q) is one of them and five times more blistering than SM. It is a common impurity in mustard mixtures and regularly found in old munitions but can also be used in pure form. Compared to the extensive literature on SM, very little experimental data is available on Q and no protein biomarkers of exposure have been reported. We herein report for the first time the adduct of Q with the nucleophilic Cys34 residue of human serum albumin (HSA) formed in vitro and introduce two novel bioanalytical procedures for detection. After proteolysis of this HSA adduct catalyzed either by pronase or by proteinase K, two biomarkers were identified by high-resolution tandem mass spectrometry (MS/HR MS), namely a dipeptide and a tripeptide, both alkylated at their Cys residue, which we refer to as HETETE-CP and HETETE-CPF. HETETE represents the Q-derived thio-alkyl moiety bearing a terminal hydroxyl group: "hydroxyethylthioethylthioethyl." Targeting both peptide markers from plasma, a micro liquid chromatography-electrospray ionization tandem mass spectrometry method working in the selected reaction monitoring mode (μLC-ESI MS/MS SRM) was developed and validated as well suited for the verification of exposure to Q. Fulfilling the quality criteria defined by the Organisation for the Prohibition of Chemical Weapons, the novel methods enable the detection of exposure to Q alone or in mixtures with SM. We further report on the relative reactivity of Q compared to SM. Based on experiments making use of partially deuterated Q as the alkylating agent, we rule out a major role for six-membered ring sulfonium ions as relevant reactive species in the alkylation of Cys34. Furthermore, the results of molecular dynamics simulations are indicative that the protein environment around Cys34 allows adduct formation with elongated but not bulky molecules such as Q, and identify important hydrogen bonding interactions and hydrophobic contacts. Graphical abstract.

Expression, purification and initial characterization of human serum albumin domain I and its cysteine 34

Human serum albumin presents in its primary structure only one free cysteine (Cys34) which constitutes the most abundant thiol of plasma. An antioxidant role can be attributed to this thiol, which is located in domain I of the protein. Herein we expressed domain I as a secretion protein using the yeast Pichia pastoris. In the initial step of ammonium sulfate precipitation, a brown pigment co-precipitated with domain I. Three chromatographic methods were evaluated, aiming to purify domain I from the pigment and other contaminants. Purification was achieved by cation exchange chromatography. The protein behaved as a non-covalent dimer. The primary sequence of domain I and the possibility of reducing Cys34 to the thiol state while avoiding the reduction of internal disulfides were confirmed by mass spectrometry. The reactivity of the thiol towards the disulfide 5,5´-dithiobis(2-nitrobenzoate) was studied and compared to that of full-length albumin. A ~24-fold increase in the rate constant was observed for domain I with respect to the entire protein. These results open the door to further characterization of the Cys34 thiol and its oxidized derivatives.

The Universal Soldier: Enzymatic and Non-Enzymatic Antioxidant Functions of Serum Albumin

As a carrier of many biologically active compounds, blood is exposed to oxidants to a greater extent than the intracellular environment. Serum albumin plays a key role in antioxidant defence under both normal and oxidative stress conditions. This review evaluates data published in the literature and from our own research on the mechanisms of the enzymatic and non-enzymatic activities of albumin that determine its participation in redox modulation of plasma and intercellular fluid. For the first time, the results of numerous clinical, biochemical, spectroscopic and computational experiments devoted to the study of allosteric modulation of the functional properties of the protein associated with its participation in antioxidant defence are analysed. It has been concluded that it is fundamentally possible to regulate the antioxidant properties of albumin with various ligands, and the binding and/or enzymatic features of the protein by changing its redox status. The perspectives for using the antioxidant properties of albumin in practice are discussed.

Acidity and nucleophilic reactivity of glutathione persulfide

Persulfides (RSSH/RSS^-) participate in sulfur trafficking and metabolic processes, and are proposed to mediate the signaling effects of hydrogen sulfide (H₂S). Despite their growing relevance, their chemical properties are poorly understood. Herein, we studied experimentally and computationally the formation, acidity, and nucleophilicity of glutathione persulfide (GSSH/GSS^-), the derivative of the abundant cellular thiol glutathione (GSH). We characterized the kinetics and equilibrium of GSSH formation from glutathione disulfide and H₂S. A pK_a of 5.45 for GSSH was determined, which is 3.49 units below that of GSH. The reactions of GSSH with the physiologically relevant electrophiles peroxynitrite and hydrogen peroxide, and with the probe monobromobimane, were studied and compared with those of thiols. These reactions occurred through S_N2 mechanisms. At neutral pH, GSSH reacted faster than GSH because of increased availability of the anion and, depending on the electrophile, increased reactivity. In addition, GSS^- presented higher nucleophilicity with respect to a thiolate with similar basicity. This can be interpreted in terms of the so-called α effect, i.e. the increased reactivity of a nucleophile when the atom adjacent to the nucleophilic atom has high electron density. The magnitude of the α effect correlated with the Brønsted nucleophilic factor, β_nuc, for the reactions with thiolates and with the ability of the leaving group. Our study constitutes the first determination of the pK_a of a biological persulfide and the first examination of the α effect in sulfur nucleophiles, and sheds light on the chemical basis of the biological properties of persulfides.

Non-redox cycling mechanisms of oxidative stress induced by PM metals

Metallic compounds contribute to the oxidative stress of ambient particulate matter (PM) exposure. The toxicity of redox inert ions of cadmium, mercury, lead and zinc, as well as redox-active ions of vanadium and chromium is underlain by dysregulation of mitochondrial function and loss of signaling quiescence. Central to the initiation of these effects is the interaction of metal ions with cysteinyl thiols on glutathione and key regulatory proteins, which leads to impaired mitochondrial electron transport and persistent pan-activation of signal transduction pathways. The mitochondrial and signaling effects are linked by the production of H₂O₂, generated from mitochondrial superoxide anion or through the activation of NADPH oxidase, which extends the range and amplifies the magnitude of the oxidative effects of the metals. This oxidative burden can be further potentiated by inhibitory effects of the metals on the enzymes of the glutathione and thioredoxin systems. Along with the better-known Fenton-based mechanisms, the non-redox cycling mechanisms of oxidative stress induced by metals constitute significant pathways for cellular injury induced by PM inhalation.

N-Acetyl-Cysteine Regenerates Albumin Cys34 by a Thiol-Disulfide Breaking Mechanism: An Explanation of Its Extracellular Antioxidant Activity

In the present paper, the extracellular antioxidant activity of N-acetyl-cysteine (NAC) is explained by considering its ability to regenerate the free form of albumin Cys34 by breaking the disulfide bond of the cysteinylated form (HSA-Cys). NAC’s capability to regenerate albumin Cys34 (HSA-SH) was studied by MS intact protein analysis in human plasma and in a concentration range of NAC easily achievable after oral and i.v. administration (5–50 µg/mL). NAC dose-dependently broke the HSA-Cys bond to form the dimer NAC-Cys thus regenerating Cys34, whose reduced state was maintained for at least 120 min. Cys was faster in restoring Cys34, according to the reaction constant determined with the glutathione disulfide (GSSG) reaction, but after 60 min the mixed disulfide HSA-Cys turned back due to the reaction of the dimer Cys-Cys with Cys34. The explanation for the different rate exchanges between Cys-Cys and Cys-NAC with Cys34 was given by molecular modeling studies. Finally, the Cys34 regenerating effect of NAC was related to its ability to improve the total antioxidant capacity of plasma (TRAP assay). The results well indicate that NAC greatly increases the plasma antioxidant activity and this effect is not reached by a direct effect but through the regenerating effect of Cys34.

Oxidative Modifications in Advanced Atherosclerotic Plaques: A Focus on In Situ Protein Sulfhydryl Group Oxidation

Although oxidative stress has been long associated with the genesis and progression of the atherosclerotic plaque, scanty data on its in situ effects on protein sulfhydryl group modifications are available. Within the arterial wall, protein sulfhydryls and low-molecular-weight (LMW) thiols are involved in the cell regulation of both Reactive Oxygen Species (ROS) and Reactive Nitrogen Species (RNS) levels and are a target for several posttranslational oxidative modifications that take place inside the atherosclerotic plaque, probably contributing to both atherogenesis and atherosclerotic plaque progression towards complicated lesions. Advanced carotid plaques are characterized by very high intraplaque GSH levels, due to cell lysis during apoptotic and/or necrotic events, probably responsible for the altered equilibrium among protein sulfhydryls and LMW thiols. Some lines of evidence show that the prooxidant environment present in atherosclerotic tissue could modify filtered proteins also by protein-SH group oxidation, and demonstrate that particularly albumin, once filtered, represents a harmful source of homocysteine and cysteinylglycine inside the plaque. The oxidative modification of protein sulfhydryls, with particular emphasis to protein thiolation by LMW thiols and its association with atherosclerosis, is the main topic of this review.

Kinetics of formation and reactivity of the persulfide in the one-cysteine peroxiredoxin from Mycobacterium tuberculosis

Hydrogen sulfide (H₂S) participates in prokaryotic metabolism and is associated with several physiological functions in mammals. H₂S reacts with oxidized thiol derivatives (i.e. disulfides and sulfenic acids) and thereby forms persulfides, which are plausible transducers of the H₂S-mediated signaling effects. The one-cysteine peroxiredoxin alkyl hydroperoxide reductase E from Mycobacterium tuberculosis (MtAhpE-SH) reacts fast with hydroperoxides, forming a stable sulfenic acid (MtAhpE-SOH), which we chose here as a model to study the interactions between H₂S and peroxiredoxins (Prx). MtAhpE-SOH reacted with H₂S, forming a persulfide (MtAhpE-SSH) detectable by mass spectrometry. The rate constant for this reaction was (1.4 ± 0.2) × 10³ m^-1 s^-1 (pH 7.4, 25 °C), six times higher than that reported for the reaction with the main low-molecular-weight thiol in M. tuberculosis, mycothiol. H₂S was able to complete the catalytic cycle of MtAhpE and, according to kinetic considerations, it could represent an alternative substrate in M. tuberculosis. MtAhpE-SSH reacted 43 times faster than did MtAhpE-SH with the unspecific electrophile 4,4'-dithiodipyridine, a disulfide that exhibits no preferential reactivity with peroxidatic cysteines, but MtAhpE-SSH was less reactive toward specific Prx substrates such as hydrogen peroxide and peroxynitrite. According to molecular dynamics simulations, this loss of specific reactivity could be explained by alterations in the MtAhpE active site. MtAhpE-SSH could transfer its sulfane sulfur to a low-molecular-weight thiol, a process likely facilitated by the low pK_a of the leaving thiol MtAhpE-SH, highlighting the possibility that Prx participates in transpersulfidation. The findings of our study contribute to the understanding of persulfide formation and reactivity.

Exploring the pH-Induced Functional Phase Space of Human Serum Albumin by EPR Spectroscopy

A systematic study on the self-assembled solution system of human serum albumin (HSA) and paramagnetic doxyl stearic acid (5-DSA and 16-DSA) ligands is reported covering the broad pH range 0.7–12.9, mainly using electron paramagnetic resonance (EPR) methods. It is tested to which extent the pH-induced conformational isomers of HSA reveal themselves in continuous wave (CW) EPR spectra from this spin probing approach in comparison to an established spin-labeling strategy utilizing 3-maleimido proxyl (5-MSL). Most analyses are conducted on empirical levels with robust strategies that allow for the detection of dynamic changes of ligand, as well as protein. Special emphasis has been placed on the EPR spectroscopic detection of a molten globule (MG) state of HSA that is typically found by the fluorescent probe 8-Anilino- naphthalene-1-sulfonic acid (ANS). Moreover, four-pulse double electron-electron resonance (DEER) experiments are conducted and substantiated with dynamic light scattering (DLS) data to determine changes in the solution shape of HSA with pH. All results are ultimately combined in a detailed scheme that describes the pH-induced functional phase space of HSA.

ROS and RNS signalling: adaptive redox switches through oxidative/nitrosative protein modifications

Over the last decade, a dual character of cell response to oxidative stress, eustress versus distress, has become increasingly recognized. A growing body of evidence indicates that under physiological conditions, low concentrations of reactive oxygen and nitrogen species (RONS) maintained by the activity of endogenous antioxidant system (AOS) allow reversible oxidative/nitrosative modifications of key redox-sensitive residues in regulatory proteins. The reversibility of redox modifications such as Cys S-sulphenylation/S-glutathionylation/S-nitrosylation/S-persulphidation and disulphide bond formation, or Tyr nitration, which occur through electrophilic attack of RONS to nucleophilic groups in amino acid residues provides redox switches in the activities of signalling proteins. Key requirement for the involvement of the redox modifications in RONS signalling including ROS-MAPK, ROS-PI3K/Akt, and RNS-TNF-α/NF-kB signalling is their specificity provided by a residue microenvironment and reaction kinetics. Glutathione, glutathione peroxidases, peroxiredoxins, thioredoxin, glutathione reductases, and glutaredoxins modulate RONS level and cell signalling, while some of the modulators (glutathione, glutathione peroxidases and peroxiredoxins) are themselves targets for redox modifications. Additionally, gene expression, activities of transcription factors, and epigenetic pathways are also under redox regulation. The present review focuses on RONS sources (NADPH-oxidases, mitochondrial electron-transportation chain (ETC), nitric oxide synthase (NOS), etc.), and their cross-talks, which influence reversible redox modifications of proteins as physiological phenomenon attained by living cells during the evolution to control cell signalling in the oxygen-enriched environment. We discussed recent advances in investigation of mechanisms of protein redox modifications and adaptive redox switches such as MAPK/PI3K/PTEN, Nrf2/Keap1, and NF-κB/IκB, powerful regulators of numerous physiological processes, also implicated in various diseases.

Tuning the Hydrolytic Stability of Next Generation Maleimide Cross-Linkers Enables Access to Albumin-Antibody Fragment Conjugates and tri-scFvs

We describe investigations to expand the scope of next generation maleimide cross-linkers for the construction of homogeneous protein-protein conjugates. Diiodomaleimides are shown to offer the ideal properties of rapid bioconjugation with reduced hydrolysis, allowing the cross-linking of even sterically hindered systems. The optimized linkers are exploited to link human serum albumin to antibody fragments (Fab or scFv) as a prospective half-life extension platform, with retention of antigen binding and robust serum stability. Finally, a triprotein conjugate is formed, by linking scFv antibody fragments targeting carcinoembryonic antigen. This tri-scFv is shown to infer a combination of greater antigen avidity and increased in vivo half-life, representing a promising platform for antibody therapeutic development.