Thioproline Serves as an Efficient Antioxidant Protecting Human Cells from Oxidative Stress and Improves Cell Viability

Absorptivity Is an Important Determinant in the Toxicity Difference between Aristolochic Acid I and Aristolochic Acid II

Inadvertent exposure to aristolochic acids (AAs) is causing chronic renal disease worldwide, with aristolochic acid I (AA-I) identified as the primary toxic agent. This study employed chemical methods to investigate the mechanisms underlying the nephrotoxicity and carcinogenicity of AA-I. Aristolochic acid II (AA-II), which has a structure similar to that of AA-I, was investigated with the same methods for comparison. Despite their structural similarities, findings from cultured human cells and gut sac experiments showed that AA-I is absorbed more effectively than AA-II (∼3 times greater for AA-I than for AA-II; p < 0.001). This increased absorption, along with the previously observed higher activity of reductive activation enzymes for AA-I, results in greater DNA damage and oxidative stress, both of which are key factors in AA-related toxicity. The similar patterns of cell mortality (34.4 ± 2.3% vs 9.7 ± 0.1% for AA-I and AA-II at 80 μM; p < 0.0001), DNA adduct formation (∼3 times greater for AA-I than for AA-II; p < 0.001), and oxidative stress levels in relation to the concentrations of AA-I and AA-II indicate that the higher absorption rate of AA-I is a significant contributor to its greater toxicity. The toxicity of AA-I was also found to be further enhanced by its (natural) coexistence with AA-II. Since AA-I and AA-II differ only by a methoxy group, future research on reducing risks associated with AA exposure should focus on strategies to lower the absorption of these compounds.

Using 2-(2-Chlorophenyl)thiazolidine-4-carboxylic Acid as a Novel Biomarker for 2-Chlorobenzalmalononitrile Exposure

This study addressed the development of a novel biomarker for 2-chlorobenzalmalononitrile (CS) gas exposure. Using liquid chromatographic and mass spectrometric techniques, we found that CS underwent rapid hydrolysis into 2-chlorobenzaldehyde (2-CBA), a highly reactive intermediate that reacted swiftly with endogenous cysteine (Cys) and Cys residues in proteins, producing a stable 2-(2-chlorophenyl)thiazolidine-4-carboxylic acid adduct (ClPh-SPro) in high yield, which may be used as a CS exposure dosimeter. In particular, it was found that most CS was rapidly hydrolyzed under physiologically relevant conditions, with over 90% of CS being converted into 2-CBA in as short as 20 min. The resultant 2-CBA then reacted swiftly with Cys (k = 0.086 M-1 s-1), forming the stable thiazolidine-4-carboxylic acid adduct, which was detected both in the intracellular fluid and in the cell-isolated proteins of CS-exposed lung cells, as well as in purified human serum albumin. It is expected that the results of this study will facilitate exposure assessment for bystanders who may have been exposed to high levels of CS gas unwillingly.

Urinary Thioproline and Thioprolinyl Glycine as Specific Biomarkers of Formaldehyde Exposure in Humans

Assessment of personal formaldehyde (FA) exposure is most commonly carried out using formate as a biomarker, as it is the major product from FA metabolism. However, formate could also have originated from the metabolism of other endogenous and exogenous substances or from dietary intake, which may give rise to overestimated results with regard to FA exposure. We have developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) coupled with an isotope-dilution method for rigorous quantitation of two major urinary FA conjugation products: thioproline (SPro) and thioprolinyl glycine (SPro-Gly), formed in the reaction between FA and endogenous cysteine or cysteinyl glycine, respectively, as marker molecules to assess personal FA exposure. Using this newly developed method, we measured the FA exposure levels in cigarette smokers, occupants of a chemistry research laboratory and typical domestic household, and visitors to a Chinese temple with a Pearson correlation coefficient greater than 0.94, showing a strong linear correlation between urinary adduct levels and the airborne FA level. It is believed that quantitation of urinary SPro and SPro-Gly may represent a noninvasive, interference-free method for assessing personal FA exposure.

Propionic Acidemia, Methylmalonic Acidemia, and Cobalamin C Deficiency: Comparison of Untargeted Metabolomic Profiles

Methylmalonic acidemia (MMA), propionic acidemia (PA), and cobalamin C deficiency (cblC) share a defect in propionic acid metabolism. In addition, cblC is also involved in the process of homocysteine remethylation. These three diseases produce various phenotypes and complex downstream metabolic effects. In this study, we used an untargeted metabolomics approach to investigate the biochemical differences and the possible connections among the pathophysiology of each disease. The significantly changed metabolites in the untargeted urine metabolomic profiles of 21 patients (seven MMA, seven PA, seven cblC) were identified through statistical analysis (p < 0.05; log2FC > |1|) and then used for annotation. Annotated features were associated with different metabolic pathways potentially involved in the disease's development. Comparative statistics showed markedly different metabolomic profiles between MMA, PA, and cblC, highlighting the characteristic species for each disease. The most affected pathways were related to the metabolism of organic acids (all diseases), amino acids (all diseases), and glycine and its conjugates (in PA); the transsulfuration pathway; oxidative processes; and neurosteroid hormones (in cblC). The untargeted metabolomics study highlighted the presence of significant differences between the three diseases, pointing to the most relevant contrast in the cblC profile compared to MMA and PA. Some new biomarkers were proposed for PA, while novel data regarding the alterations of steroid hormone profiles and biomarkers of oxidative stress were obtained for cblC disease. The elevation of neurosteroids in cblC may indicate a potential connection with the development of ocular and neuronal deterioration.

Proline Analogues

Within the canonical repertoire of the amino acid involved in protein biogenesis, proline plays a unique role as an amino acid presenting a modified backbone rather than a side-chain. Chemical structures that mimic proline but introduce changes into its specific molecular features are defined as proline analogues. This review article summarizes the existing chemical, physicochemical, and biochemical knowledge about this peculiar family of structures. We group proline analogues from the following compounds: substituted prolines, unsaturated and fused structures, ring size homologues, heterocyclic, e.g., pseudoproline, and bridged proline-resembling structures. We overview (1) the occurrence of proline analogues in nature and their chemical synthesis, (2) physicochemical properties including ring conformation and cis/trans amide isomerization, (3) use in commercial drugs such as nirmatrelvir recently approved against COVID-19, (4) peptide and protein synthesis involving proline analogues, (5) specific opportunities created in peptide engineering, and (6) cases of protein engineering with the analogues. The review aims to provide a summary to anyone interested in using proline analogues in systems ranging from specific biochemical setups to complex biological systems.

Targeted Quantification of Glutathione/Arginine Redox Metabolism Based on a Novel Paired Mass Spectrometry Probe Approach for the Functional Assessment of Redox Status

Glutathione (GSH) redox control and arginine metabolism are critical in regulating the physiological response to injury and oxidative stress. Quantification assessment of the GSH/arginine redox metabolism supports monitoring metabolic pathway shifts during pathological processes and their linkages to redox regulation. However, assessing the redox status of organisms with complex matrices is challenging, and single redox molecule analysis may not be accurate for interrogating the redox status in cells and in vivo. Herein, guided by a paired derivatization strategy, we present a new ultraperformance liquid chromatography tandem mass spectrometry (UPLC-MS/MS)-based approach for the functional assessment of biological redox status. Two structurally analogous probes, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) and newly synthesized 2-methyl-6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (MeAQC), were set for paired derivatization. The developed approach was successfully applied to LPS-stimulated RAW 264.7 cells and HDM-induced asthma mice to obtain quantitative information on GSH/arginine redox metabolism. The results suggest that the redox status was remarkably altered upon LPS and HDM stimulation. We expect that this approach will be of good use in a clinical biomarker assay and potential drug screening associated with redox metabolism, oxidative damage, and redox signaling.

4-Thiaproline accelerates the slow folding phase of proteins containing cis prolines in the native state by two orders of magnitude

The cis/trans isomerization of peptidyl-prolyl peptide bonds is often the bottleneck of the refolding reaction for proteins containing cis proline residues in the native state. Proline (Pro) analogues, especially C4-substituted fluoroprolines, have been widely used in protein engineering to enhance the thermodynamic stability of peptides and proteins and to investigate folding kinetics. 4-thiaproline (Thp) has been shown to bias the ring pucker of Pro, to increase the cis population percentage of model peptides in comparison to Pro, and to diminish the activation energy barrier for the cis/trans isomerization reaction. Despite its intriguing properties, Thp has been seldom incorporated into proteins. Moreover, the impact of Thp on the folding kinetics of globular proteins has never been reported. In this study, we show that upon incorporation of Thp at cisPro76 into the thioredoxin variant Trx1P the half-life of the refolding reaction decreased from ~2 h to ~35 s. A dramatic acceleration of the refolding rate could be observed also for the protein pseudo wild-type barstar upon replacement of cisPro48 with Thp. Quantum chemical calculations suggested that the replacement of the Cγ H2 group by a sulfur atom in the pyrrolidine ring, might lower the barrier for cis/trans rotation due to a weakened peptide bond. The protein variants retained their thermodynamic stability upon incorporation of Thp, while the catalytic and enzymatic activities of the modified Trx1P remained unchanged. Our results show that the Pro isostere Thp might accelerate the rate of the slow refolding reaction for proteins containing cis proline residues in the native state, independent from the local structural environment.

Proline and Proline Analogues Improve Development of Mouse Preimplantation Embryos by Protecting Them against Oxidative Stress

The culture of embryos in the non-essential amino acid L-proline (Pro) or its analogues pipecolic acid (PA) and L-4-thiazolidine carboxylic acid (L4T) improves embryo development, increasing the percentage that develop to the blastocyst stage and hatch. Staining of 2-cell and 4-cell embryos with tetramethylrhodamine methyl ester and 2',7'-dichlorofluorescein diacetate showed that the culture of embryos in the presence of Pro, or either of these analogues, reduced mitochondrial activity and reactive oxygen species (ROS), respectively, indicating potential mechanisms by which embryo development is improved. Inhibition of the Pro metabolism enzyme, proline oxidase, by tetrahydro-2-furoic-acid prevented these reductions and concomitantly prevented the improved development. The ways in which Pro, PA and L4T reduce mitochondrial activity and ROS appear to differ, despite their structural similarity. Specifically, the results are consistent with Pro reducing ROS by reducing mitochondrial activity while PA and L4T may be acting as ROS scavengers. All three may work to reduce ROS by contributing to the GSH pool. Overall, our results indicate that reduction in mitochondrial activity and oxidative stress are potential mechanisms by which Pro and its analogues act to improve pre-implantation embryo development.

Novel styryl-thiazole hybrids as potential anti-Alzheimer's agents

In this study, combining the thiazole and cinnamoyl groups into the styryl-thiazole scaffold, a series of novel styryl-thiazole hybrids (6a-p) was rationally designed, synthesized, and evaluated by the multi-target-directed ligands strategy as potential candidates for the treatment of Alzheimer's disease (AD). Hybrids 6e and 6i are the most promising among the synthesized hybrids since they are able to significantly increase cell viabilities in Aβ1-42-exposed-human neuroblastoma cell line (6i at the concentration of 50 μg mL-1 and 6e at the concentration of 25 μg mL-1 resulted in ∼34% and ∼30% increase in cell viabilities, respectively). Compounds 6e and 6i exhibit highly AChE inhibitory properties in the experimental AD model at 375.6 ± 18.425 mU mL-1 and 397.6 ± 32.152 mU mL-1, respectively. Moreover, these data were also confirmed by docking studies and in vitro enzyme inhibition assays. Compared to hybrid 6e and according to the results, 6i also has the highest potential against Aβ1-42 aggregation with over 80% preventive activity. The in silico prediction of the physicochemical properties confirms that 6i possesses a better profile compared to 6e. Therefore, compound 6i presents a promising multi-targeted active molecular profile for treating AD considering the multifactorial nature of AD, and it is reasonable to deepen its mechanisms of action in an in vivo experimental model of AD.

Delayed metabolic disturbances in the myocardium after exertional heat stroke: contrasting effects of exertion and thermal load

Epidemiological studies report higher risks of cardiovascular disease in humans exposed to heat stroke earlier in life. Previously, we explored mechanistic links between heat stroke and developing cardiac abnormalities using a preclinical mouse model of exertional heat stroke (EHS). Profound metabolic abnormalities developed in the ventricles of females but not males after 2 wk of recovery. Here we tested whether this lack of response in males could be attributed to the lower exercise performances or reduced thermal loads they experienced with the same running protocol. We systematically altered environmental temperature (Te) during EHS to manipulate heat exposure and exercise performance in the males. Three groups of adult C57BL/6 male mice were studied: "EHS-34" (Te = 34°C), "EHS-41" (Te = 41°C), and "EHS-39.5" (Te = 39.5°C). Mice ran until symptom limitation (unconsciousness), reaching max core temperature (Tc,max). After a 2-wk recovery, the mice were euthanized, and the ventricles were removed for untargeted metabolomics. Results were compared against age-matched nonexercise controls. The EHS-34 mice greatly elevated their exercise performance but reached lower Tc,max and lower thermal loads. The EHS-41 mice exhibited equivalent thermal loads, exercise times, and Tc,max compared with EHS-39.5. The ventricles from EHS-34 mice exhibited the greatest metabolic disturbances in the heart, characterized by shifts toward glucose metabolism, reductions in acylcarnitines, increased amino acid metabolites, elevations in antioxidants, altered TCA cycle flux, and increased xenobiotics. In conclusion, delayed metabolic disturbances following EHS in male myocardium appear to be greatly amplified by higher levels of exertion in the heat, even with lower thermal loads and max core temperatures.NEW & NOTEWORTHY Epidemiological data demonstrate greater cardiovascular risk in patients with previous heat stroke exposure. Using a preclinical mouse model of exertional heat stroke, male mice were exposed to one of three environmental temperatures (Te) during exercise. Paradoxically, after 2 wk, the mice in the lowest Te, exhibiting the largest exercise response and lowest heat load, had the greatest ventricular metabolic disturbances. Metabolic outcomes resemble developing left ventricular hypertrophy or stress-induced heart disease.

Consequences of incorporating thiaproline and its oxidized derivatives into collagen triple helices

(2R)-4-thiaproline (Thp) is an analog of proline, replacing Cγ in the pyrrolidine ring with sulfur. Its thiazolidine ring easily interconverts between endo and exo puckers due to a small energy barrier, which leads to destabilize polyproline helices. Collagen, composed of three polyproline II helices, mainly consists of X-Y-Gly triplets, where X is often proline and Y is frequently (2S,4R)-hydroxyproline. In this study, we incorporated Thp into either position-X or position-Y to investigate the consequences of such a replacement on the triple helix. Circular dichroism and differential scanning calorimetry analyses showed that the Thp-containing collagen-mimetic peptides (CMPs) can fold into stable triple helices, in which the substitution at position-Y exhibits a larger destabilization effect. Additionally, we also prepared the derivative peptides by oxidizing Thp in the peptide to N-formyl-cysteine or S,S-dioxide Thp. The results showed that the oxidized derivatives at position-X only slightly affect collagen stability, but those at position-Y induce a large destabilization effect. The consequences of incorporating Thp and its oxidized derivatives into CMPs are position dependent. Computational results suggested that the ease of interconversion between exo and endo puckers for Thp and the twist conformation of S,S-dioxide Thp may cause the destabilization effect at position-Y. We have revealed new insights into the impacts of Thp and its oxidized derivatives on collagen and demonstrated that Thp can be used to design collagen-related biomaterials.

Neutrophil metabolomics in severe COVID-19 reveal GAPDH as a suppressor of neutrophil extracellular trap formation

Severe COVID-19 is characterized by an increase in the number and changes in the function of innate immune cells including neutrophils. However, it is not known how the metabolome of immune cells changes in patients with COVID-19. To address these questions, we analyzed the metabolome of neutrophils from patients with severe or mild COVID-19 and healthy controls. We identified widespread dysregulation of neutrophil metabolism with disease progression including in amino acid, redox, and central carbon metabolism. Metabolic changes in neutrophils from patients with severe COVID-19 were consistent with reduced activity of the glycolytic enzyme GAPDH. Inhibition of GAPDH blocked glycolysis and promoted pentose phosphate pathway activity but blunted the neutrophil respiratory burst. Inhibition of GAPDH was sufficient to cause neutrophil extracellular trap (NET) formation which required neutrophil elastase activity. GAPDH inhibition increased neutrophil pH, and blocking this increase prevented cell death and NET formation. These findings indicate that neutrophils in severe COVID-19 have an aberrant metabolism which can contribute to their dysfunction. Our work also shows that NET formation, a pathogenic feature of many inflammatory diseases, is actively suppressed in neutrophils by a cell-intrinsic mechanism controlled by GAPDH.

Bioaccumulation and DNA Adduct Formation of Aristolactam I: Unmasking a Toxicological Mechanism in the Pathophysiology of Aristolochic Acid Nephropathy

Prolonged exposure to aristolochic acid (AA) through AA-containing herbal medicines or AA-tainted food is putting a large portion of the global population at risk of developing renal fibrosis and tumors of the upper urinary tract. In an effort to better understand the organotropic property of AA, we studied the cytotoxicity, absorption, oxidative-stress inducing potential, and DNA adduct formation capability of aristolactam I (ALI), one of the major urinary metabolites of aristolochic acid I (AAI) in human cells. Despite ALI having a slightly lower cytotoxicity than that of AAI, the analysis revealed, for the first time, that ALI is bioaccumulated 900 times more than that of AAI inside cultured kidney cells. Furthermore, ALI induced a significantly larger glutathione depletion than that of AAI in the exposed cells. Together with the formation of ALI-DNA adduct at a reasonably high abundance, results of this study unmasked a previously disregarded causative role of ALI in the organotropic tumor-targeting property of AA.

Synthesis, In Silico and Pharmacological Evaluation of New Thiazolidine-4-Carboxylic Acid Derivatives Against Ethanol-Induced Neurodegeneration and Memory Impairment

Introduction

Several studies revealed that alcohol utilization impairs memory in adults; however, the underlying mechanism is still unclear. The production of inflammatory markers and reactive oxygen species (ROS) plays a major role in neurodegeneration, which leads to memory impairment. Therefore, targeting neuroinflammation and oxidative distress could be a useful strategy for abrogating the hallmarks of ethanol-induced neurodegeneration. Moreover, several studies have demonstrated multiple biological activities of thiazolidine derivatives including neuroprotection.

Methods

In the current study, we synthesized ten (10) new thiazolidine-4-carboxylic acid derivatives (P1-P10), characterized their synthetic properties using proton nuclear magnetic resonance (¹H-NMR) and carbon-13 NMR, and further investigated the neuroprotective potential of these compounds in an ethanol-induced neuroinflammation model.

Results

Our results suggested altered levels of antioxidant enzymes associated with an elevated level of tumor necrosis factor-alpha (TNF-α), nuclear factor-κB (p-NF-κB), pyrin domain-containing protein 3 (NLRP3), and cyclooxygenase-2 (COX-2) in ethanol-treated animals. Ethanol treatment also led to memory impairment in rats, as assessed by behavioral tests. To further support our notion, we performed molecular docking studies, and all synthetic compounds exhibited a good binding affinity with a fair bond formation with selected targets (NF-κB, TLR4, NLRP3, and COX-2).

Discussion

Overall, our results revealed that these derivatives may be beneficial in reducing neuroinflammation by acting on different stages of inflammation. Moreover, P8 and P9 treatment attenuated the neuroinflammation, oxidative stress, and memory impairment caused by ethanol.

LC-MS/MS Quantitation of Formaldehyde-Glutathione Conjugates as Biomarkers of Formaldehyde Exposure and Exposure-Induced Antioxidants: A New Look on an Old Topic

Humans are continuously exposed to formaldehyde via both endogenous and exogenous sources. Prolonged exposure to formaldehyde is associated with many human diseases, such as lung cancer and leukemia. The goal of this study is to develop biomarkers to measure formaldehyde exposure, which could be used to predict the risk of associated diseases. As glutathione (GSH) is well-known for its crucial role in the detoxification of a wide variety of xenobiotics, including formaldehyde, we rigorously quantitated in this study the conjugates formed when formaldehyde reacted with GSH using liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) coupled with an isotope dilution method. The results showed for the first time that (S)-1-(((R)-2-amino-3-(carboxymethylamino)-3-oxopropylthio)methyl)-5-oxopyrrolidine-2-carboxylic acid (PGF) and thioproline-glycine (SPro-Gly) are major metabolites in both nonenzymatic reactions and formaldehyde-exposed human cells. In particular, over 35% of the formaldehyde from external sources was found to convert to SPro-Gly in the exposed cells. Interestingly, data showed that these exposure-induced adducts exhibited good antioxidative properties, which can protect cells from hydrogen peroxide mediated oxidative insult. It is anticipated that the findings of this study could shed light on developing PGF and SPro-Gly as dietary supplements and on the development of noninvasive methods to assess health risks associated with formaldehyde exposure.

Synergistic Interaction of Polycyclic Aromatic Hydrocarbons, Phthalate Esters, or Phenol on DNA Adduct Formation by Aristolochic Acid I: Insights into the Etiology of Balkan Endemic Nephropathy

Balkan endemic nephropathy (BEN) is a multifactorial environmental disease, with chronic exposure to aristolochic acids (AAs) through AA-contaminated food being one of the major etiological mechanisms. However, the bulk of previous research has only focused on investigating the possible roles of individual pollutants in disease development and the etiological mechanism of BEN remains controversial. In this study, we investigated the exposure concentration and duration dependence of coexposure to phthalate esters and lignite coal-derived phenol and polycyclic aromatic hydrocarbons (PAHs) on the metabolism and DNA adduct formation of aristolochic acid I (AAI). Results showed that both the metabolic activation and DNA adduct formation of AAI in cultured human kidney cells were affected by their coexposure to the above-mentioned environmental pollutants. Furthermore, our results suggest that chemicals leached from lignite coal likely played a role by triggering AA-activating enzymes to produce more of the promutagenic DNA adducts, thus further elevating the nephrotoxicity and carcinogenicity of AAs and increasing the risk of BEN. It is believed that the results of this study provide a better understanding of the etiological mechanism of BEN and offer insights into methods and policies to lower the risk of this devastating disease.

Nitrite Triggers Reprogramming of the Oral Polymicrobial Metabolome by a Commensal Streptococcus

Commensal streptococci regulate health and homeostasis within oral polymicrobial communities. Remarkably, high salivary nitrite concentrations have also been associated with improved health in the oral cavity. We previously demonstrated that nitrite assists hydrogen peroxide-producing oral commensal streptococci in regulating homeostasis via the generation of reactive nitrogen species (RNS), which have antimicrobial activity on oral pathogens. However, it is unknown how nitrite and commensal streptococci work in concert to influence the metabolome of oral polymicrobial communities. In this study, we report that nitrite aids commensal streptococci in the inhibition of multi-kingdom pathogens that reside in distinct oral niches, which supports commensal dominance. More importantly, we show that commensal streptococci utilize nitrite to drive the metabolic signature of multispecies biofilms in a manner that supports commensal metabolism and resistance to RNS, and restricts metabolic processes that are required for pathogen virulence. Taken together, our study provides insight into how commensal streptococci use nitrite to trigger shifts in the oral polymicrobial metabolome to support health and homeostasis.

The importance of sulfur-containing motifs in drug design and discovery

INTRODUCTION

Sulfur-containing functional groups are privileged motifs that occur in various pharmacologically effective substances and several natural products. Various functionalities are found with a sulfur atom at diverse oxidation states, as illustrated by thioether, sulfoxide, sulfone, sulfonamide, sulfamate, and sulfamide functions. They are valuable scaffolds in the field of medicinal chemistry and are part of a large array of approved drugs and clinical candidates.

AREA COVERED

Herein, the authors review the current research on the development of organosulfur-based drug discovery. This article also covers details of their roles in the new lead compounds reported in the literature over the past five years 2017-2021.

EXPERT OPINION

Given its prominent role in medicinal chemistry and its importance in drug discovery, sulfur has attracted continuing interest and has been used in the design of various valuable compounds that demonstrate a variety of biological and pharmacological feature activities. Overall, sulfur's role in medicinal chemistry continues to grow. However, many sulfur functionalities remain underused in small-molecule drug discovery and deserve special attention in the armamentarium for treating diverse diseases. Research efforts are also still required for the development of a synthetic methodology for direct access to these functions and late-stage functionalization.

Chemistry of Outlandish Natural Products Belonging to Sulfur Metabolism: Unrevealed Green Syntheses and Separation Strategies from the Cavallini’s Old School

The last century has been very important from the point of view of research and investigation in the fields of the chemistry and biochemistry of sulfur-containing natural products. One of the most important contributions to the discovery and study of human sulfur-containing metabolites was performed by the research group of Professor Doriano Cavallini at Sapienza University of Rome, during the last 80 years. His research brought to light the discovery of unusual sulfur metabolites that were chemically synthesized and determined in different biological specimens. Most of his synthetical strategies were performed in aqueous conditions, which nowadays can be considered totally in line with the recent concepts of the green chemistry. The aim of this paper is to describe and summarize synthetic procedures, and purification and analytical methods from the Cavallini’s school, with the purpose to provide efficient and green methodologies for the preparation and obtainment of peculiar unique sulfur-containing metabolites.

Metabolic and Metabo-Clinical Signatures of Type 2 Diabetes, Obesity, Retinopathy, and Dyslipidemia

Macro- and microvascular complications of type 2 diabetes (T2D), obesity, and dyslipidemia share common metabolic pathways. In this study, using a total of 1,300 metabolites from 996 Qatari adults (57% with T2D) and 1,159 metabolites from an independent cohort of 2,618 individuals from the Qatar BioBank (11% with T2D), we identified 373 metabolites associated with T2D, obesity, retinopathy, dyslipidemia, and lipoprotein levels, 161 of which were novel. Novel metabolites included phospholipids, sphingolipids, lysolipids, fatty acids, dipeptides, and metabolites of the urea cycle and xanthine, steroid, and glutathione metabolism. The identified metabolites enrich pathways of oxidative stress, lipotoxicity, glucotoxicity, and proteolysis. Second, we identified 15 patterns we defined as "metabo-clinical signatures." These are clusters of patients with T2D who group together based on metabolite levels and reveal the same clustering in two or more clinical variables (obesity, LDL, HDL, triglycerides, and retinopathy). These signatures revealed metabolic pathways associated with different clinical patterns and identified patients with extreme (very high/low) clinical variables associated with extreme metabolite levels in specific pathways. Among our novel findings are the role of N-acetylmethionine in retinopathy in conjunction with dyslipidemia and the possible roles of N-acetylvaline and pyroglutamine in association with high cholesterol levels and kidney function.

Decoding the Structure of Non-Proteinogenic Amino Acids: The Rotational Spectrum of Jet-Cooled Laser-Ablated Thioproline

The broadband rotational spectrum of jet-cooled laser-ablated thioproline was recorded. Two conformers of this system were observed and identified with the help of DFT and ab initio computations by comparison of the observed and calculated rotational constants and ¹⁴N quadrupole coupling constants as well as the predicted energies compared to the observed relative populations. These conformers showed a mixed bent/twisted arrangement of the five-membered ring similar to that of the related compound thiazolidine with the N–H bond in axial configuration. The most stable form had the COOH group in an equatorial position on the same side of the ring as N-H. The arrangement of the C=O group close to the N-H bond led to a weak interaction between them (classified as type I) characterized by a noncovalent interaction analysis. The second form had a trans-COOH arrangement showing a type II O–H···N hydrogen bond. In thioproline, the stability of conformers of type I and type II was reversed with respect to proline. We show how the conformation of the ring depends on the function associated with the endocyclic N atom when comparing the structures of isolated thioproline with its zwitterion observed in condensed phases and with peptide forms.

Evidence for Proline Catabolic Enzymes in the Metabolism of Thiazolidine Carboxylates

Thiazolidine carboxylates such as thiazolidine-4-carboxylate (T4C) and thiazolidine-2-carboxylate (T2C) are naturally occurring sulfur analogues of proline. These compounds have been observed to have both beneficial and toxic effects in cells. Given that proline dehydrogenase has been proposed to be a key enzyme in the oxidative metabolism of thioprolines, we characterized T4C and T2C as substrates of proline catabolic enzymes using proline utilization A (PutA), which is a bifunctional enzyme with proline dehydrogenase (PRODH) and l-glutamate-γ-semialdehyde dehydrogenase (GSALDH) activities. PutA is shown here to catalyze the FAD-dependent PRODH oxidation of both T4C and T2C with catalytic efficiencies significantly higher than with proline. Stopped-flow experiments also demonstrate that l-T4C and l-T2C reduce PutA-bound FAD at rates faster than proline. Unlike proline, however, oxidation of T4C and T2C does not generate a substrate for NAD⁺-dependent GSALDH. Instead, PutA/PRODH oxidation of T4C leads to cysteine formation, whereas oxidation of T2C generates an apparently stable Δ⁴-thiazoline-2-carboxylate species. Our results provide new insights into the metabolism of T2C and T4C.

Probing the Hidden Role of Mitochondrial DNA Damage and Dysfunction in the Etiology of Aristolochic Acid Nephropathy

Aristolochic acid nephropathy (AAN) is a unique type of progressive renal interstitial fibrotic disease caused by prolonged exposure to aristolochic acids (AAs) through AA-containing herbal medicines or AA-tainted food. Despite decades of research and affecting millions of people around the world, the pathophysiology of AAN remains incompletely understood. In this study, we tested the potential causative role of mitochondrial dysfunction in AAN development. Our findings revealed AA exposure induces an exposure concentration and duration dependent lowering of adenosine triphosphate in both cultured human kidney and liver cells, highlighting an AA exposure effect on mitochondrial energy production in the kidney and liver, which both are highly metabolically active and energy-demanding organs. Analysis with liquid chromatography-tandem mass spectrometry coupled with stable isotope dilution method detected high levels of mutagenic 8-oxo-2'-deoxyguanosine and 7-(deoxyadenosine-N⁶-yl)-aristolactam adduct on mitochondrial DNA isolated from AA-treated cells, unmasking a potentially important causative, but previously unknown role of mitochondrial DNA mutation in the pathophysiology of AAN development.

Analysis of Polycyclic Aromatic Hydrocarbons and Phthalate Esters in Soil and Food Grains from the Balkan Peninsula: Implication on DNA Adduct Formation by Aristolochic Acid I and Balkan Endemic Nephropathy

Balkan endemic nephropathy (BEN) is a chronic tubulointerstitial nephropathy affecting residents of rural farming areas in many Balkan countries. Although it is generally believed that BEN is an environmental disease caused by multiple geochemical factors with much attention on aristolochic acids (AAs), its etiology remains controversial. In this study, we tested the hypothesis that environmental contamination and subsequent food contamination by polycyclic aromatic hydrocarbons (PAHs) and phthalate esters are AA toxicity factors and important to BEN development. We identified significantly higher concentrations of phenanthrene, anthracene, diethyl phthalate (DEP), dibutyl phthalate (DBP), and benzyl butyl phthalate (BBP) in both maize and wheat grain samples collected from endemic villages than from nonendemic villages. Other PAHs and phthalate esters were also detected at higher concentrations in the soil samples from endemic villages. Subsequent genotoxicity testing of cultured human kidney cells showed an alarming phenomenon that phenanthrene, DEP, BBP, and DBP can interact synergistically with AAs to form elevated levels of AA-DNA adducts, which are associated with both the nephrotoxicity and carcinogenicity of AAs, further increasing their disease risks. This study provides direct evidence that prolonged coexposure to these environmental contaminants via dietary intake may lead to greater toxicity and accelerated development of BEN.

Quantitation of γ-Glutamylcysteine-Formaldehyde Conjugate in Formaldehyde- and Oxidative Stress-Exposed Cells by Liquid Chromatography-Tandem Mass Spectrometry

Humans are constantly exposed to formaldehyde (FA) of both exogenous and endogenous sources, and FA exposure is associated with the development of many human diseases, including cancers. Marker molecules that can provide information on exposure history and amounts will assist disease risk assessment and early interventions. To develop marker signatures of FA exposure, we explored in this study the conjugation reaction of FA with γ-glutamylcysteine (GGC), one of the precursors to glutathione biosynthesis, under physiologically relevant conditions. The results showed that the reaction produced a stable metabolite of FA, (S)-1-((((R)-2-amino-2-carboxyethyl)thio)methyl)-5-oxopyrrolidine-2-carboxylic acid (COCA). Using liquid chromatography-tandem mass spectrometry coupled to a stable isotope-dilution method, we then quantitated for the first time the formation of this novel metabolite in FA- and Fe²⁺-EDTA-exposed human cells. The results revealed the exposure time- and concentration-dependent formation of COCA in FA- or Fe²⁺-EDTA-exposed cells, suggesting that COCA may serve as a biomarker of FA and oxidative stress exposure. Furthermore, the study sheds light on a previously unknown protective role of GGC against FA and oxidative stress.

The effect of tempeh flour from tree bean (Parkia timoriana (dc) Merr.) seeds on lipid profile in protein-energy malnutrition rats

Children with protein-energy malnutrition (PEM) need a food product that can be easily absorbed by the body because they have impaired nutrient utilization and lipid profile disorder. The provision of fermented products such as tempeh is an alternative approach. Seeds of tree bean (Parkia timoriana (DC) Merr.) are a good source of various nutrients needed by children with PEM. This study aimed to prove the effectiveness of the administration of tempeh flour from tree bean (Parkia timoriana (DC) Merr.) seeds on the lipid profile in PEM rats. PEM in rats was induced by a zero-protein diet containing granulated sugar (20 g), vegetable oil (1 g), and cornstarch (24 g). The animals were categorized into a normal control group (K+), a PEM group without treatment (K−); a group fed tempeh flour from tree bean (Parkia timoriana (DC) Merr.) seeds 1.5 g.kg-1 BW (P1); and a group fed tempeh flour from tree bean (Parkia timoriana (DC) Merr.) seeds 3 g.kg-1 BW (P2). The intervention was administered for 30 days. There was a significant increase in HDL levels (p = 0.001) and significant decreases in cholesterol (p = 0.001), triglyceride (p = 0.001), LDL levels (p = 0.001), and atherogenic index of plasma (AIP) (p = 0.000) in treatment groups. In conclusion, the administration of tempeh flour from tree bean (Parkia timoriana (DC) Merr.) seeds increases HDL levels and decreases cholesterol, triglycerides, and LDL level, and atherogenic index of plasma (AIP) in rats with PEM.

Sulfur-containing therapeutics in the treatment of Alzheimer's disease

Sulfur is widely existent in natural products and synthetic organic compounds as organosulfur, which are often associated with a multitude of biological activities. OBenzothiazole, in which benzene ring is fused to the 4,5-positions of the thiazolerganosulfur compounds continue to garner increasing amounts of attention in the field of medicinal chemistry, especially in the development of therapeutic agents for Alzheimer's disease (AD). AD is a fatal neurodegenerative disease and the primary cause of age-related dementia posing severe societal and economic burdens. Unfortunately, there is no cure for AD. A lot of research has been conducted on sulfur-containing compounds in the context of AD due to their innate antioxidant potential and some are currently being evaluated in clinical trials. In this review, we have described emerging trends in the field, particularly the concept of multi-targeting and formulation of disease-modifying strategies. SAR, pharmacological targets, in vitro/vivo ADMET, efficacy in AD animal models, and applications in clinical trials of such sulfur compounds have also been discussed. This article provides a comprehensive review of organosulfur-based AD therapeutic agents and provides insights into their future development.