Deuterium labeling improves the therapeutic index of 3,3'-diselenodipropionic acid as an anticancer agent: insights from redox reactions

3,3'-Diselenodipropionic acid (DSePA), a selenocystine derivative, has been previously reported as an oral supplement for anticancer/radio-modulation activities. The present study is focused on devising a strategy to synthesize and characterize the deuterated derivative of DSePA and on understanding the effect of deuteration on its therapeutic index by comparing its cytotoxicity in cancerous versus non-cancerous cell types. In this context, the synthesis of 3,3'-diselenodipropionic acid-D8 (D-DSePA) was accomplished in ∼42% yield. Further, the results clearly established that the deuteration of DSePA significantly reduced its cytotoxicity in non-cancerous cell types while retaining its cytotoxicity in cancerous cell lines. Together, D-DSePA displayed a ∼5-fold higher therapeutic index than the non-deuterated derivative for anticancer activity. The biochemical and NMR studies confirmed that the better biocompatibility of D-DSePA than its non-deuterated derivative in non-cancerous cells was due to its ability to undergo slower redox reactions and to cause lesser inhibition of intracellular redox enzymes.

Synthesis and glutathione peroxidase (GPx)-like activity of selenocystine (SeC) bioconjugates of biotin and lipoic acid

The conjugation of active biomolecules provides insight into their bioreactivity, leading to many applications in biotechnology and materials science. Herein, we report L-selenocystine (SeC) bioconjugates of lipoic acid (universal antioxidant) and biotin (Vitamin-H). The SeC-bioconjugates, SeC-Biotin (1) and SeC-Lipoic acid (2) were synthesized using solid phase peptide synthesis (SPPS) method and were characterized by multinuclear 1D (1H, 13C, 77Se) and 2D (1H-1H COSY and 1H-13C TOCSY) NMR spectroscopy, ESI-MS spectrometry, and RP-HPLC. The GPx-like enzyme mimicking activity of the SeC-bioconjugates 1 and 2 has been investigated through the coupled reductase assay method for the catalytic reductions of hydrogen peroxide into water. A significant enhancement in GPx-like enzymatic activity was observed for both novel bioconjugates SeC-Biotin (1) and SeC-Lipoic acid (2) as compared to diphenyl diselenide (Ph2Se2), L-selenocystine (SeC), biotin, lipoic acid, and ebselen.

Study of the inhibition effects on glutathione peroxidase immobilized on MNPs using a stopped-flow microfluidic system

A stopped-flow microfluidic system to monitor glutathione peroxidase (GPx) activity and evaluate potential inhibitors of the enzyme has been developed based on the integration of the microfluidic chip in the reaction/detection zone. This integration supposes the physical alignment at the optimal location of the microfluidic channel, both the magnetically retained enzyme microreactor (MREµR) and the remote luminescence detection using a focused bifurcated fiber optic bundle (BFOB) connected to a conventional spectrofluorometer detector. The method is based on the coupling of two competitive oxidative chemical reactions, in which glutathione (GSH) and homovanillic acid (HVA) competed for their interaction with hydrogen peroxide in the presence of the magnetically retained GPx-MNPs. The biocatalytic reaction was followed by monitoring the fluorescence of the biphenyl-HVA dimer formed. The dynamic range of the calibration graph was 0.45-10 µmol L^-1, expressed as GSH concentration with a detection limit of 0.1 µmol L^-1 (r² = 0.9954, n = 10, r = 3). The precision expressed as the relative standard deviation (RSD%) was between 0.5 and 3.9%. The stopped-flow microfluidic system showed a sampling frequency of 25 h^-1. The method was applied to the study of GPx inhibition provided by three inhibitory compounds, two metallic ions Hg(II) and Cu(II) and t-butyl hydroperoxide, and their presence in liquid samples, as water, milk, and edible oil. Recovery values between 88.7 and 99.4% were achieved in all instances.

Incorporating Selenium into Heterocycles and Natural Products─From Chemical Properties to Pharmacological Activities

Selenium (Se)-containing compounds have emerged as potential therapeutic agents for the treatment of a range of diseases. Through tremendous effort, considerable knowledge has been acquired to understand the complex chemical properties and biological activities of selenium, especially after its incorporation into bioactive molecules. From this perspective, we compiled extensive literature evidence to summarize and critically discuss the relationship between the pharmacological activities and chemical properties of selenium compounds and the strategic incorporation of selenium into organic molecules, especially bioactive heterocycles and natural products. We also provide perspectives regarding the challenges in selenium-based medicinal chemistry and future research directions.

3,3'-Diselenodipropionic acid (DSePA) induces reductive stress in A549 cells triggering p53-independent apoptosis: A novel mechanism for diselenides

The aim of present study was to investigate the anticancer mechanisms of 3,3'-diselenodipropionic acid (DSePA), a redox-active organodiselenide in human lung cancer cells. DSePA elicited a significant concentration and time-dependent cytotoxicity in human lung cancer cell line A549 than in normal WI38 cells. The cytotoxic effect of DSePA was preceded by an acute decrease in the level of basal reactive oxygen species (ROS) and a concurrent increase in levels of reducing equivalents (like GSH/GSSG and NADH/NAD) within cells. Further, a series of experiments were performed to measure the markers of intrinsic (Bax, cytochrome c and caspase-9), extrinsic (TNFR, FADR and caspase-8) and endoplasmic reticulum (ER) stress (protein ubiquitylation, calcium flux, Bip, CHOP and caspase-12) pathways in DSePA treated cells. DSePA treatment significantly increased the levels of all the above markers. Moreover, DSePA did not alter the expression and phosphorylation (Ser15) of p53 but caused a significant damage to mitochondria. Pharmacological modulation of GSH level by BSO and NAC in DSePA treated cells led to partial abrogation and augmentation of cell kill respectively. This established the role of reductive stress as a trigger for the apoptosis induced by DSePA treatment. Finally, in vitro anticancer activity of DSePA was also corroborated by its in vivo efficacy of suppressing the growth of A549 derived xenograft tumor in SCID mice. In conclusion, above results suggest that DSePA induces apoptosis in a p53 independent manner by involving extrinsic and intrinsic pathways together with ER stress which can an interesting strategy for lung cancer therapy.

S-Denitrosylase-like Activity of Cyclic Diselenides Conjugated with Xaa-His Dipeptide: Role of Proline Spacer as a Key Activity Booster

This study developed dipeptide-conjugated 1,2-diselenan-4-amine (1), i.e., 1-Xaa-His, as a new class of S-denitrosylase mimic. The synthesized compounds, especially 1-Pro-His, remarkably promoted S-denitrosylation of nitrosothiols (RSNO) via a catalytic cycle involving the reversible redox reaction between the diselenide and its corresponding diselenol ([SeH,SeH]) form with coexisting reductant thiols (R'SH), during which the [SeH,SeH] form as a key reactive species reduces RSNO to the corresponding thiol (RSH). Structural analyses of 1-Pro-His suggested that the peptide backbone of [SeH,SeH] is rigidly bent to form a γ-turn, possibly including an NH···Se hydrogen bond between the imidazole ring of His and selenol group, thus stabilizing the [SeH,SeH] form thermodynamically, and dramatically enhancing the catalytic activity. Furthermore, the synthetic compounds were found to prohibit S-nitrosylation-induced protein misfolding in the presence of RSNO, eventually implying their potential as a drug seed for misfolding diseases caused by the dysregulation of the S-denitrosylation system.

Redox reactions of organoselenium compounds: Implication in their biological activity

Antioxidant activity of organoselenium compounds belonging to different classes i.e. functionalized aliphatic, aromatic and cyclic selenoethers, are compared on the basis of their ability to scavenge reactive oxygen species like hydroxyl and peroxyl radicals and to exhibit glutathione peroxidase (GPx) like catalytic activity. The comparative analysis has revealed that the antioxidant activity of the organoselenium compounds show direct correlation with the energy of the highest occupied molecular orbital (HOMO) and neighboring group participation that stabilizes the reaction intermediate. Finally, structural features responsible for improving the rate of reaction of organoselenium compounds with free radical/molecular oxidants have been discussed on the basis of the compounds screened at our institute.

Selenoglutathione Diselenide: Unique Redox Reactions in the GPx-Like Catalytic Cycle and Repairing of Disulfide Bonds in Scrambled Protein

Selenoglutathione (GSeH) is a selenium analogue of naturally abundant glutathione (GSH). In this study, this water-soluble small tripeptide was synthesized in a high yield (up to 98%) as an oxidized diselenide form, i.e., GSeSeG (1), by liquid-phase peptide synthesis (LPPS). Obtained 1 was applied to the investigation of the glutathione peroxidase (GPx)-like catalytic cycle. The important intermediates, i.e., GSe^- and GSeSG, besides GSeO₂H were characterized by ⁷⁷Se NMR spectroscopy. Thiol exchange of GSeSG with various thiols, such as cysteine and dithiothreitol, was found to promote the conversion to GSe^- significantly. In addition, disproportionation of GSeSR to 1 and RSSR, which would be initiated by heterolytic cleavage of the Se-S bond and catalyzed by the generated selenolate, was observed. On the basis of these redox behaviors, it was proposed that the heterolytic cleavage of the Se-S bond can be facilitated by the interaction between the Se atom and an amino or aromatic group, which is present at the GPx active site. On the other hand, when a catalytic amount of 1 was reacted with scrambled 4S species of RNase A in the presence of NADPH and glutathione reductase, native protein was efficiently regenerated, suggesting a potential use of 1 to repair misfolded proteins through reduction of the non-native SS bonds.

The Small Glutathione Peroxidase Mimic 5P May Represent a New Strategy for the Treatment of Liver Cancer

Glutathione peroxidase (GPx) is an antioxidant protein containing selenium. Owing to the limitations of native GPx, considerable efforts have been made to develop GPx mimics. Here, a short 5-mer peptides (5P) was synthesized and characterized using matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Enzyme coupled assays were used to evaluate GPx activity. The cell viability and apoptosis of H22 cells were tested, and mice bearing H22 cell-derived tumors were used to determine the effects of 5P on tumor inhibition. In comparison with other enzyme models, 5P provided a suitable substrate with proper catalytic site positions, resulting in enhanced catalytic activity. In our mouse model, 5P showed excellent inhibition of tumor growth and improved immunity. In summary, our findings demonstrated the design and synthesis of the small 5P molecule, which inhibited tumor growth and improved immunity. Notably, 5P could inhibit tumor growth without affecting normal growth. Based on these advantages, the novel mimic may have several clinical applications.

Stable selones in glutathione-peroxidase-like catalytic cycle of selenonicotinamide derivative

Stable selone formation in 2,2′-diselenobis[3-amidopyridine], reduces unwanted sulfur exchange reaction in glutathione peroxidase like catalytic cycle and enhances its enzyme activity.