3,3'-Diselenodipropionic acid (DSePA) forms 1:1 complex with Hg (II) and prevents oxidative stress in cultured cells and mice model
Graphical abstract
Mercury is one of the most common heavy metal causing toxicity in humans and livestock. The present study demonstrates that diselenodipropionic acid a synthetic and safer derivative of selenocystine sequesters mercury with a high binding constant and thereby prevents oxidative stress in cellular and in vivo model systems.
Introduction
Mercury is one of the most common heavy metal causing toxicity in humans and livestock [[1], [2], [3]]. It is one amongst the top ten chemicals listed by world health organisation (WHO) for health hazard [[1], [2], [3]]. Mercury naturally exists in three different chemical forms as elemental (Hg), inorganic (HgCl2) and organic (methyl Hg) [[1], [2], [3], [4], [5], [6]]. Different forms have different target organs such as inorganic mercury deposits in kidney whereas organic form acts as strong neurotoxin [[1], [2], [3], [4], [5], [6]]. Mercury poisoning can occur through inhalation, ingestion, injection, and absorption by different parts of the body [[1], [2], [3], [4], [5], [6]]. The occupational exposure involves mainly the inorganic form of mercury (HgCl2) [[4], [5], [6]]. Some of the well documented effects of HgCl2 poisoning are hepatic toxicity, renal toxicity, gastrointestinal toxicity and reproductive toxicity [[1], [2], [3], [4], [5], [6], [7], [8]]. The recent reports indicate that HgCl2 can also cross blood brain barrier causing deleterious damage to brain [9]. The toxicity of HgCl2 is attributed to its inhibitory effect on selenoenzymes such as thioredoxin reductase (TrxR) and glutathione peroxidase (GPx), glutathione (GSH) and thiol containing proteins [[10], [11], [12], [13]]. These enzymes are known to maintain redox homeostasis within the cells by scavenging the reactive oxygen species (ROS) [[10], [11], [12], [13]]. Hence, the inhibition or inactivation of these enzymes through HgCl2 leads to overproduction of ROS causing oxidative damages and in turn toxicity [13–15]. Accordingly, oxidative stress has been attributed as the major factor for HgCl2 induced toxicity.
Of late, selenium has gained a lot of importance in reducing the HgCl2 mediated toxicity in cells and animal model systems [[16], [17], [18], [19]]. Selenium is a micronutrient required for regulation of normal physiological functions of our body through its incorporation into proteins as selenocysteine (21st amino acid) [20,21]. Such proteins are called selenoproteins and till date 25 such proteins have been identified in humans. TrxR and GPx are some of the well characterized selenoproteins [20,21]. Inorganic selenium has a very high binding affinity to mercury and forms a complex which is not absorbed by the body and hence helps in the elimination of this toxic metal from the body [16,17,22,23]. Apart from inorganic selenium, synthetic organoselenium compounds are also reported for their ability to overcome the mercury induced toxicities in cellular and mice models [17,19,24,25]. However, most of these compounds have limitations in terms of their inherent toxicity and water insolubility. This warrants the need to identify newer safer selenium compounds for the management of mercury toxicity in animals and humans.
Our group has previously reported that 3,3′-diselenodipropionic acid (DSePA), a synthetic organoselenium compound is a relatively safer derivative for supplementation under in vivo conditions [26]. DSePA is a water soluble diselenide and exhibits ROS scavenging and GPx like activity in cell free system [27]. Its reported lethal dosages (LD50) in mice model are 88 mg/kg body weight and 200 mg/kg body weight through intraperitoneal (IP) and oral routes of administration [26,28]. Subsequently this compound has been reported for multi-pharmacological activity including antioxidant, radio-protective and NO generating activities in cellular and mice models [[27], [28], [29], [30], [31], [32], [33]]. Based on these reports, it was thought that DSePA could be an ideal candidate to evaluate its potential against HgCl2 induced toxicity in cellular and mice model system. Accordingly, DSePA was first investigated for its interaction with HgCl2 in cell free system using advance spectroscopic techniques. Subsequently, the protective effect of DSePA against HgCl2 induced toxicity was studied in Chinese Hamster Ovary (CHO) epithelial cells and in hepatic and renal tissues of murine model. CHO cell line was selected considering the fact that it is the representative of the reproductive organ, one of the critical targets of mercury toxicity [8]. Scheme 1 shows the chemical structure of DSePA.
Section snippets
Chemicals
The synthesis and characterization of DSePA has been reported previously [27]. The reagents like HgCl2, β-nicotinamide adenine dinucleotide 2′-phosphate reduced tetra sodium salt hydrate (NADPH), dimethyl sulfoxide (DMSO), dihydrolipoic acid (DLA), N-Ethylmaleimide (NEM), o-phthaldialdehyde (OPT), 5,5′-dithio-bis-[2-nitrobenzoic acid] (DTNB), reduced glutathione (GSH), cumene hydroperoxide, meta-phosphoric acid (HPO3), cellytic M®, TRI reagent, dipotassium hydrogen phosphate (K2HPO4), potassium
Interaction of DSePA with HgCl2 by UV–Visible spectroscopy
Although, selenium compounds are known to sequester Hg (II) ions through complexation [[16], [17], [18], [19],22,23], very few reports are available on the interaction of Hg (II) with diselenide compounds. Therefore, the detail interaction between DSePA and HgCl2 was studied in cell free system by employing the steady-state spectroscopic technique. The absorption spectrum of the aqueous solution of DSePA (25 μM) was found to be featureless with absorption ranging from 200 to 350 nm, while that
Discussion
Although selenium compounds have been reported for their ability to overcome heavy metal toxicity in biological models, there is a constant need of newer and safer compounds [8,[16], [17], [18], [19],[22], [23], [24], [25]]. In our previous studies, we have shown that DSePA is a potent antioxidant compound and is relatively safer as compared to other selenium compounds commonly used as supplement for health improvement [[26], [27], [28], [29],33]. Additionally, researchers from other groups
Author statement
The work described has neither been published previously nor is under consideration for publication elsewhere. We also confirm that all the authors have approved the submission of this manuscript. All data generated or analyzed during this study are included in this published article and its supplementary information files.
Declaration of Competing Interest
None.
Acknowledgement
The authors acknowledge Dr. Awadhesh Kumar, Head, RPC Division BARC and Dr. A. K. Tyagi, Associate Director, Chemistry Group, BARC for their support and encouragement.
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