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Spectrometric and Chromatographic Study of Reactive Oxidants Hypochlorous and Hypobromous Acids and Their Interactions with Taurine

Activated neutrophils and eosinophils generate a variety of reactive oxygen species (ROS). Hypochlorous acid (HOCl) and hypobromous acid (HOBr) are the major reactive oxidants generated by these cells at sites of inflammation [1]. Both agents, components of the human innate immune system, exert strong antimicrobial activity, but their excessive production leads to tissue damaging [2, 3]. Taurine is the most abundant free amino acid in the leukocyte cytosol (30 mM) and is the major scavenger for both hypohalous acids, HOCl and HOBr [4]. Mammalian taurine (2-aminoethanesulfonic acid) is synthesised in the pancreas via the cysteine sulfinic acid pathway [5]. In this pathway (Fig. 1), the sulfhydryl group of cysteine is first oxidized to cysteine sulfinic acid by the enzyme cysteine dioxygenase. Cysteine sulfinic acid, in turn, is decarboxylated by sulfinoalanine decarboxylase to form hypotaurine [6]. Moreover, taurine has significant anti-inflammatory properties [7] and participates in different physiological processes as it stabilizes cell membranes [8], regulates fatty tissues metabolism [9] and levels of calcium ions in blood [10]. Due to biological functions of taurine, this compound is intensely studied due to its possibility to regulate oxidation stress [1, 6, 11-15]. Protective role of taurine against cadmium induced oxidative stress in murine erythrocytes was demonstrated [16-18]. Treatment with taurine before cadmium intoxication prevented the toxin-induced oxidative impairments in the erythrocytes of the experimental animals [16].
Derivatization with subsequent high performance liquid chromatographic (HPLC) [19-28] separation is the most commonly used techniques for taurine quantification. o-phthaldialdehyde (OPA) [29], 2,4-dinitrofluorobenzene (DNFB) [30], 1-dimethylaminonaphthalene-5-sulfonyl chloride (dansyl-Cl) [31], 4-dimethylaminoazobenzene-4'-sulfonyl chloride (DABS-Cl) [32], are probably the most commonly used derivatization agents [33, 34]. Hyphenated techniques, such as HPLC Fourier transform infrared (spectroscopy) and HPLC inductively coupled plasma atomic emission spectroscopy have been proved to be effective for the analysis of taurine in biological samples. However, they have not been widely used due to the complexity of the instruments [35]. Due to ability of taurine to form complexes with ninhydrin [36-39], o-phthalaldehyde [40] and l-fluoro-2,4-dinitrobenzene [41], it is possible to detect it also spectrophotometrically. Spectrophotometric measurements are usually cost effective and easy to perform. This fact is important especially in the light of the fact that biological significance of taurine derivatives remains almost unknown. It has been demonstrated that haloamines play role in the inflammatory processes as proinflammatory mediators [1]. In addition, taurine halocomplexes show also antimicrobial properties.
The main aim of this study was to characterize complexes of taurine with hypochlorous and hypobromous acids using spectrometry (Figs. 2a and b). Primarily, we focused on the basic characteristics of the studied complexes using UV-VIS spectrometry. Further, the characterized complexes were analysed using high performance liquid chromatography with UV detection.

Simulation of Oxidative Stress by Peroxide

There were carried out three series of simulations (eight samples) of oxidative stress by peroxide (1.72, 3.34, 6.85, 13.75, 27.5, 55, 110 and 220 mM). Individual concentrations of peroxide were diluted with taurine and its complexes concentrations of 0.008, 0.02, 0.03, 0.063, 0.125, 0.25, 0.5 and 1 mM. The dependences of peak heights of taurine and its complexes on the concentration of the compound of interest are shown in Figs. 7a, b and c). We demonstrated significant antioxidant effect of taurine and its complexes with phenol, hypochlorite and/or hypobromite (Fig. 7d). Taurine itself reacts with peroxide more intensely than in a bound form, which can be associated with the highest signal decrease. Complexes stabilized structure taurine against peroxide radicals, resulting in slower decreasing of peak heights. The most stable was taurine complexes with phenol and hypobromite. These results confirm that taurine complexes with phenol and hypobromite have significant antioxidant effects. In addition, TauBr and TauCl are major haloamines generated by eosinophils and neutrophils at a site of inflammation. Both haloamines share anti-inflammatory and anti-oxidant properties. TauBr, similarly to TauCl, decreases the production of proinflammatory mediators. Their anti-inflammatory and anti-oxidant activities are enhanced by their ability to induce the expression of heme oxygenase-1 (HO-1) [1]. Taurine chloramine is the major chloramine generated in activated neutrophils via the reaction between the overproduced hypochlorous acid and the stored taurine. Taurine chloramine has anti-inflammatory and cytoprotective effects in inflamed tissues by inhibiting the production of inflammatory mediators. Taurine chloramine increases heme oxygenase activity and also protects against hydrogen peroxide (H2O2)-derived necrosis in macrophages [46, 47].

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