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Abstract
The protective activity of hypotaurine (HTAU) and cysteine sulphinic acid (CSA) on peroxynitrite-mediated oxidative damage has been assessed by monitoring different target molecules, i.e. tyrosine, dihydrorhodamine-123 (DHR) and glutathione (GSH). The inhibition of tyrosine oxidation exerted by HTAU and CSA both in the presence and the absence of bicarbonate can be ascribed to their ability to scavenge hydroxyl (•OH) and carbonate (CO3•−) radicals. HTAU and CSA also reduce tyrosyl radicals, suggesting that this repair function of sulphinates might operate as an additional inhibiting mechanism of tyrosine oxidation. In the peroxynitrite-dependent oxidation of DHR, the inhibitory effect of HTAU was lower than that of CSA. Moreover, while HTAU and CSA competitively inhibited the direct oxidation of GSH by peroxynitrite, HTAU was again poorly effective against the oxidation of GSH mediated by peroxynitrite-derived radicals. The possible involvement of secondary reactions, which could explain the difference in antioxidant activity of HTAU and CSA, is discussed.
| Abbreviations | ||
| CSA | = | cysteine sulphinic acid |
| DHR | = | dihydrorhodamine-123 |
| DTNB | = | 5,5-dithio-bis(2-nitrobenzoic acid) |
| DTPA | = | diethylenetriaminepentaacetic acid |
| GSH | = | glutathione |
| HRP | = | horseradish peroxidase |
| HTAU | = | hypotaurine |
| Abbreviations | ||
| CSA | = | cysteine sulphinic acid |
| DHR | = | dihydrorhodamine-123 |
| DTNB | = | 5,5-dithio-bis(2-nitrobenzoic acid) |
| DTPA | = | diethylenetriaminepentaacetic acid |
| GSH | = | glutathione |
| HRP | = | horseradish peroxidase |
| HTAU | = | hypotaurine |
Notes
1. IUPAC recommended names for peroxynitrite anion (ONOO–) and peroxynitrous acid (ONOOH) are oxoperoxonitrate (1–) and hydrogen oxoperoxonitrate, respectively. The term peroxynitrite is used to refer to the sum of ONOO– and ONOOH
2. The amount of peroxynitrite which reacted directly with sulphinates (equation 1) was calculated using the following equation:
where k1 is the rate constant of the reaction of peroxynitrite with HTAU (77.4 m−1s−1) or CSA (76.4 m–1s–1) at pH 7.4 and 25°C [40]; kdec=0.26 s–1 is the rate constant of peroxynitrite decomposition at pH 7.4 and 25°C [13]; [peroxynitrite]i and [sulphinate]i are the initial concentrations of the reactants
3. Different experimental conditions have been previously used to produce sulphonyl radicals: 65 mm CSA and 1 mg/ml HRP in the presence of 0.3 or 3 mm H2O2 [59]. It was also reported that the oxidation was largely non-enzymatic since heat-denatured HRP, as well as hematin, supported the reaction
4. In a simple competition model the percentage inhibition of GSH oxidation in the presence of sulphinates (RSO2−) can be calculated using the following equation:
where: x=% of inhibition of GSH oxidation; kRSO2 is the rate constant of the reaction of sulphinates with peroxynitrite at pH 7.4, 25°C (77.4 and 76.4 m–1s–1 for HTAU and CSA, respectively) [40]; kGSH=650 m–1s–1 is the rate constant of the reaction of GSH with peroxynitrite at pH 7.4, 25°C [13]. With [GSH] = 2.5 mm, resulted 32.0 and 48.7% of inhibition at [RSO2–] of 10 and 20 mm, respectively