Reactivity of hypotaurine and cysteine sulfinic acid toward carbonate radical anion and nitrogen dioxide as explored by the peroxidase activity of Cu,Zn superoxide dismutase and by pulse radiolysis

Abstract

Hypotaurine and cysteine sulfinic acid are known to be readily oxidized to the respective sulfonates, taurine and cysteic acid, by several oxidative agents that may be present in biological systems. In this work, the relevance of both the carbonate anion and nitrogen dioxide radicals in the oxidation of hypotaurine and cysteine sulfinic acid has been explored by the peroxidase activity of Cu,Zn superoxide dismutase (SOD) and by pulse radiolysis. The extent of sulfinate oxidation induced by the system SOD/H₂O₂ in the presence of bicarbonate (CO₃^•– generation), or nitrite (^•NO₂ generation) has been evaluated. Hypotaurine is efficiently oxidized by the carbonate radical anion generated by the peroxidase activity of Cu,Zn SOD. Pulse radiolysis studies have shown that the carbonate radical anion reacts with hypotaurine more rapidly (k = 1.1 × 10⁹ M⁻¹s⁻¹) than nitrogen dioxide (k = 1.6 × 10⁷ M⁻¹s⁻¹). Regarding cysteine sulfinic acid, it is less reactive with the carbonate radical anion (k = 5.5 × 10⁷ M⁻¹s⁻¹) than hypotaurine. It has also been observed that the one-electron transfer oxidation of both sulfinates by the radicals is accompanied by the generation of transient sulfonyl radicals (RSO₂^•). Considering that the carbonate radical anion could be formed in vivo at high level from bicarbonate, this radical can be included in the oxidants capable of performing the last metabolic step of taurine biosynthesis. Moreover, the protective effect exerted by hypotaurine and cysteine sulfinate on the carbonate radical anion-mediated tyrosine dimerization indicates that both sulfinates have scavenging activity towards the carbonate radical anion. However, the formation of transient reactive intermediates during sulfinate oxidation by carbonate anion and nitrogen dioxide radical may at the same time promote oxidative reactions.

Keywords::

• sulfinate
• sulfonyl radical
• taurine
• antioxidant
• dityrosine
• scavenger

Declaration of interest

The authors report no declarations of interest. The authors alone are responsible for the content and writing of the paper.

Notes

¹From the pK_a values of CO₃^•– (< 0) and that of the sulfinic group of HTAU (2.5), the second order rate constant for this reaction should be almost the same at neutral pH.

²The sulfinic group of HTAU and CSA, with pK_a values of 2.5 and 1.5, respectively, reacts in the anionic form within the pH range studied here.

³In a simple competition model, the percentage inhibition of tyrosine dimerization in the presence of sulfinates (RSO₂^–) can be calculated using the following equation:

where:

x = % of inhibition of tyrosine dimerization

k_RSO2− is the rate constant of the reaction of sulfinates with carbonate anion radical (1.1 × 10⁹ M⁻¹s⁻¹ and 5.5 × 10⁷ M⁻¹s⁻¹ for HTAU and CSA, respectively) (this work)

k_Tyr = 4.5 × 10⁷ M⁻¹s⁻¹ is the rate constant of the reaction of tyrosine with carbonate anion radical [45].

With [Tyr] = 1 mM, and [RSO₂^–] = 1 mM resulted 96% and 54% inhibition of tyrosine dimerization for HTAU and CSA, respectively.