On the kinetics and mechanism of the reaction of cysteine and hydrogen peroxide in aqueous solution

The kinetics and mechanism of oxidation of cysteine by hydrogen peroxide have been recently investigated by Luo, Smith, and Anderson. 1. Luo D. Smith S.W. Anderson B.D. Kinetics and mechanism of the reaction of cysteine and hydrogen peroxide in aqueous solution. J Pharm Sci. 2005; 94: 304-316 Abstract Full Text Full Text PDF PubMed Scopus (173) Google Scholar The disappearance of cysteine (CSH) and hydrogen peroxide (H2O2) and the appearance of cystine (CSSC) were monitored by HPLC and analysis of that data lead the authors to propose the mechanism of Scheme I: rapid equilibrium to form cysteinate (CS−), followed by irreversible oxidation of CS− by H2O2 to give cysteine sulfenic acid (CSOH), and nucleophilic attack on CSOH by CS− to yield CSSC. Nonlinear least-squares fitting of the observed concentration-time profiles yielded values of k1=15.2±0.1/M · s and k2=720±70/M · s for the second-order rate constants at 25°C and pH 6.0. 1. Luo D. Smith S.W. Anderson B.D. Kinetics and mechanism of the reaction of cysteine and hydrogen peroxide in aqueous solution. J Pharm Sci. 2005; 94: 304-316 Abstract Full Text Full Text PDF PubMed Scopus (173) Google Scholar Regrettably, we do not agree with the analysis of Luo, Smith, and Anderson. We suggest herein that their data afford no insight into the mechanism that follows the rate-determining step, oxidation of CS− by H2O2. Furthermore, as cited in the original study, the kinetics and mechanism of that step have been previously established. 2. Barton J.P. Packer J.E. Sims R.J. Kinetics of the reaction of hydrogen peroxide with cysteine and cysteamine. J Chem Soc, Perkin Trans 2: Phys Org Chem. 1973; 11: 1547-1549 Crossref Scopus (72) Google Scholar , 3. Winterbourn C.C. Metodiewa D. Reactivity of biologically important thiol compounds with superoxide and hydrogen peroxide. Free Rad Biol Med. 1999; 27: 322-328 Crossref PubMed Scopus (641) Google Scholar , 4. Radi R. Beckman J.S. Bush K.M. Freeman B.A. Peroxynitrite oxidation of sulfhydryls. The cytotoxic potential of superoxide and nitric oxide. J Biol Chem. 1991; 266: 4244-4250 Abstract Full Text PDF PubMed Google Scholar