Intramolecular Electron Transfer between Tyrosyl Radical and Cysteine Residue Inhibits Tyrosine Nitration and Induces Thiyl Radical Formation in Model Peptides Treated with Myeloperoxidase, H2O2, and NO2-

We investigated the effects of a cysteine residue on tyrosine nitration in several model peptides treated with myeloperoxidase (MPO), H(2)O(2), and nitrite anion (NO(2)(-)) and with horseradish peroxidase and H(2)O(2). Sequences of model peptides were acetyl-Tyr-Cys-amide (YC), acetyl-Tyr-Ala-Cys-amide (YAC), acetyl-Tyr-Ala-Ala-Cys-amide (YAAC), and acetyl-Tyr-Ala-Ala-Ala-Ala-Cys-amide (YAAAAC). Results indicate that nitration and oxidation products of tyrosyl residue in YC and other model peptides were barely detectable. A major product detected was the corresponding disulfide (e.g. YCysCysY). Spin trapping experiments with 5,5'-dimethyl-1-pyrroline N-oxide (DMPO) revealed thiyl adduct (e.g. DMPO-SCys-Tyr) formation from peptides (e.g. YC) treated with MPO/H(2)O(2) and MPO/H(2)O(2)/NO(2)(-). The steady-state concentrations of DMPO-thiyl adducts decreased with increasing chain length of model peptides. Blocking the sulfydryl group in YC with methylmethanethiosulfonate (that formed YCSSCH(3)) totally inhibited thiyl radical formation as did substitution of Tyr with Phe (i.e. FC) in the presence of MPO/H(2)O(2)/NO(2)(-). However, increased tyrosine nitration, tyrosine dimerization, and tyrosyl radical formation were detected in the MPO/H(2)O(2)/NO(2)(-)/YCSSCH(3) system. Increased formation of S-nitrosated YC (YCysNO) was detected in the MPO/H(2)O(2)/(*)NO system. We conclude that a rapid intramolecular electron transfer reaction between the tyrosyl radical and the Cys residue impedes tyrosine nitration and induces corresponding thiyl radical and nitrosocysteine product. Implications of this novel intramolecular electron transfer mechanism in protein nitration and nitrosation are discussed.