S-Nitrosothiols: Formation, Decomposition, Reactivity and Possible Physiological Effects

Three biologically-active aminothiols cysteamine (CA), DL-cysteine (CYSH) and DL-homocysteine, were studied in this thesis. These aminothiols react with nitrous acid (HNO2), prepared in situ, to produce S-nitrosothiols (RSNOs): S-nitrosocyteamine (CANO), S-nitrosocysteine (CYSNO) and S-nitrosohomocysteine (HCYSNO). They also react with S-nitrosoglutathione (GSNO) and S-nitroso-N-acetylpenicillamine (SNAP) through a transnitrosation reaction to produce their corresponding RSNOs. A detailed kinetics and mechanistic study on the formation of these RSNOs and their subsequent decomposition to release nitric oxide (NO) were studied. For all three aminothiols the stoichiometry of their reaction with nitrous acid is strictly 1:1 with the formation of one mole of RSNO from one mole of HNO2. In all cases, the nitrosation reaction is first order in nitrous acid, thus implicating it as a nitrosating agent in mildly acidic pH conditions. Acid catalyzes nitrosation after nitrous acid has saturated, implicating another nitrosating agent, the nitrosonium cation, NO+ (which is produced from the protonation of nitrous acid) as a contributing nitrosating species in highly acidic environments. The acid catalysis at constant nitrous acid concentrations suggests that the nitrosonium cation nitrosates at a much higher rate than nitrous acid. Nitric oxide itself was not detected as a nitrosant. Bimolecular rate constants for the nitrosation of CA, CYSH and HCYSH were deduced to be 17.9, 6.4, 0.09 M-1 s-1 for the nitrosation by nitrous acid and 8.25 x 1010, 2.89 x 1010 and 6.57 x 1010 M-1 s-1 for the nitrosation by nitrosonium cation respectively. A linear correlation was obtained between the rate constants and the pKa of the sulfur center of the aminothiols for nitrosation by NO+. The stabilities of the three RSNOs were found to be affected by metal ions. They were unstable in the presence of metal ions, with half-lives of few seconds. However, in the presence of metal ion chelators, they were found to be relatively stable with half-lives of 10, 30 and 198 hours for CYSNO, CANO and HCYSNO respectively. The relative stability of HCYSNO may be an advantage in the prevention of its metabolic conversion to homocysteine thiolactone, the major culprit in HCYSH pathogenesis. This dissertation has thus revealed new potential therapeutic way for the modulation of HCYSH related cardiovascular diseases