Properties of selected S-nitrosothiols compared to nitrosylated WR-1065

Detailed mechanistic investigation into the S-nitrosation of cysteamine

The nitrosation of cysteamine (H₂NCH₂CH₂SH) to produce cysteamine-S-nitrosothiol (CANO) was studied in slightly acidic medium by using nitrous acid prepared in situ. The stoichiometry of the reaction was H₂NCH₂CH₂SH + HNO₂ → H₂NCH₂CH₂SNO + H₂O. On prolonged standing, the nitrosothiol decomposed quantitatively to yield the disulfide, cystamine: 2H₂NCH₂CH₂SNO → H₂NCH₂CH₂S-SCH₂CH₂NH₂ + 2NO. NO₂ and N₂O₃ are not the primary nitrosating agents, since their precursor (NO) was not detected during the nitrosation process. The reaction is first order in nitrous acid, thus implicating it as the major nitrosating agent in mildly acidic pH conditions. Acid catalyzes nitrosation after nitrous acid has saturated, implicating the protonated nitrous acid species, the nitrosonium cation (NO⁺) 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. Bimolecular rate constants for the nitrosation of cysteamine by nitrous acid and by the nitrosonium cation were deduced to be 17.9 ± 1.5 (mol/L)^-1 s^-1 and 6.7 × 10⁴ (mol/L)^-1 s^-1, respectively. Both Cu(I) and Cu(II) ions were effective catalysts for the formation and decomposition of the cysteamine nitrosothiol. Cu(II) ions could catalyze the nitrosation of cysteamine in neutral conditions, whereas Cu(I) could only catalyze in acidic conditions. Transnitrosation kinetics of CANO with glutathione showed the formation of cystamine and the mixed disulfide with no formation of oxidized glutathione (GSSG). The nitrosation reaction was satisfactorily simulated by a simple reaction scheme involving eight reactions.

Direct vasoactive properties of thienopyridine-derived nitrosothiols

Thienopyridines (ticlopidine, clopidogrel, and prasugrel) require in vivo metabolism to exhibit a critical thiol group in the active form that binds to the P2Y12 platelet receptor to inhibit platelet activation. We hypothesized that formation of thienopyridine-derived nitrosothiols (ticlopidine-SNO, clopidogrel-SNO, and prasugrel-SNO) occurs directly from the respective parent drug. Pharmaceutical-grade thienopyridine (ticlopidine, clopidogrel chloride, clopidogrel sulfate, clopidogrel besylate, or prasugrel) was added to nitrite in aqueous solution to form the respective thienopyridine-SNO (Th-SNO). An isolated aortic ring preparation was used to test vasoactivity of the Th-SNO derivatives. Increasing nitrite availability resulted in increased Th-SNO formation for all drugs (other than ticlopidine). Th-SNO induced significant endothelium-independent relaxation of preconstricted aortic rings. Clopidogrel-chloride-SNO displayed rapid-release kinetics in a chemical environment, which was reflected by immediate and transient vasorelaxation when compared with the SNO derivatives of the other thienopyridines. Accounting for differences in yield, clopidogrel-chloride-SNO exhibited the greatest propensity to immediately relax vascular tissue. Th-SNO derivatives exhibit nitrovasodilator properties by supplying NO that can directly activate vascular soluble guanylate cyclase to induce vasorelaxation. Differences in SNO yield and vasoactivity exist between thienopyridine preparations that might be important to our understanding of the direct pharmacological effectiveness of thienopyridines on vascular and platelet function.

Effects of Amifostine on Glycerol-Pretreated Rabbit Kidneys

Glycerol-induced acute renal failure is an experimental model for myoglobinuric nephropathy. Amifostine is a cytoprotective agent which scavenges the free radicals. Since there is enhanced production of reactive oxygen metabolites in glycerol-induced acute renal failure, we wanted to examine whether amifostine has a protective role against vascular reactivity and histological changes in kidneys isolated from glycerol-pretreated rabbits. Perfusion pressure was recorded from kidneys obtained from rabbits injected with glycerol 3 hr before the experiments and from glycerol-pretreated and non-pretreated rabbits injected with amifostine 30 min. before the experiments. Acetylcholine-induced (10(-8)-10(-5) M) vasodilatation was tested following the construction of submaximal vasoconstriction by phenylephrine. Histological investigation was performed using light microscope. Acetylcholine-induced vasodilatation was found to be significantly decreased in glycerol, glycerol+amifostine and amifostine groups compared to controls at all concentrations. Reduction in acetylcholine-induced vasodilation was more prominent in amifostine group compared to amifostine+glycerol group. There was histological renal damage in all experimental groups and this damage was more pronounced in glycerol+amifostine group. In conclusion, contrary to expectation, amifostine per se led to histological damage and potentiated the histological damage caused by glycerol and produced a decrease in acetylcholine-induced vasodilatation. The mechanisms by which amifostine exerts its effects are not known.