Aquart Danielle V, Dasgupta Tara P
Department of Chemistry, University of the West Indies, Mona Campus, Kingston 7, Jamaica.
Biophys Chem. 2004 Feb 1;107(2):117-31. doi: 10.1016/j.bpc.2003.08.011.
The reductive decomposition of both SNAP and SNOCap by ascorbate in aqueous solution (in the presence of EDTA) was thoroughly investigated. Nitric oxide (NO) release from the reaction occurs in an ascorbate concentration and pH dependent manner. Rates and hence NO release increased drastically with increasing pH, signifying that the most highly ionized form of ascorbate is the more reactive species. The experiments were monitored spectrophotometrically, and second-order rate constants calculated at 37 degrees C for the reduction of SNAP are k(b)=9.81+/-1.39 x 10(-3) M(-1) s(-1) and k(c)=662+/-38 M(-1) s(-1) and for SNOCap are k(b)=2.57+/-1.29 x 10(-2) M(-1) s(-1) and k(c)=49.7+/-1.3 M(-1) s(-1). k(b) and k(c) are the second-order rate constants via the ascorbate monoanion (HA-) and dianion (A2-) pathways, respectively. Activation parameters were also calculated and are DeltaHb++ =93+/-7 kJ mol(-1), DeltaSb++ =15+/-2 J K(-1) mol(-1) and DeltaHc++ =51+/-5 kJ mol(-1), DeltaSc++ =-28+/-3 J K(-1) mol(-1) with respect to the reactions involving SNAP. Those for the reaction between SNOCap and ascorbate were calculated to be DeltaHb++ =63+/-11 kJ mol(-1), DeltaSb++ =-71+/-20 J K(-1) mol(-1) and DeltaHc++ =103+/-7 kJ mol(-1), DeltaSc++ =118+/-8 J K(-1) mol(-1). The effect of Cu2+/Cu+ ions on the reductive decompositions of these S-nitrosothiols was also investigated in absence of EDTA. SNOCap exhibits relatively high stability at near physiological conditions (37 degrees C and pH 7.55) even in the presence of micromolar concentrations of Cu2+, with decomposition rate constant being 0.011 M(-1) s(-1) in comparison to SNAP which is known to be more susceptible to catalytic decomposition by Cu2+ (second-order rate constant of 20 M(-1) s(-1) at pH 7.4 and 25 degrees C). It was also observed that the reductive decomposition of SNAP is not catalyzed by alkali metal ions, however, there was an increase in rate as the ionic strength increases from 0.2 to 0.5 mol dm(-3) NaCl.
对水溶液中（存在乙二胺四乙酸的情况下）抗坏血酸盐对SNAP和SNOCap的还原分解进行了全面研究。反应中一氧化氮（NO）的释放以抗坏血酸盐浓度和pH值依赖的方式发生。随着pH值升高，反应速率以及NO释放量急剧增加，这表明抗坏血酸盐的高度离子化形式是更具反应活性的物种。实验通过分光光度法进行监测，并计算了37℃下SNAP还原反应的二级速率常数，k(b)=9.81±1.39×10⁻³ M⁻¹ s⁻¹，k(c)=662±38 M⁻¹ s⁻¹；对于SNOCap，k(b)=2.57±1.29×10⁻² M⁻¹ s⁻¹，k(c)=49.7±1.3 M⁻¹ s⁻¹。k(b)和k(c)分别是通过抗坏血酸单阴离子（HA⁻）和二阴离子（A²⁻）途径的二级速率常数。还计算了活化参数，对于涉及SNAP的反应，ΔHb⁺⁺ =93±7 kJ mol⁻¹，ΔSb⁺⁺ =15±2 J K⁻¹ mol⁻¹，ΔHc⁺⁺ =51±5 kJ mol⁻¹，ΔSc⁺⁺ = -28±3 J K⁻¹ mol⁻¹。对于SNOCap与抗坏血酸盐之间反应的计算结果为，ΔHb⁺⁺ =63±11 kJ mol⁻¹，ΔSb⁺⁺ = -71±20 J K⁻¹ mol⁻¹，ΔHc⁺⁺ =103±7 kJ mol⁻¹，ΔSc⁺⁺ =118±8 J K⁻¹ mol⁻¹。在不存在乙二胺四乙酸的情况下，还研究了Cu²⁺/Cu⁺离子对这些S - 亚硝基硫醇还原分解的影响。即使存在微摩尔浓度的Cu²⁺，SNOCap在接近生理条件（37℃和pH 7.55）下仍表现出相对较高的稳定性，其分解速率常数为0.011 M⁻¹ s⁻¹，而SNAP已知更容易受到Cu²⁺催化分解（在pH 7.4和25℃下二级速率常数为20 M⁻¹ s⁻¹）。还观察到，碱金属离子不会催化SNAP的还原分解，然而，随着离子强度从0.2 mol dm⁻³ NaCl增加到0.5 mol dm⁻³，反应速率有所增加。
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