Horinouchi Rina, Yamamoto Yorihiro, Fujisawa Akio
School of Bioscience and Biotechnology, Tokyo University of Technology, 1404-1 Katakura-cho, Hachioji, Tokyo 192-0982, Japan.
J Clin Biochem Nutr. 2021 Jul;69(1):16-19. doi: 10.3164/jcbn.20-101. Epub 2021 Jan 16.
Singlet oxygen prefers to react with an electron-rich double bonds. We observed that the oxidation rate for uric acid with singlet oxygen increased with increasing pH and the oxidation rate dramatically was elevated at around pH 5.4 and 9.8, which are the acidity constants of uric acid, pK and pK, respectively. Furthermore, we observed that the absorbance near 200 nm and the molar extinction coefficient (ɛ) increased with increasing pH, similar to the change in oxidation rate. Computer calculations by Chong [Chong, 2013; 1(1)] revealed that uric acid elongates its C=N conjugated diene structure with increasing pH. This is correlated with an increase in the UV absorbance of C=C double bonds near 200 nm, and may indicate higher electron density in the double bonds. Therefore, we concluded that the increased oxidation rate is due to elongation of the C=N conjugated polyene system at higher pH. On the other hand, the major products were 4-hydroxyallantoin and parabanic acid (hydrolyzed to oxaluric acid at pH 10.7), suggesting that the reaction pathways were the same regardless of pH. Finally, possible reaction schemes are presented.
单线态氧更倾向于与富电子双键发生反应。我们观察到，尿酸与单线态氧的氧化速率随pH值升高而增加，且在pH约为5.4和9.8时氧化速率显著提高，这两个pH值分别是尿酸的酸度常数pK₁和pK₂。此外，我们观察到200 nm附近的吸光度和摩尔消光系数(ɛ)随pH值升高而增加，这与氧化速率的变化相似。Chong的计算机计算结果[Chong, 2013; 1(1)]表明，尿酸的C=N共轭二烯结构随pH值升高而伸长。这与200 nm附近C=C双键的紫外吸光度增加相关，可能表明双键中的电子密度更高。因此，我们得出结论，氧化速率的增加是由于在较高pH值下C=N共轭多烯体系的伸长。另一方面，主要产物是4-羟基尿囊素和仲班酸(在pH 10.7水解为草尿酸)，这表明无论pH值如何，反应途径都是相同的。最后，给出了可能的反应方案。
