Scavenging Effects of Metal Complexes of Water-Soluble Thiacalix[4]arenetetrasulfonate on Superoxide Anion Radicals

The scavenging effects of metal complexes of thiacalix[4]arenetetrasulfonate (Me-TCAS[4], Me=H2, Fe3+, Mn3+, Mn2+, Cu2+, and Zn2+) on superoxide anion radicals (O2−) generated from the xanthine–xanthine oxidase system were investigated by the nitroblue tetrazolium (NBT) method and electron spin resonance (ESR) spin-trapping method using 5,5-dimethyl-1-pyrroline-N-oxide as a trapping reagent. As a reference, calix[4]arenetetrasulfonate (H2-CAS[4]), calix[6]arenehexasulfonate (H2-CAS[6]) and calix[8]areneoctasulfonate (H2-CAS[8]) were also examined. The results by the NBT method indicated that Fe3+- and Mn3+-TCAS[4] exhibited the highest O2− scavenging activity among Me-TCAS[4] and H2-CAS[n] (n=4, 6, 8) in this study. The IC50 values of Fe3+- and Mn3+-TCAS[4] for O2− scavenging activity were estimated to be 5.3 and 7.8 μM, respectively, and were almost the same as those of tannin acid, catechin and their derivatives, which are known as very effective scavengers of O2−. Scavenging activities were in the order of Fe3+- and Mn3+-TCAS[4]>>Mn2+-, Cu2+-, and Zn2+-TCAS[4]>>H2-TCAS[4] and H2-CAS[n] (n=4, 6, 8). Each activity of Me-TCAS[4] (Me=Fe3+, Mn3+, Mn2+, Cu2+, and Zn2+) was higher than that of the corresponding metal ion, indicating that H2-TCAS[4] has the ability to raise the activity of the metal ion itself by forming a complex. Also, the ESR spin-trapping method revealed that Fe3+- and Mn3+-TCAS[4] showed high O2− scavenging activities, similarly to the results by the NBT method.