Baicalein and pentamethyl-hydroxychromane (PMC) have been investigated for use as antioxidants. However, antioxidants may stimulate free radical formation under certain conditions. The aim of our study was to determine whether PMC and baicalein exhibit both pro-oxidant and antioxidant activities in human promyelocytic leukemia (HL-60) cells. In this study, electron spin resonance spectrometry was used to investigate the effects of baicalein and PMC on free radical formation. In HL-60 cells, baicalein and PMC produced hydroxyl and phenoxyl radicals, respectively, but each inhibited radical formation by the other. The PMC pro-oxidant activity required H2O2, whereas baicalein produced hydroxyl radicals during the cell resting state only. The antioxidant effect of baicalein on PMC-induced oxidative stress in HL-60 cells may involve myeloperoxidase inhibition, which produces the myeloperoxidase- protein radical. Our investigation of the antioxidant effects of baicalein on arachidonic acid (AA)-induced oxidative stress in HL-60 cells showed that the baicalein-phenoxyl radical was the primary product, and that either carbon-centered or acyl radicals were the secondary products. However, the antioxidant effects of PMC on AA-induced oxidative stress produced only nonradical products. In conclusion, we showed that baicalein displayed both pro-oxidant and antioxidant activities in HL-60 cells. PMC exhibited no pro-oxidant activity during the cells' resting state but produced the PMC-phenoxyl radical in the presence of H2O2.The reaction of baicalein with AA in HL-60 cells produced baicalein-derived phenoxyl radicals that may initiate various pro-oxidative reactions. However, PMC does not produce radicals when it acts as an antioxidant. Thus, PMC is more beneficial as an antioxidant than baicalein.
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