Fig. 2.

Shikonin reactivity with O₂^•−. The reaction of shikonin with O₂^•− was tested by EPR in a competition assay with CYPMPO (see Materials and Methods). (A) a representative EPR spectrum of the CYPMPO-O₂^•− adduct. (B) Shikonin or alkannin was used at concentrations of 5 and 10 µM, and Trolox, at 20 and 40 µM. Resulted signal decreases are shown as a plot of I₀/I – 1 against [antioxidant]₀/[CYPMPO]₀. The slopes of the lines represent k_AOx/k_CYPMPO for shikonin (upper panel), alkannin (lower panel), and Trolox (squares, both panels) as a standard. I₀: EPR peak height in the presence of spin trap alone; I: EPR signal height in the presence of spin trap plus antioxidant. Plotted values are means ± SD (n = 5).