Reaction of β-Alkannin (Shikonin) with Reactive Oxygen Species: Detection of β-Alkannin Free Radicals

Abstract

β-Alkannin (shikonin), a compound isolated from the root of Lithospermum erythrorhizon Siebold Zucc., has been used as a purple dye in ancient Japan and is known to exert an anti-inflammatory activity. This study aimed to understand the biological activity in terms of physico-chemical characteristics of β-alkannin. Several physico-chemical properties including proton dissociation constants, half-wave potentials and molecular orbital energy of β-alkannin were elucidated. This compound shows highly efficient antioxidative activities against several types of reactive oxygen species (ROS), such as singlet oxygen (1O2), superoxide anion radical (O2), hydroxyl radical (OH) and tert-butyl peroxyl radical (BuOO) as well as iron-dependent microsomal lipid peroxidation. During the reactions of β-alkannin with 1O2, O2 and BuOO, intermediate organic radicals due to β-alkannin were detectable by ESR spectrometry. Compared with the radicals due to naphthazarin, the structural skeleton of β-alkannin, the β-alkannin radical observed as an intermediate in the reactions with 1O2 and O2 was concluded to be a semiquinone radical. On the other hand, during the reactions of β-alkannin and naphthazarin with BuOO, ESR spectra different from the semiquinone radical were observed, and proposed to result from the abstraction of hydrogen atoms from phenolic hydroxyl groups of β-alkannin by BuOO. Based on the ROS-scavenging abilities of β-alkannin, the compound was concluded to react directly with ROS and exhibits antioxidative activity, which in turn exerts anti-inflammatory activity.