Antioxidants from a Chinese medicinal herb – Lithospermum erythrorhizon

Seven compounds, deoxyshikonin (1), β,β-dimethylacrylshikonin (2), isobutylshikonin (3), shikonin (4), 5,8-dihydroxy-2-(1-methoxy-4-methyl-3-pentenyl)-1,4-naphthalenedione (5), β-sitosterol (6) and a mixture of two caffeic acid esters [7 (7a,7b)] were isolated from Lithospermum erythrorhizon Sieb et. Zucc. and identified by spectroscopic methods. Among them, compound 5 was isolated from this plant species for the first time. The antioxidant activities of the seven compounds were compared and evaluated through Rancimat method, reducing power and radical scavenging activity. Results showed that, except compound 6, another 6 compounds all exhibited obvious antioxidant activities against four different methods. Compounds 4 and 7 exerted much more potent antioxidant effects on retarding the lard oxidation than that of BHT and both were found to exhibit strong reducing power. Their antioxidant activities, assessed by Rancimat method and reducing power, decreased in the following order, respectively: compound 7 > 4 > BHT > 2 > 3 > 5 > 1 > 6 and compound 7 > BHT > 4 > 2 ∼ 3 ∼ 5 > 1> 6. In addition, compounds 1–5 all exerted very good radical scavenging activities toward ABTS+ but showed moderate inhibition of DPPH, while compound 7 presented as a powerful radical scavenger against both ABTS+ and DPPH. Thus, our results suggested that L. erythrorhizon could be a promising rich source of natural antioxidants.