Antioxidant activities of alkannin, shikonin and Alkanna tinctoria root extracts in oil substrates

Alkannin and shikonin (A/S) are potent pharmaceutical substances with a wide spectrum of biological properties and comprise the active ingredients of several pharmaceutical and cosmetic preparations, and are used as food colorants. From a structural point of view, A/S bear both the quinone (naphthoquinone) and the phenolic moiety. Several hydroquinone compounds have been studied for their antioxidant effects, but very little is known about alkannin and Alkanna tinctoria root extracts that contain several naphthoquinone derivatives (mainly alkannin esters). The antioxidant activity of A/S and their derivatives, bearing both the quinone and the hydroquinone moiety, was studied in this paper. Several oils and fats were used as oil substrates for the antioxidant assay, namely, lard, corn oil, olive oil and sunflower oil. Dichloromethane extract of A. tinctoria roots, containing mainly alkannin esters, showed satisfactory antioxidant activity in lard. Shikonin and monomeric alkannin presented very good antioxidant activity in lard, but their polymerization resulted in partial loss of this activity. The combination of the dichloromethane extract of A. tinctoria roots with caffeic acid showed a synergistic effect. The oxidation of olive oil was retarded when monomeric or polymeric shikonin was present. It can be concluded that A/S and their derivatives, used as active ingredients in pharmaceutical, cosmetic and food preparations, may simultaneously exert antioxidant effects in oil substrates in addition to wound healing, anti-inflammatory and antimicrobial properties. The antioxidant activity of alkannin is low in corn oil, while monomeric shikonin, combined with citric acid, presents a very moderate antioxidant activity in sunflower oil. (C) 2004 Elsevier Ltd. All rights reserved.