Isohexenylnaphthazarins from Lithospermum canescens (Michx.) Lehm. and Arnebia euchroma (Royle) Jonst. in vitro culture

Abstract

The alkannins, shikonins and related isohexenylnaphthazarin compounds are natural lipophilic red pigments. They occur in many species that belong to the Boraginaceae family. Because of the importance of the shikonin-related compounds for pharmaceutical and cosmetic applications, the species from boraginaceous, among them Lithospermum and Arnebia genus, are investigated using biotechnology approaches. Our investigation aimed at isolating and elucidating structures of pigments of shikonin type and another compounds from hairy roots of Lithospermum canescens and Arnebia euchroma callus and cell suspension culture. Four alkannines: isobutylalkannin, β,β-dimethylacrylalkannin, acetylalkannin and β-hydroxyisovalerylalkannin, together with ACS and shikonofuran C and shikonofuran D were isolated and determined from the n-hexane extract of L. canescens transgenic roots by modern spectral means (MS, NMR), while several phytosterols, esterified acids as well as hydrocarbons were characterised by GC-MS. This is the first detailed report on chemical composition of transgenic roots from species of Lithospermum genus. The phytochemical investigation of n-hexane extract from callus and cell suspension culture of A. euchroma with respect to pigment compounds resulted in the isolation of the following alkannin derivatives: deoxyalkannin, alkannin, acetylalkannin, isobutyrylalkannin, β-hydroxyisovalerylalkannin, α-methylbutyrylalkannin, propionylalkannin, teracrylalkannin. Their structures were determined by MS and NMR methods. All mentioned pigments except deoxyalkannin were isolated for the first time from Arnebia in vitro cultures. Isobutyrylalkannin and propionylalkannin are reported in the present work as novel metabolites within the Arnebia genus.