Survey of pyrrolizidine alkaloids in the tribe Lithospermeae (Boraginaceae) from Pan-Himalaya and their chemotaxonomic significance

Family Boraginaceae has drawn the attention of many botanists for many years because of its systematic position and the presence of toxic pyrrolizidine alkaloids (PAs). Some researcher used these alkaloids as chemotaxonomic markers at the family and subfamily level in several botanical families. Therefore, this explorative study carried out with an aim to determine the distribution of selected PAs and their chemotaxonomic significance in the tribe Lithospermeae. For this, Lithospermeae species along with other Boraginaceae members from different parts of Pan-Himalayan regions in Pakistan and China were brought to Beijing Normal University in Beijing, China. For HPLC fingerprint analysis, dried silica gel aerial parts (500 mg) of twenty-four mature flowering species of Lithospermeae and the available PAs: supinine, heliotrine, lycopsamine, europine, and echimidine were used as chemotaxonomic markers. In the laboratory, the plants of Lithospermeae were investigated for pyrrolizidine alkaloids by using high performance liquid chromatography (HPLC). Five pyrrolizidine alkaloids were identified qualitatively in 24 species of tribe Lithospermeae from Pan-Himalaya. The results of the study support the previous research that PAs can be used as a taxonomic marker. The PAs lycopsamine and echimidine were common in all Onosma species except O. sinicum Diels, O. waddellii Duthie, and O. griffithii Vatke. So we proposed that lycopsamine and echimidine can be used as taxonomic markers for Onosma species from Pan-Himalayan regions. In the case of the genus Maharanga, M. emodi (Wall.) A. DC. and M. bicolor (Wall. ex G. Don) A. DC. show the same composition of PAs lycopsamine and echimidine. But these two species could be easily distinguished from each other; heliotrine is present in M. emodi but absent in M. bicolor. Arnebia guttata Bunge was the only species in the genus Arnebia which was detected for only europine. A. benthamii (Wall. ex) G. Don. I.M. Johnst. and A. euchroma (Royle) I.M. Johnst. were similar in PA composition for heliotrine, lycopsamine, and echimidine. These two species can easily be distinguished from each other: europine is present in the former genus but absent in the latter one. Moreover, the result of HPLC fingerprint analysis shows that supinine is distributed throughout all investigated species from the Pan-Himalayan regions except Lithospermum arvense L. So we proposed that supinine could serve as a chemotaxonomic marker in the tribe Lithospermeae.