Variation of the Biologically Active Constituent Harpagoside in Harpagophytum procumbens and H. zeyheri
Harpagophytum procumbens and H. zeyheri (Pedaliaceae) are widely distributed in southern Africa. They are highly valued traditional medicines used in the treatment of inflammation and pain-related ailments such as rheumatoid arthritis, ulcers and boils . The biological activity of the species has been ascribed to the presence of iridoid glycosides such as harpagoside, harpagide and procumbide. However, only harpagoside is currently used as a 'biomarker' to determine the quality of raw material and commercial products. Harpagophytum procumbens is regarded as more commercially valuable as it contains higher levels of the biologically active constituent, harpagoside, however H. zeyheri is being used as an adulterant to meet high market demands . To explore the extent of variation, three hundred samples were collected throughout the geographical distribution of the two species and the harpagoside content quantified using LC-MS. The results confirm that both species contain harpagoside and that it is present in higher amounts in H. procumbens with a range of 0.17 – 4.37%, whereas in H. zeyheri, the range was 0.00 – 3.07%. According to the standard set by the European Pharmacopoeia, the harpagoside content is required to be at least 1.2%. All of the H. procumbens samples tested contained harpagoside, although only 61 met the 1.2% specification. Only 38 H. zeyheri samples contained harpagoside and only 26 met the specifications indicating that it is not a suitable substitute. This study confirmed that the two taxa are highly variable and intricate species as reflected by the significant variation in harpagoside content. This study provides valuable information regarding the difficulties of quality control and can also be used as a basis for the selection of raw material for propagation. Acknowledgements: The National Research Foundation of South Africa is thanked for its financial contribution to the study. References:  Van Wyk B-E, Gericke NP (2000) Peoples Plants. A guide to Useful Plants of Southern Africa. Briza Publications, Pretoria, South Africa.  Stewart KM, Cole D, (2005)J Ethnopharmacol, 100: 225 – 236.