A quality control perspective on Devil's claw, Harpagophytum procumbens and H. zeyheri: phytochemical analysis and DNA barcoding

Devil's claw is a popular natural anti-inflammatory phytomedicine. The preparation of Devil's claw products is complicated by the similarity between the morphotypes of Harpagophytum procumbens and H. zeyheri. This study investigated the potential of a validated ultra-high performance liquid chromatography coupled to mass spectrometry (UHPLC-MS) method to investigate the harpagoside content of methanol extracts of the secondary tubers of both species. In addition, DNA barcoding was used to determine the identity of the Harpagophytum species included in commercial Devil's claw products. Herbal reference material of H. procumbens and H. zeyheri, originally harvested in southern Africa, was morphologically authenticated and commercial products containing Harpagophytum were purchased on the internet using the search terms “Harpagophytum” or “Devil's claw.” High-performance thin-layer chromatography (HPTLC) and UHPLC-MS was used for the qualitative and quantitative determination of harpagoside content in commercial products. The UHPLC-MS method developed had a limit of detection (LOD) of 0.165 µg/mL and a limit of quantification (LOQ) of 0.499 µg/mL. Quantitative UHPLC-MS analysis revealed high variability in the harpagoside content for both species; H. procumbens (0.17-4.37%) and H. zeyheri (0.00-3.07%). Only 41% of H. procumbens and 17% of H. zeyheri raw materials contained harpagoside. Using HPTLC, 90% of the commercial products tested were determined to contain harpagoside. The results were congruent with the UHPLC-MS analysis where the harpagoside content was out of specification for 10% of analyzed commercial products. The harpagoside content in commercial products varied from not detected to 3.50%. PCR amplification followed by comparative sequencing analysis of trnL-F, and ycf1 plastid gene amplicons indicated specific sites in trnL-F at positions 118 and 146 and at position 538 in ycf1 amplicons suitable to distinguish the two Harpagophytum species. These sites were used to barcode the herbal products and Character-based Barcode Recognition Obtained with Nucleotide eXposés (BRONX) analyses verified the taxonomic identity of the Harpagophytum in the samples. The results revealed that 76% (16/21) of the commercial products tested were incorrectly labelled as H. procumbens and, instead, contained H. zeyheri. Our study suggests that DNA barcoding in combination with the UHPLC-MS and HPTLC could significantly support herbal product authentication. This study contributed knowledge and methods that can be implemented to enhance the quality of commercial Devil's claw products.