Antioxidant and DNA damage protective activities of selected endophytic actinobacteria isolated from Harpagophytum procumbens: A Kalahari desert-adapted plant

Given that synthetic antioxidants are currently in use and have been demonstrated to have known negative side effects, it is important to look for natural antioxidant sources. Endophytic actinobacteria, especially those associated with medicinal plants, are one understudied source. The goal of this study was to investigate whether the secondary metabolites of endophytic actinobacteria isolated from Harpagophytum procumbens, commonly known as the Devil's claw, have antioxidant and DNA damage protective properties. H. procumbens secondary tubers were collected from the Khomas and Hardap regions of Namibia, yielding a total of 23 isolated actinobacteria. The identities of the actinobacterial isolates were determined by sequencing the 16S rRNA gene. This revealed 6 families affiliated to 7 genera that included the dominant Streptomyces genus (57%) whereas rare genera (Agromyces, Nocardiopsis, Rubrobacter, Patulibacter, Rhodococcus, Curtobacterium) and 2 unidentified strains accounted for 43%. The antioxidant results revealed that the ethyl acetate extracts of Streptomyces species strain A36, had the highest phenolic content of 1.993 ± 0.004 and total antioxidant activity of 0.258 ± 0.001. Meanwhile, extracts from Streptomyces spp. strain B12, had the highest radical scavenging activity of 95% at 0.1 mg·mL -1 while B43, an unidentified actinobacterium, had the highest total reducing power of 0.849 ± 0.003. In addition, 61% of extracts showed the ability to protect DNA from damage when exposed to hydrogen peroxide. None of the samples contained terpenoids, while 13 (72%) contain alkaloids, 17 (94%) contain phenols and 8 (44%) contained flavonoids. The present study concludes that endophytic actinobacteria associated with H. procumbens are a potential source of bioactive compounds with pharmacological properties.