Isolation of bioactive compounds from selected plants of South African flora (Helichrysum and Aspalathus species) for application in the preparation of biocompatible metal nanoparticles

Abstract

The Cape Floral Region of South Africa is among the world floristic regions. Its rich plant diversity makes it ecologically and economically significant for research in natural products and phyto-mediated nanoparticle synthesis. The endemic Aspalathus linearis (Fabaceae) and the Helichrysum foetidum (Asteraceae) are well distributed in the western cape of South Africa are used as herbal tea and medicinal plants respectively. Recent studies attributed their medicinal use to high antioxidants from chalcones and flavonoids. Flavonoids are known to play significant roles in natural product drugs, and in phyto-assisted metallic nanoparticles for use in nanomedicine. These functional properties were explored for the isolation of bioactive compounds from Aspalathus linearis and Helichrysum foetidum and their bio-evaluations for protection of skin cells against UVB-induced oxidative stress, mitigation of prediabetes, and toxicity on cancer cells. Also, their role in the preparation of biocompatible metal nanoparticles was established. UVB-induced skin diseases and high blood glucose are among increasing global health challenges that may lead to socio-economic disorders. A non-sedentary lifestyle is necessary for balanced glucose metabolism from ingested carbohydrates which in turn exposes the skin to prolong UV radiation from sunlight and inflammations of skin cells. Several chemically synthesized drugs mostly prescribed to counteract or mitigate the development of these disease conditions are associated with some side effects on administration. In the light of this, recent researchers have been motivated to formulate immune cosmetic prototypes and antidiabetic drugs at the nanoscale for effective delivery. Biosynthesised nanodrugs are believed to have near-zero side effects and are effective for targeted drug delivery due to their small sizes and large surface area of metal carriers. Gold metal ranks the highest among the metallic nanoparticles used due to its higher biocompatibility. The silver metal has also been indicated as a diagnostic tool for suspected diseases and therapy. However, their conventional synthesis methods by physical and chemical methods also pose concerns from the use of hazardous chemicals, high experimental cost, and long reaction time. Alternative methods that apply green chemistry for biosynthesis from plant phytochemicals have now been considered. In this study, A. linearis and H. foetidum were exhaustively extracted to obtain the total extract for each plant. Each extract was separately subjected to serial chromatography using different solvent systems and elution modes to isolate their respective compounds. Successive chromatography of A. linearis and chemical investigation of its polar fractions yielded a novel compound 4.1 named linearthin, a dihydrochalcone not previously reported in addition to the known compounds 4.2 (aspalathin), 4.3 (nothofagin). The chromatography of H. foetidum afforded and five known compounds, 7.1 (quercetin), 7.2 (helichrysetin), 7.4 (helichrysin), 7.5 (luteolin), 7.6 (Kaur-16-en-19-oic acid, 15-methoxylate), and 7.3 (3-methoxyqurecetin + luteolin mixiture). The structural elucidations of the compounds were confirmed from 1D, 2D NMR, HRMS, UV, and IR spectra data. Investigations on the protective effects of phytochemicals (chalcones) of A. linearis against UV-induced oxidative stress on skin cells is limited in the literature. In the same vein, the potentials of phytochemicals from H. foetidum in the management of diabetes is yet to be fully studied. Furthermore, the involvement of pure compounds isolated from both plants in the nanochemistry of metal nanoparticles has not been reported before this study. Hence, this study investigated the protection offered by A. linearis phytochemicals against UV-induced oxidative damage on skin cells of human immortalized keratinocytes (HaCaT) and melanocytes (SKMEL-1) via cell viability, cytotoxicity, and caspase-3 activation using ApoTox-Glo™ Triplex Assay. It also evaluated the phytochemicals of H. foetidum for glucose uptake, and inhibition of carbohydrate hydrolyzing enzymes (α-glucosidase and α-amylase enzymes) in vitro using HEK293 kidney cell and the application of the plants in the synthesis of metallic nanoparticles for drug discovery. From the chemical profile of A. linearis, and its bioassays in skin cells, the total extract (TE), 4.1, 4.2 and 4.3, all elicited cytoprotection from oxidative damage which is suggestive of their applications for skincare products. Interestingly, 4.1 was most cytoprotective against UVB irradiation of HaCaT cell line (over 24 hours) with an IC50 of 282 μg/mL and in the SK-MEL-1 cell line with IC50 values of 248.3 and 142.6 μg/mL over 4 and 24 hours, respectively. On the other hand, HaCaT cells exposed to 4.2 over 4 hours before UVB irradiation showed the highest degree of cytoprotection with an IC50 of 398.9 μg/mL among the four studied samples. Glucose uptake activities of HEK293 kidney cells by compounds of H. foetidum was significantly improved by the total extract (HF), 7.1 and 7.6 . All tested samples showed higher α-glucosidase inhibition compared to the acarbose standard. HF, 7.2 and 7.4 were most potent with IC50 at 16.3± 0.1 μg/mL, 19.4± 0.3 μg/mL, and 48.3 ± 0.1 μg/mL respectively. However, only 7.1 showed higher inhibition of α-amylase than the standard. Nano screening of the phytochemicals of both plants revealed that the total extract of both plants, 4.2, 7.2, and 7.4 most nano active. Subsequent biosynthesis with the nano active constituents of each plant led to the synthesis, characterisation, in vitro stability, and bio-evaluation of quasi-spherical shapes gold nanoparticles (2 – 12 nm) from H. foetidum, gold and silver nanoparticles (0.9 – 4.3 nm) from A. linearis. Summarily, this is the first scientific report on the biosynthesis of nanoparticles from compounds of H. foetidum and A. linearis with potential application in the management of diabetes and cancer cells. In addition to the in vitro evaluation of their respective compounds for glucose utilisation and against UVB-induced oxidative stress.