Phytochemical composition and antioxidant and antimicrobial activities of Solanum retroflexum leaf extracts

M.Tech. (Food Technology)Abstract: Solanum retroflexum is a herbal plant that is consumed in some parts of the Venda region in Limpopo Province, South Africa. In this study, the phytochemical composition, antioxidant and antimicrobial activity of leaf extracts from the plant were investigated. Extraction of the metabolites was carried out with pressurized hot water extraction (PHWE) and different concentrations of aqueous methanol (40 %, 60 %, 80 %, methanol/water, v/v). Ultra-performance liquid chromatography coupled to quadruple-time-of-flight mass spectrometer (UPLC-qTOF-MS) technique was used to achieve an untargeted metabolite fingerprinting of the S. retroflexum leaf extracts. In total, 30 phytochemicals including alkaloids, flavonoids and cinnamic acids derivatives were identified and characterised from the aqueous methanolic leaf extracts. The identified compounds were from different classes namely, phenolics (22), flavonoids (6) and alkaloids (2). The different concentrations of aqueous methanol resulted in very similar phytochemical composition. As expected, chlorogenic acids were identified in S. retroflexum leaf extracts. Interestingly, both trans- and cis- isomers of chlorogenic acid were identified in this study. Among the flavonoids, quercetin and kaemferol were present, while two alkaloids, Solasonine and Solamargine were identified in the aqueous methanol leaf extracts. With PHWE, optimum extraction occurred at the highest temperature (250 °C). A total of 6 health-promoting metabolites were extracted and these included Quercetin-3-rutinoside, Kaempferol-3-0-rutinoside, Kaempferol-3-0-glucoside, 3-Caffeoylquinic acid, 5-Caffeoylquinic acid and 3, 4-di-Caffeoylquinic acid. Quercetin rutinoside was extracted at all extraction temperatures, while alkaloids were absent in the PHWE extracts. Solanum retroflexum aqueous methanol leaf extracts showed strong scavenging activities against DPPH and ABTS radicals. Similarly, the FRAP assay showed some antioxidant activity of the leaf extracts. The concentration of methanol did not substantially affect the antioxidant activity with all the assays, while the highest concentration of total phenol content did not exhibit high antioxidant activity in this study. The antimicrobial activity of S. retroflexum aqueous methanol leaf extracts was investigated with the disc diffusion and minimal inhibitory concentration assays. About 14 microorganisms constituting of 6 Gram-negative and 8 Gram-positive bacteria were used..