Chemical constituents and antioxidant capacities of Asparagus africanus Lam.

Phytochemical study of the roots and aerial parts of Asparagus africanus Lam. have led to the identification of thirty-three specialized metabolites (1–33) including two undescribed steroidal saponins: (25 S)− 5β-spirostan-3β-ol-3-O-β-d-xylopyranosyl-(1→2)-β-d-glucopyranoside (4) and (25 S)− 5β-spirostan-3β-ol-3-O-β-d-glucopyranosyl-(1→6)-[β-d-glucopyranosyl-(1→2)-]β-d-glucopyranoside (5) and eight undescribed norlignans Aspafricanol A-E (6–10), Aspafricanene (11) and Aspafricanone A-B (12–13). Their structures were characterized by using a combination of 1D, and 2D NMR spectroscopic and mass spectrometry (HR-ESI-MS) analyses, as well as by comparison with literature values. The antioxidant potential of the phenolic constituents was evaluated by the 2,2-diphenyl-1-picrylhydrazyl (DPPH), hydroxyl radical antioxidant capacity (HORAC) and cupric ion reducing antioxidant capacity (CUPRAC) methods. The results revealed higher antioxidant capacity for syringaresinol (21) in DPPH, HORAC and CUPRAC assays with IC50 values of 15.9 ± 0.3, 86.0 ± 6.9 and 18.0 ± 1.7 µg/mL, respectively, compared to the positive controls (ascorbic acid, quercetin and Trolox). Additionally, compounds 7, 9 and 28 showed similar or stronger activities than the positive controls in DPPH (IC50 = 33.3 ± 5.7, 40.3 ± 0.6 and 46.7 ± 2.9 µg/mL, respectively), HORAC (IC50 = 74.7 ± 4.6, 66.7 ± 5.8 and 99.3 ± 1.1 µg/mL, respectively), and CUPRAC assays (IC50 = 128.7 ± 9.6, 123.3 ± 3.0 and 59.0 ± 1.7 µg/mL, respectively).