Synthesis and antioxidant evaluation of (S,S) and (R,R) secoisolariciresinol diglucosides (SDGs)

Secoisolariciresinol diglucosides (SDGs) (S,S) SDG 1 (major isomer in flaxseed) and (R,R) SDG 2 (minor isomer in flaxseed) were synthesized from vanillin via secoisolariciresinol and glucosyl donor 7 through a concise route that involved chromatographic separation of diastereomeric diglucoside derivatives (S,S) 8 and (R,R) 9. Synthetic (S,S) SDG 1 and (R,R) SDG 2 exhibited potent antioxidant properties (EC50 equal  292.17 plus and less 27.71 lM and 331.94 plus and less 21.21 lM, respectively), which compared well with that of natural (S,S) SDG 1 (EC50 equal  275.24 plus and less 13.15 lM). These values are significantly lower than those of ascorbic acid (EC50 equal 1129.32 plus and less 88.79 lM) and alpha tocopherol (EC50 equal  944.62 plus and less 148.00 lM). Compounds (S,S) SDG 1 and (R,R) SDG 2 also demonstrated powerful scavenging activities against hydroxyl [natural (S,S) SDG 1: 3.68 plus and less 0.27; synthetic (S,S) SDG-1: 2.09 plus and less 0.16; synthetic (R,R) SDG 2: 1.96 plus and less 0.27], peroxyl [natural (S,S) SDG 1: 2.55 plus and less 0.11; synthetic (S,S) SDG 1: 2.20 plus and less 0.10; synthetic (R,R) SDG 2: 3.03 plus and less 0.04] and DPPH [natural (S,S) SDG 1: EC50 equal  83.94 plus and less 2.80 lM; synthetic (S,S) SDG 1: EC50 equal  157.54 plus and less 21.30 lM; synthetic (R,R) SDG 2: EC50 equal  123.63 plus and less 8.67 lM] radicals. These results confirm previous studies with naturally occurring (S,S) SDG 1 and establish both (S,S) SDG 1 and (R,R) SDG 2 as potent antioxidants and free radical scavengers for potential in vivo use. (Authors abstract)

Secoisolariciresinol diglucoside [SDG, (S,S) SDG 1] is a major component of the lignans in flaxseed. (S,S) SDG 1 is a potent antioxidant in vitro and in vivo, and a powerful in vitro scavenging agent against hydroxyl free radicals.  Increased generation of reactive oxygen species (ROS) such as superoxide anion (O2), hydroxyl radical (OH), and hydrogen peroxide leads to tissue damage under various pathological conditions.   These species result in cellular damage through oxidative modification of cellular membrane lipids, proteins, and the genomic DNA.  SDG as an antioxidant may have therapeutic potential under various experimental and disease conditions that are associated with oxidative tissue damage, such as in cancer patients undergoing radiotherapy. In previous studies, flaxseed ameliorated the radiation induced inflammation and oxidative stress in mice, and irradiated mice fed with flaxseed diets enriched with (S,S) SDG 1 showed improved hemodynamic indices and survival over the control. The reducing power of synthesized (S,S) SDG 1 and (R,R) SDG 2 as well as their free radical scavenging activity against hydroxyl, peroxyl, and DPPH free radicals were tested in this study.

The reducing power of synthetic (S,S) SDG 1, synthetic (R,R)- SDG-2, natural (S,S) SDG 1, ascorbic acid, and a-tocopherol was determined by the reduction of K3FeCN6 in the presence of FeCl3, as measured by the absorbance of the resulting ferric-ferrous complex. The reducing power of synthetic (S,S) SDG 1, synthetic (R,R) SDG 2, and natural (S,S) SDG 1 was significantly concentration-dependent at higher concentrations; however, at all concentrations tested, the SDGs had comparable or higher reducing power than known antioxidants ascorbic acid and tocopherol, with a notable increase in potency in the 200–500 lM range. The ability of synthetic (S,S) SDG 1 and (R,R) SDG 2 to scavenge hydroxyl and peroxyl radicals as manifested by their inhibition of the oxidation of fluorescein was assessed by the Hydroxyl Radical Antioxidant Capacity (HORAC, gallic acid standard) and Oxygen Radical Antioxidant Capacity assays (ORAC, Trolox standard), respectively. Fluorescein oxidation by hydroxyl radicals was decreased by synthetic (S,S) SDG 1 and synthetic (R,R) SDG 2 in a concentration-dependent manner and was found to be twofold higher than gallic acid.

The free radical scavenging activities of synthetic (S,S) SDG 1 and (R,R) SDG 2 were determined using a 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging assay and were compared to those of natural (S,S) SDG 1, ascorbic acid, and a-tocopherol. Synthetic (R,R) SDG 2 (123.63 plus and less 8.67 lM) and synthetic (S,S) SDG 1 (157.54 plus and less 21.30 lM) were not significantly different. These values were similar to those exhibited by natural (S,S) SDG 1 (83.94 plus and less 2.80 lM) and a-tocopherol (132.81 plus and less 12.57 lM) but considerably lower than that shown by ascorbic acid (439.56 plus and less 11.81 lM).  Further in vitro and in vivo studies to determine the mechanism of action and usefulness of the SDGs as antioxidants and protectors against radiation-induced tissue damage are in progress. (Editors comments)