Marine macroalgae are widely recognized as very prolific sources of both biologically active and structurally unique secondary metabolites. Terpenoids, acetogenins, and mixed metabolites of uncommon carbon skeletons and functionalities belong to the major groups of metabolites from these plants. Meroterpenoids are mixed biosynthetic products which contain terpenoids and polyketides fragments. Particularly, meroterpenoids of chromene structural class were found abundantly whithin the brown algae and revealed a variety of bioactivities such as cytotoxicity, antioxidant activity, anthelmintic activity, and inducement of the larval settlement of a hydrozoan.
As a part of our search for novel bioactive compounds from marine algae, the brown alga Sargassum siliquastrum was collected off the shore of Jeju Island. The collected samples of Sargassum siliquastrum were briefly dried under shade and repeatedly extracted for 2 days with a mixture (1:1) of acetone-CH2Cl2 (1.5 L X 2) and MeOH (1.5 L X 2), respectively. The combined crude extracts of Sargassum siliquastrum were fractionated into n-hexane, 85% aq. MeOH, n-BuOH, and water fractions.
Antioxidant activities of n-hexane, 85% aq. MeOH, n-BuOH, and water fractions including crude extracts of Sargassum siliquastrum were evaluated using 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical, authentic ONOO-, and ONOO- generated from SIN-1. Scavenging activity of each fraction on DPPH radical increased in the order of 85% aq. MeOH > n-hexane > n-BuOH > H2O fractions, while that of each fraction on authentic ONOO- and induced ONOO- from SIN-1 increased in the order of 85% aq. MeOH > n-BuOH > H2O > n-hexane fractions.
On the basis of the above results, purification of the 85% aq. MeOH fraction using C18 reversed-phase column, followed by HPLC, resulted in the isolation of three new meroterpenoids (compounds 5-7) and four known meroterpenoids (compounds 1-4) of chromene class. The structure of the isolated compounds was established by extensive 2D NMR experiments such as 1H gDQCOSY, TOCSY, NOESY, gHMQC, and gHMBC, and by comparison with published spectral data.
In our measurement for evaluating antioxidant activity of compounds 1-7 using DPPH radical, authentic ONOO-, and ONOO- generated from SIN-1, all of them exhibited significantly high activities.
The protective effect of compounds from Sargassum siliquastrum on H2O2-induced oxidative stress was examined in HT1080 cells. The levels of intracellular reactive oxygen species (ROS) were measured using 2’,7’-dichlorofluorescin diacetate (DCFH-DA). All solvent fractions including crude extracts of Sargassum siliquastrum not only significantly decreased levels of intracellular ROS in a dose-dependent manner but also exhibited a protective effect on oxidative damage of purified genomic DNA. In addition, they increased intracellular GSH level in a dose dependent manner. Compound (1-7) significantly decreased those of intracellular ROS while they increased that of intracellular GSH at concentration of 5 ㎍/㎖. Therefore, these results suggested that these active compounds from Sargassum siliquastrum could be developed as a candidate for potential natural antioxidant related to oxidative stress.
Antiobesity effect of each fraction was estimated by measuring levels of glycerol and triglyceride. When 3T3-L1 adipocytes are treated with a good antiobestic material, glycerol secretion is increased and triglyceride (TG) accumulation is reduced, compared to control adipocytes. Among tested samples, 85% aq. MeOH and n-BuOH fractions revealed very good antiobesitic effect, significantly increasing glycerol secretion and decreasing TG accumulation, respectively. Based upon these results, it is suggested that further investigation should be performed on the isolation and identification of the antiobesitic components in its active fraction.