Antioxidant and antiproliferative properties of a tocotrienol-rich fraction from grape seeds
Introduction
In recent years, there has been much interest and research into the influence of diet on chronic diseases including cancer, coronary heart disease, atherosclerosis, and diabetes (Halliwell and Gutteridge, 1984, Steinberg et al., 1989). Recent epidemiological studies have suggested that increased consumption of whole grains, legumes, fruits, and vegetables is inversely associated with the risk of chronic diseases (Hu, 2002). This association may be attributed to natural antioxidants such as vitamin C, vitamin E, polyphenol, and flavonoids, which prevent free radical damage (Diplock et al., 1998, Shahidi, 2004).
Agricultural and industrial residues are attractive sources of potential natural antioxidants. Grape seeds, a byproduct of the winemaking or juice-processing industry, constitute about 5% by weight of the grape and contain 10–20% oil with a high vitamin E content, which is important for human health. Commercial grape seed oil contains 399–785 mg/kg vitamin E, depending on the variety and environmental growing conditions (Crews et al., 2006).
Vitamin E is a generic term for tocopherols and tocotrienols, which possess a saturated phytyl tail and an unsaturated isoprenoid side chain, respectively. Tocopherols and tocotrienols are closely related chemically; however, they have widely varying degrees of biological activities (Theriault, Chao, Wang, Gapor, & Adeli, 1999). α-Tocopherol is regarded as intracellular antioxidants due to their activity in inhibiting the peroxidation of polyunsaturated fatty acids in biological membranes. Although α-tocopherol is the most active form in the vitamin E group in vivo, hypocholesterolemic, antitumor, neuroprotective, and antioxidant activities of tocotrienols or a tocotrienol-rich fraction (TRF) have recently received much attention (Khanna et al., 2003, Nesaretnam et al., 2007, Qureshi et al., 2000). Recent papers have only demonstrated the biological effects of TRF from barley, palm, and rice bran oils. Qureshi, Burger, Peterson, and Elson (1986) reported that α-tocotrienol from barley was an inhibitor of HMG-CoA reductase, which is the rate-limiting enzyme of the cholesterol biosynthetic pathway. In several recent studies, γ- and δ-tocotrienols from palm oil were shown to inhibit the growth of human breast cancer cells in culture (Nesaretnam et al., 2004). Furthermore, Serbinova, Tsuchiya, Goth, Packer, and Kagan (1993) reported higher antioxidant activity with tocotrienol than with α-tocopherol against lipid peroxidation in rat liver microsomes. However, no study has examined the biological effects of grape seed-derived tocotrienol-rich fraction (TRF) compared to those of tocopherols. Here we aimed to evaluate the antioxidant and antiproliferative activities of TRF obtained from grape seeds in relation to those of α-tocopherol.
Section snippets
Chemicals
1,1-Diphenyl-2-picrylhydrazyl (DPPH), diammonium salt of 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), potassium ferricyanide, ferric chloride, ferrous chloride, ferozine [3-(2-pyridyl)-5,6-bis-(4-phenylsulphonic acid)-1,2,4-triazine], linoleic acid, and thiazolyl blue terazolium bromide (MTT) were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Tocopherol and tocotrienol standards were obtained from Merck (Darmstadt, Germany). All other reagents and solvents used were
Yields of methanolic extracts
The vitamin E isomers present in grape seeds (var. Campbell early) include α-tocopherol (αT), α-tocotrienol (αT3), γ-tocopherol (γT), and γ-tocotrienol (γT3). The vitamin E profiles and purities of hexane- and methanol-soluble fractions and TRF are presented in Table 1. Hexane was chosen to extract lipid-soluble substances from grape seeds such as tocopherols, tocotrienols, sterols, fatty acid esters, and triglycerides. Methanol was chosen to extract vitamin E from the hexane-soluble fraction
Acknowledgment
This study was supported by MAF/ARPC through the Grape Research Project Group.
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