Disparate effects of similar phenolic phytochemicals as inhibitors of oxidative damage to cellular DNA

Phenolic phytochemicals are natural plant substances whose cellular effects have not been completely determined. Nordihydroguaiaretic acid (NDGA) and curcumin are two phenolic phytochemicals with similar molecular structures, suggesting that they possess comparable chemical properties particularly in terms of antioxidant activity. To examine this possibility in a cellular system, this study evaluated the capacities of NDGA and curcumin to function as antioxidants in inhibiting oxidative damage to DNA. Jurkat T-lymphocytes were pre-incubated for 30 min with 0-25 microM of either NDGA or curcumin to allow for uptake. The phenolic phytochemical-treated cells were then oxidatively challenged with 25 microM hydrogen peroxide (H2O2). Afterwards, cells were subjected to alkaline micro-gel electrophoresis (i.e. comet assay) to assess the extent of single-strand breaks in DNA. In a concentration-dependent manner, NDGA inhibited H2O2-induced DNA damage, whereas curcumin did not. In fact, incubating Jurkat T-lymphocytes with curcumin alone actually induced DNA damage. This effect of curcumin on DNA did not appear to reflect the DNA fragmentation associated with apoptosis because there was no proteolytic cleavage of poly-(ADP-ribose)-polymerase, which is considered an early marker of apoptosis. Curcumin-induced damage to DNA was prevented by pre-treatment of the cells with the lipophilic antioxidant, alpha-tocopherol, suggesting that curcumin damaged DNA through oxygen radicals. Therefore, it is concluded that NDGA has antioxidant activity but curcumin has prooxidant activity in cultured cells based on their opposite effects on DNA.

Potent antioxidant activity of a dithiocarbamate-related compound from a marine hydroid

Recently, we discovered a novel class of natural products, named the tridentatols, in a marine hydroid. Close examination of their molecular structures suggested that they may have antioxidant activity. This observation prompted us to evaluate in vitro the capacity of one of these tridentatols, viz. tridentatol A, to inhibit lipid peroxidation using human low density lipoprotein (LDL) as an experimental model. LDL was incubated with 5 microM cupric chloride (Cu2+) in the absence and presence of tridentatol A or a reference antioxidant standard, i.e. vitamin E. The onset of rapid formation of conjugated lipid hydroperoxides was delayed in a concentration-dependent manner by tridentatol A. More specifically, LDL incubated with Cu2+ had a lag-phase time (the elapsed time before the onset of rapid formation of conjugated lipid hydroperoxides) of 150 min. However, when 0.5 microM tridentatol A was present during incubation, the lag phase time was extended to 225 min. With 1 microM tridentatol A, the lag phase time was 300 min. The same concentrations of vitamin E produced noticeably lower lag phase times. Thus, compared with vitamin E, tridentatol A better protected against the formation of conjugated lipid hydroperoxides in LDL. Direct colorimetric measurements of both lipid hydroperoxides and thiobarbituric acid-reactive substances confirmed the greater potency of tridentatol A relative to vitamin E. Furthermore, tridentatol A negated the Cu2+-induced increase in electrophoretic mobility of LDL to a greater extent than vitamin E. In conclusion, tridentatol A is a powerful antioxidant against lipid peroxidation of LDL and is significantly more potent than vitamin E in this regard.

A phenolic antioxidant from the freshwater orchid, Habenaria repens

Recently, an unusual compound named habenariol was isolated from the freshwater orchid, Habenaria repens. Its phenolic structure suggested that habenariol should have substantial antioxidant activity. This possibility was investigated by evaluating the capacity of habenariol to inhibit copper-induced lipid peroxidation of human low density lipoprotein (LDL), a popular experimental model. LDL was incubated with 5 microM cupric chloride in the presence and absence of habenariol or a positive control, viz., alpha-tocopherol. Both kinetic and end-point spectrophotometric assays were used to determine extent of lipid peroxidation of LDL. In the kinetic assay, the time elapsing before the onset of rapid formation of conjugated lipid hydroperoxides in LDL (marked by a sharp increase in UV absorbance) was prolonged by habenariol, indicative of an antioxidant effect. In the end-point assay, direct colorimetric measurement confirmed habenariol's ability to inhibit formation of lipid hydroperoxides. However, in both assays, habenariol was less potent than alpha-tocopherol in inhibiting lipid peroxidation of LDL.