Antioxidant activity of Nigella sativa essential oil

Hydroxyl radicals, generated by reaction of an iron-EDTA complex with H2O2 in the presence of ascorbic acid, attack deoxyribose to form products that, upon heating with thiobarbituric acid at low pH, yield a pink chromogen. Added hydroxyl radical "scavengers" compete with deoxyribose for the hydroxyl radicals produced and diminish chromogen formation. A rate constant for reaction of the scavenger with hydroxyl radical can be deduced from the inhibition of color formation. For a wide range of compounds, rate constants obtained in this way are similar to those determined by pulse radiolysis. It is suggested that the deoxyribose assay is a simple and cheap alternative to pulse radiolysis for determination of rate constants for reaction of most biological molecules with hydroxyl radicals. Rate constants for reactions of ATP, ADP, and Good's buffers with hydroxyl radicals have been determined by this method.

Samples of the expressed fixed oil from different sources of Nigella sativa seeds were examined by thin-layer and gas chromatography for content of fixed oils and thymoquinone, and these substances were tested as possible inhibitors of eicosanoid generation and membrane lipid peroxidation. The crude fixed oil and pure thymoquinone both inhibited the cyclooxygenase and 5-lipoxygenase pathways of arachidonate metabolism in rat peritoneal leukocytes stimulated with calcium ionophore A23187, as shown by dose-dependent inhibition of thromboxane B2 and leukotriene B4, respectively. Thymoquinone was very potent, with approximate IC50 values against 5-lipoxygenase and cyclo-oxygenase of < 1 microgram/ml and 3.5 micrograms/ml, respectively. Both substances also inhibited non-enzymatic peroxidation in ox brain phospholipid liposomes, but thymoquinone was about ten times more potent. However, the inhibition of eicosanoid generation and lipid peroxidation by the fixed oil of N. sativa is greater than is expected from its content of thymoquinone (ca. 0.2% w/v), and it is possible that other components such as the unusual C20:2 unsaturated fatty acids may contribute also to its anti-eicosanoid and antioxidant activity. These pharmacological properties of the oil support the traditional use of N. sativa and its derived products as a treatment for rheumatism and related inflammatory diseases.

Free radical oxidative stress has been implicated in the pathogenesis of a variety of human diseases. Natural antioxidant defences have been found to be defective in many of the same diseases. This has led to suggestions that oxidative damage and therefore disease progression may be retarded by supplementing natural antioxidant defences. Potential antioxidant therapy includes natural antioxidant enzymes and vitamins or synthetic agents with antioxidant activity. Diseases where antioxidant therapy may be beneficial include diabetes mellitus, reperfusion injury, inflammatory diseases and the prevention of chronic processes such as atherosclerosis and carcinogenesis. Further well controlled prospective clinical trials of antioxidants are required to establish the efficacy and tolerability of antioxidant therapy in the treatment of human diseases.