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Green tea is a powerful preventative agent for cancer and other diseases, due to its large flavonoid content. A new, sensitive analytical method for determining one of the most common flavonoids, quercetin, in human plasma after drinking canned green tea will help to determine its mode of action in the body. Not so long ago, few people had heard of free radicals. They were limited to the secret world of scientists and were rarely spoken of outside the lab. How things change. Now most of us know that free radicals are the enemy of the people, being responsible for many diseases, including cancer, atherosclerosis and heart disease. They also cause aging. So tackle free radicals and we can live longer, healthier lives (but not to immortality, as some believe). Flavonoids are receiving good press lately, due to their effective antioxidant behaviour. They neutralise free radicals in the body before they can damage our tissue and DNA. Fortunately, flavonoids are in good supply because they are present at effective levels in many appetizing plant-based foods, including fruits, green vegetables, peanuts, chocolate and red wine. Some people confuse flavonoids with flavanoids. Flavonoids such as flavone and flavonol are a subset of flavanoids, which are the large group of plant metabolites derived from flavan. One of the most abundant flavonoids in the diet is quercetin, which is present in many foods, including green tea. Quercetin itself has many biological activities, including anti-inflammatory, anti-cancer and antioxidant functions. It generally occurs in foods in the form of conjugates, with sugar groups attached to the hydroxyl group(s), rather than as the parent compound. The American Cancer Society explains the beneficial aspect of green tea, again implicating flavonoids as the protective agents. However, recently, a second anti-cancer mechanism of green tea has been proposed, in which flavonoids inactivate a DNA-altering compound in the body. In practice, it may well be that flavonoids express their antioxidant activity in several ways. In order to determine the metabolism of quercetin in the body, methods for measuring small amounts should be in place. Currently, HPLC methods with various types of detection (UV, fluorescence, electrochemical, mass spectrometry) exist but they are limited by relatively high detection limits. Now, Asian researchers have developed a further method, based on HPLC with electrochemical detection, (HPLC-ECD) that significantly reduced the detection limit. It was used to measure quercetin in commercial samples of canned green tea, then in human plasma after people had drunk the tea. All teas come from the tea plant, Camellia sinensis, green tea being prepared by drying and heating the leaves. Black tea is produced after further fermentation and oxidation steps, which might explain why it has less antioxidants than green tea. As described by the US National Cancer Institute, green tea is recognised as a cancer preventative. Fumiyo Kusu and colleagues from the Department of Analytical Chemistry at the Tokyo University of Pharmacy and Life Science and Dongri Jin from the Chemistry Department of Science and Engineering at Yanbian University in China described their new method in Biomed. Chromatogr. 2004, 18 . In order to lower the detection limit, they used a semi-micro HPLC column (with an internal diameter of 1.0 mm) which they say reduced diffusion of the sample that occurs in conventional columns, increasing the sensitivity. So quercetin could be measured in only 100 µl of plasma, to a detection limit of 0.3 pg, 7-fold smaller than the previous best published method (2.2 pg by LC/MS/MS). In canned green tea, quercetin is present mainly as glycoside conjugates, so samples were acid hydrolyzed to produce free quercetin for analysis. Using ECD, the concentration of quercetin in the tea was found to be 6.6 µg/ml. For the human studies, plasma samples were taken at various time intervals after volunteers had drunk the tea. This time, plasma was hydrolysed with enzymes, to release the free quercetin from its sulphate and glucuronide conjugated forms and accurate concentration profiles were obtained. The sulphate form maximised in plasma one hour after ingestion of the tea, and the glucuronide form after 1.5 hours. Both fell away to low levels after about 6 hours. No free, unconjugated quercetin was found at all. Because of the low detection limit, the pharmacokinetics can be followed after small doses, equivalent to those that occur in some foods. This will allow detailed studies of quercetin in humans after taking various foods and drinks, and should shed more light on its protective nature. Related links:
Article by Steve Down |
 Flavonoids in green tea provide many health benefits  The structure of quercetin  Tea is probably the most popular non-alcoholic drink worldwide, placing great demand on growers |
