|Posted on November 1, 2014 at 11:40 AM|
Written by Emma Lewis
Green tea has been used as both a beverage and a medicine for more than 4000 years. It was first brewed in 2737 BC during the reign of Emperor Shennong in China. The tea is made from the leaves of the Camellia sinensis which have undergone minimal oxidation during processing. In a book written by the Tang dynasty of China, the positive effects of tea on vital organs - especially the heart – is described.
There is increasing evidence that green tea is beneficial in the prevention of the production of cholesterol and hence cardiovascular disease. Polyphenolic compounds make up most of the chemical components of the tea and have been linked with the disruption of cholesterol biosynthesis. Jun Xu and his colleagues at the Sun Yat-Sen University have investigated the effects of four different polyphenols found in green tea on three essential enzymes for the synthesis of cholesterol. They discovered that two polyphenols, epicatechin-3-gallate (ECG) and epigallocatechin-3-gallate (EGCG), acted as competitive inhibitors with all three enzymes simultaneously, whereas the other two had no effect on the rate of cholesterol production. Competitive enzyme inhibition is where the polyphenol binds with the active site of the enzyme, preventing the enzyme binding to other substrate molecules and completing the reactions necessary for the synthesis of cholesterol.
Computer modelling was then used to determine exactly how these molecules actually bind to the enzymes. It seems that the ECG and EGCG have two side groups which form strong electrostatic attractions with the enzyme active sites. The linkages between these side groups are flexible which allows the polyphenols to form the optimum shape for effective binding. Lower blood cholesterol leads to lower risk of blood clots and further cardiovascular diseases – so it seems that green tea is good for you.
However, a review into the toxicity of polyphenols has shown that in large quantities, they can actually have an adverse effect on the body – causing liver and kidney damage. Experiments in which rodents and dogs were given very large doses of polyphenols resulted in many of them dying from liver poisoning. It should be stressed that this is only a concern for those taking supplements which can contain up to 50 times as much polyphenol as a single cup of tea.
Many cancer patients drink green tea or take supplements believing its positive association with health. For cancer patients taking bortezomib (brand name: Velcade) which is used to combat multiple myeloma, the second most common blood cancer, it kills cancer cells through apoptosis which is the process of programmed cell death (used by the body especially in development to separate the toes of an embryo for example).
The polyphenols in green tea – in particular EGCG – can deactivate the drug. Axel Schönthal, a cancer biologist from the University of Southern California says that there was ‘100% deactivation of Velcade at levels easily achieved by patients who take green tea supplements, and possibly those who drink large amounts of tea’. Tumour apoptosis was seen to be blocked rather than enhanced (as they expected) by the EGCG.
When the Schönthal team analysed the reaction between EGCG and Velcade, they found that the EGCG interacts with a boronic acid unit which is part of the Velcade molecule and hence prevents the drug binding to the targeted cancer cell. This result may give some insight into the puzzling statistic that one third of those treated with Velcade find it ineffective. Schönthal suggested that those who take green tea supplements experience fewer side effects and therefore could be encouraged to continue taking them when in actual fact the decrease in side effects is due to the drug not working.
Obviously, the supplements magnify the potentially adverse effects so it may be seen as too far to suggest that people should not drink green tea in moderation. However, this new insight into the effects of polyphenols shows that even the most ancient and well trusted remedies may contain a sinister side.
Graphic by Luke O'Sullivan. Original graphic.