Eating bracken

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Eating bracken

Post by ThreeperMan on Sun 12 Jul 2009, 15:05

Science: How bracken's deadly chemical breaks the back of DNA
16 April 1994
From New Scientist Print Edition. Subscribe and get 4 free issues.

Just in case you were thinking of eating bracken - don't! The warning comes from Japanese chemists who have found that a simple molecule in the bracken fern changes in the body into a deadly chemical that causes cancer. The carcinogen removes an adenine group from DNA, making its backbone so unstable that it quickly disintegrates.

The discovery may explain the high incidence of throat cancer among the Japanese, who eat the shoots of bracken (Pteri-dium aquilinum), as well as the prevalence of intestinal and bladder cancer among animals that are given this fern as fodder.

Makoto Ojika and colleagues at Nagoya University had previously isolated a compound from bracken which they suspected was a carcinogen. Now Ojika's group, together with a team headed by Yukio Sugiura at Kyoto University, have found how the compound, called ptaquiloside, causes cancer (Journal of the American Chemical Society, vol 116, p 479).

Attached to ptaquiloside is a three-carbon cyclopropyl ring, which is the molecule's reactive site. When ptaquiloside is dissolved in a weakly alkaline solution, it loses a simple glucose unit to form a simpler molecule called a dienone. According to the Japanese researchers, the dienone is the key to the carcinogenic action.

To discover how the dienone worked, Ojika and Sugiura observed how it reacted with a small chain called deoxytetra-nucleotide d(GTAC). This is a model of DNA because it incorporates the four nucleic acid bases: guanine (G), thymine (T), adenine (A) and cytosine (C). The chemists dissolved the d(GTAC) and dienone together in a mixed solvent consisting of three parts water and one part acetonitrile, and kept the solution at 0 °C and a pH of 7.5 for 15 hours.

Their findings show that the dienone reacts with the adenine group in such a way that the cyclopropyl ring opens and the free end attaches itself to a nitrogen atom on adenine. The d(GTAC) then sheds the attacked group but, in doing so, is left with a hydroxyl (OH) radical on the polymer backbone. This destabilises the DNA, causing the chain to break. The chemists also observed that the dienone could attack a guanine group and cause the same change, although this reaction was much slower.

Having discovered that the adenine base was the preferred target of attack, the Japanese chemists tried to discover which adenine group on DNA was most prone to attack. After testing more than 1000 strands with different combinations of guanine, thymine, adenine and cytosine, they found that the most likely attack would be on an (AAAT) site in which the attacked adenine A was flanked on one side by a thymine group and on the other side by two other adenine groups.

The (AAAT) sequence is also known to be sensitive to attack by the anticancer drug known as CC-1065, which also contains a cyclopropyl group. The Japanese chemists conclude that CC-1065 destroys cancer cells in the same way that ptaquiloside attacks healthy cells.

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