Tuesday, November 20, 2012

The Japanese can't make their own taxol from their yew





Species name: Taxus cuspidata

Common name: Japanese yew

Location: Ontario

You might be wondering why I'm featuring the same plant twice in the same week, but you would have made one vital assumption that is incorrect: just because two species look the same doesn't mean they are the same. The Japanese yew, featured here (native to Japan, as you might have guessed), is a very close relative to the Canada yew, featured in my blog post on Monday (read all about it HERE). In fact, by assuming that the photos above represented the same species as that of the Canada yew, you would be making the same error that many botanists have made in the past; it was only recently demonstrated that there are distinct species within the genus Taxus. The genus was originally thought to be monotypic, and instead having distinct geographic races or subspecies that are on their way to becoming their own species, but have not yet become distinct enough to warrant the title. The Sumatran and the Mexican yews are the most different from the type species, the European yew. These were the first to obtain their own species designation, with every other species following suit and being confirmed through DNA sequencing.

So if these species are so morphologically similar, how do you tell them apart? Well, the needles and their arrangement on the branches is the first clue. The needles of the Canada yew are arranged around the branches when young, but the development of the bark on the branches almost flattens out the arrangement of the needles as it matures. If you look at the branches that are brown in colour instead of green (showing that bark has been formed), the needles are in two rows on each side, but are flattened on the branch. It has almost become two-dimensional. The difference in the maturation of the Japanese yew branches is that the needles remain spirally-arranged on the branch; they never flatten out unless something has been grazing the branches. With age, the needles also turn upwards, so all of the oldest needles on the inside of the shrub or small tree are pointing upwards, even though their point of attachment on the branch is all the way around it.

The second hint that these are different species is their sheer size difference. The Canada yew barely reaches a height of 2 meters off the ground and completely lacks a central stem. While the Japanese yew pictured above doesn't really look like it has a central stem, it is clearly more "tree-like", and the branches have the ability to support their own weight as opposed to drooping onto the ground like the Canada yew branches do. The Japanese yew can also grow to a maximum height of 18 meters.

The last major differences are not visible to the eye, but rather "visible" to a machine called a gas chromatography mass spectrophotometer, or GCMS. These machines are also sometimes called the "mass spec," and if you watch CSI at all you will have heard of them! They really do exist, and they are used in much the same way as suggested on TV (but not nearly as quickly, and the results are a bit harder to interpret than what the shows make them out to be!). You dissolve a bit of an unknown substance into a solvent (usually some sort of alcohol, although other organic solutions are sometimes used like chloroform), and inject a very small quantity into the machine. The machine separates the individual components of the liquid mixture by molecular size, and displays a graph of molecular weight versus abundance. The shape of the peak tells you what types of chemical bonds are present, and the location along the size axis tell you how big the molecules are. The height of the peaks tell you how much of any given substance are present. So what does this have to do with the Japanese yew? Well, it's a rather unfortunate tale for the Japanese; if they want to make their own paclitaxel, they're going to have to start planting Canada yew. The Japanese yew doesn't contain the right combination of taxanes to have the desired medical effects. In fact, the Japanese yew is exponentially more toxic than the Canada yew; it has the distinction of being the "Japanese horse killer." Just five needles are enough to kill a small dog. If you choose to plant this ornamental tree in your garden, please keep animals and small children away from it! And don't EVER use it as a Christmas tree!