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One Step Nearer to Taxol

taxadien-5-alpha-ol Although Taxol® (paclitaxel) offers significant benefits to cancer patients, its initial isolation from the bark of the Pacific yew tree (Taxus brevifolia) raised serious ecological concerns since the trees are killed in the harvesting process. These concerns led to the development of both synthetic and semisynthetic routes to paclitaxel, but the drug is now manufactured more efficiently using plant cell cultures.

Scientists at MIT and Tufts University have now engineered a new strain of E. coli bacteria that can produce taxadiene and taxadien-5-α-ol, key precursors to paclitaxel. Although E. coli does not naturally produce taxadiene, it does produce isopentenyl pyrophosphate (IPP), a compound that is two steps back in the plant biosynthetic pathway. The team identified four bottlenecks in the eight-step E. coli biosynthesis of IPP and engineered the bacteria to produce multiple copies of the genes encoding the enzymes responsible for carrying out these four steps. To enable the bacteria to carry out the two additional steps needed to covert IPP to taxadiene, the researchers added the plant genes for the appropriate enzymes. By altering the copy numbers of genes to find the most efficient combination, the team were able to produce a strain of E. coli that produces more than 1000 times more taxadiene (ca 1g/L) than any other engineered strain. They then added an extra step towards the synthesis of paclitaxel, achieving the first microbial conversion of taxadiene to taxadien-5-α-ol.

There are another fifteen to twenty steps to go to achieve a microbial synthesis of pactitaxel but, if these can be achieved, as well as producing pactitaxel, the engineered bacteria should allow access to a variety of terpenoid natural products.

The study is published in the journal Science.


Mice Can Do What Poppies Can

Preparations from the opium poppy, papaver somniferum, have been used for thousands of years to relieve pain. In the early 1800s, Sertürner isolated one of the constituent alkaloids, morphine, which was later shown to be almost entirely responsible for the analgesic activity of crude opium extracts. Since then, morphine and other opiates have been shown to bind to specific receptors, leading to speculation that mammals may be able to biosynthesise morphine. It has also been recognised that humans excrete small but steady amounts of morphine in their urine, but it has been unclear whether this comes from dietary sources or whether the presence of morphine in urine provides evidence that people can actually biosynthesise morphine. The discovery of the simple isoquinoline alkaloid, tetrahydropapaveroline (THP), which can be formed from catecholamines, in both human and rodent brain and in human urine also suggested a possible biosynthetic linkage.

A team led by Nadja Grobe from the Donald Danforth Plant Science Center and Marc Lamshöft from the Institute of Environmental Research, University of Technology Dortmund have now shown conclusively for the first time that mammals are able to synthesise morphine. When either unlabelled or deuterated THP, a potential precursor of morphine, was administered to mice, analysis of their urine showed that the compound had been extensively metabolised. Salutaridine, a known biosynthetic precursor of morphine in the opium poppy, was found amongst the metabolites and when deuterated salutaridinol, the biosynthetic reduction product of salutaridine, was administered to the mice, it was shown to be converted to deuterated thebaine, which was excreted in their urine. Deuterated thebaine was also administered and deuterated morphine together with the related compounds, codeine and oripavine, was recovered in urine.

The study, which is published in the journal PNAS, provides the first evidence that mammals have the capacity to convert THP, known to be present in the brain, into morphine. Although the pathway has now been shown to exist, it is still not clear whether mammals naturally produce morphine as an analgesic.