How microbes consume levodopa, the primary treatment for Parkinson’s disease, could reveal more about how the microbiome impacts our health, said study.
The first time Vayu Maini Rekdal manipulated microbes, he made a decent sourdough bread. At the time, young Maini Rekdal, and most people who head to the kitchen to whip up a salad dressing, pop popcorn, ferment vegetables, or caramelize onions, did not consider the crucial chemical reactions behind these concoctions.

Even more crucial are the reactions that happen after the plates are clean. When a slice of sourdough travels through the digestive system, the trillions of microbes that live in our gut help the body break down that bread to absorb the nutrients. Since the human body cannot digest certain substances–all-important fiber, for example–microbes step up to perform chemistry no human can.

“But this kind of microbial metabolism can also be detrimental,” said Maini Rekdal, a graduate student in the lab of Professor Emily Balskus and first-author on their new study published in Science. According to Maini Rekdal, gut microbes can chew up medications, too, often with hazardous side effects. “Maybe the drug is not going to reach its target in the body, maybe it’s going to be toxic all of a sudden, maybe it’s going to be less helpful,” Maini Rekdal said.

In their study, Balskus, Maini Rekdal, and their collaborators at the University of California San Francisco, describe one of the first concrete examples of how the microbiome can interfere with a drug’s intended path through the body.

Parkinson’s disease attacks nerve cells in the brain that produce dopamine, without which the body can suffer tremors, muscle rigidity, and problems with balance and coordination. L-dopa delivers dopamine to the brain to relieve symptoms. But only about 1 to 5% of the drug actually reaches the brain.
This number–and the drug’s efficacy–varies widely from patient to patient. Since the introduction of L-dopa in the late 1960s, researchers have known that the body’s enzymes (tools that perform necessary chemistry) can break down L-dopa in the gut, preventing the drug from reaching the brain. So, the pharmaceutical industry introduced a new drug, carbidopa, to block unwanted L-dopa metabolism. Taken together, the treatment seemed to work.

“Even so,” Maini Rekdal said, “there’s a lot of metabolism that’s unexplained, and it’s very variable between people.” That variance is a problem: Not only is the drug less effective for some patients, but when L-dopa is transformed into dopamine outside the brain, the compound can cause side effects, including severe gastrointestinal distress and cardiac arrhythmias. If less of the drug reaches the brain, patients are often given more to manage their symptoms, potentially exacerbating these side effects.
Maini Rekdal suspected microbes might be behind the L-dopa disappearance. Since previous research showed that antibiotics improve a patient’s response to L-dopa, scientists speculated that bacteria might be to blame. Still, no one identified which bacterial species might be culpable or how and why they eat the drug.

So, the Balskus team launched an investigation. The unusual chemistry–L-dopa to dopamine–was their first clue.

Few bacterial enzymes can perform this conversion. But, a good number bind to tyrosine–an amino acid similar to L-dopa. And one, from a food microbe often found in milk and pickles (Lactobacillus brevis), can accept both tyrosine and L-dopa.

Using the Human Microbiome Project as a reference, Maini Rekdal and his team hunted through bacterial DNA to identify which gut microbes had genes to encode a similar enzyme. Several fit their criteria; but only one strain, Enterococcus faecalis (E. faecalis), ate all the L-dopa, every time.

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