Differences in metabolism of levodopa between patients with Parkinson’s disease may be caused by variations in gut bacteria, according to investigators.
Specifically, patients with a higher abundance of Enterococcus faecalis may be converting levodopa into dopamine via decarboxylation before it can cross the blood-brain barrier, reported Emily P. Balskus, PhD, of Harvard University in Cambridge, Mass.
Although existing decarboxylase inhibitors, such as carbidopa, can reduce metabolism of levodopa by host enzymes, these drugs are unable to inhibit the enzymatic activity of E. faecalis in the gut, Dr. Balskus said at the annual Gut Microbiota for Health World Summit, sponsored by the American Gastroenterological Association and the European Society for Neurogastroenterology and Motility.
“[Carbidopa] is actually completely ineffective at inhibiting decarboxylation in human fecal suspension,” Dr. Balskus said, referring to research led by PhD student Vayu Maini Rekdal. “We think that this could indicate that patients who are taking carbidopa are not inhibiting any bacterial metabolism that they may have.”
While previous research showed that E. faecalis could decarboxylate levodopa, Dr. Balskus and colleagues linked this process with the tyrosine decarboxylase gene (TyrDC), and showed that the of abundance E. faecalis and TyrDC correlate with levodopa metabolism.
Unlike the human enzyme responsible for decarboxylation of levodopa, the E. faecalis enzyme preferentially binds with L-tyrosine. This could explain why existing decarboxylase inhibitors have little impact on decarboxylation in the gut, Dr. Balskus said.
She also noted that this unique characteristic may open doors to new therapeutics. In the lab, Dr. Balskus and colleagues tested a decarboxylase inhibitor that resembled L-tyrosine, (S)-alpha-fluoromethyltyrosine (AFMT). Indeed, AFMT completely inhibited of decarboxylation of levodopa in both E. faecalis cells and complex human microbiome samples.
“We think this is pretty exciting,” Dr. Balskus said.
Early animal studies support this enthusiasm, as germ-free mice colonized with E. faecalis maintain higher serum levels of levodopa with concurrent administration of AFMT.
“We think that this could indicate that a promising way to improve levodopa therapy for Parkinson’s patients would be to develop compounds that inhibit bacterial drug metabolism activity,” Dr. Balskus said.
Concluding her presentation, Dr. Balskus emphasized the importance of a biochemical approach to microbiome research. “Studying enzymes opens up new, exciting opportunities for microbiome manipulation. We can design or develop inhibitors of enzymes, use those inhibitors as tools to study the roles of individual metabolic activities, and potentially use them as therapeutics. We are very excited about the possibility of treating or preventing human disease not just by manipulating processes in our own cells, but by targeting activities in the microbiota.”
Dr. Balskus reported funding from HHMI, the Bill and Melinda Gates Foundation, the David and Lucile Packard Foundation, and Merck.