The human gut microbiota metabolizes the Parkinson’s disease medication Levodopa (L-dopa), potentially reducing drug availability and causing side effects. However, the organisms, genes, and enzymes responsible for this activity in patients and their susceptibility to inhibition by host-targeted drugs are unknown. Here, we describe an interspecies pathway for gut bacterial L-dopa metabolism. Conversion of L-dopa to dopamine by a pyridoxal phosphate-dependent tyrosine decarboxylase from Enterococcus faecalis is followed by transformation of dopamine to m-tyramine by a molybdenum-dependent dehydroxylase from Eggerthella lenta. These enzymes predict drug metabolism in complex human gut microbiotas. Although a drug that targets host aromatic amino acid decarboxylase does not prevent gut microbial L-dopa decarboxylation, we identified a compound that inhibits this activity in Parkinson’s patient microbiotas and increases L-dopa bioavailability in mice. INTRODUCTION Parkinson’s disease is a debilitating neurological condition affecting more than 1% of the global population aged 60 and above. The primary medication used to treat Parkinson’s disease is levodopa (L-dopa). To be effective, L-dopa must enter the brain and be converted to the neurotransmitter dopamine by the human enzyme aromatic amino acid decarboxylase (AADC). However, the gastro-intestinal tract is also a major site for L-dopa decarboxylation, and this metabolism is problematic because dopamine generated in the periphery cannot cross the blood-brain barrier and causes unwanted side effects. Thus, L-dopa is coadministered with drugs that block pe-ripheral metabolism, including the AADC in-hibitor carbidopa. Even with these drugs, up to 56% of L-dopa fails to reach the brain. Moreover, the efficacy and side effects of L-dopa treatment are extremely heterogeneous across Parkinson’s patients, and this variability cannot be completely explained by differences in host metabolism RATIONALE Previous studies in humans and animal models have demonstrated that the gut microbiota can metabolize L-dopa. The major proposed pathway involves an initial decarboxylation of L-dopa to dopamine, followed by conversion of dopamine to m-tyramine by means of a distinctly microbial dehydroxylation reaction. Although these metabolic activities were shown to occur in complex gut microbiota samples, the specific organisms, gene, and enzymes responsible were unknown. The effects of host-targeted inhibitors such as carbidopa on gut microbial L-dopa metabolism were also unclear. As a first step toward understanding the gut microbiota’s effect on Parkinson’s disease therapy, we sought to elucidate the molecular basis for gut microbial L-dopa and dopamine metabolism. RESULTS Hypothesizing that L-dopa decarboxylation would require a pyridoxal phosphate (PLP)–dependent enzyme, we searched gut bacterial genomes for candidates and identified a conserved tyrosine decarboxylase (TyrDC) in Enterococcus faecalis. Genetic and biochemical experiments revealed that TyrDC simultaneously decarboxylates both L-dopa and its preferred substrate, tyrosine. Next, we used enrichment culturing to isolate a dopamine dehydroxylating strain of Eggerthella lenta, a species previously implicated in drug metabolism. Transcriptomics linked this activity to a molybdenum cofactor–dependent dopamine dehydroxylase (Dadh) enzyme. Unexpectedly, the presence of this enzyme in gut bacterial genomes did not correlate with dopamine metabolism; instead, we identified a single-nucleotide polymorphism (SNP) in the dadh gene that predicts activity. The abundance of E. faecalis, tyrDC, and the individual SNPs of dadh correlated with L-dopa and dopamine metabolism in complex gut microbiotas from Parkinson’s patients, indicating that these organisms, genes, enzymes, and even nucleotides are relevant in this setting. We then tested whether the host-targeted AADC inhibitor carbidopa affected L-dopa decarboxylation by E. faecalis TyrDC. Carbidopa displayed greatly reduced potency toward bacteria and was completely ineffective in complex gut microbiotas from Parkinson’s patients, suggesting that this drug likely does not prevent microbial L-dopa metabolism in vivo. To identify a selective inhibitor of gut bacterial L-dopa decarboxylation, we leveraged our molecular understanding of gut microbial L-dopa metabolism. Given TyrDC’s preference for tyrosine, we examined tyrosine mimics and found that (S)-α-fluoromethyltyrosine (AFMT) prevented L-dopa decarboxylation by TyrDC and E. faecalis as well as complex gut microbiota samples from Parkinson’s patients. Coadministering AFMT with L-dopa and carbidopa to mice colonized with E. faecalis also increased the peak serum concentration of L-dopa. This observation is consistent with inhibition of gut microbial L-dopa metabolism in vivo. CONCLUSION We have characterized an interspecies pathway for gut bacterial L-dopa metabolism and demonstrated its relevance in human gut microbiotas. Variations in these microbial activities could possibly contribute to the heterogeneous responses to L-dopa observed among patients, including decreased efficacy and harmful side effects. Our findings will enable efforts to elucidate the gut microbiota’s contribution to treatment outcomes and highlight the promise of developing therapies that target both host and gut microbial drug metabolism.
