Background Parkinson’s disease (PD) is a progressive neurodegenerative disorder characterized by both motor and non-motor symptoms. Gastrointestinal tract dysfunction is one of the non-motor features, where constipation is reported as the most common gastrointestinal symptom. Aromatic bacterial metabolites are attracting considerable attention due to their impact on gut homeostasis and host’s physiology. In particular, Clostridium sporogenes is a key contributor to the production of these bioactive metabolites in the human gut. Results Here, we show that C. sporogenes deaminates levodopa, the main treatment in Parkinson’s disease, and identify the aromatic aminotransferase responsible for the initiation of the deamination pathway. The deaminated metabolite from levodopa, 3-(3,4-dihydroxyphenyl)propionic acid, elicits an inhibitory effect on ileal motility in an ex vivo model. We detected 3-(3,4-dihydroxyphenyl)propionic acid in fecal samples of Parkinson’s disease patients on levodopa medication and found that this metabolite is actively produced by the gut microbiota in those stool samples. Conclusions Levodopa is deaminated by the gut bacterium C. sporogenes producing a metabolite that inhibits ileal motility ex vivo. Overall, this study underpins the importance of the metabolic pathways of the gut microbiome involved in drug metabolism not only to preserve drug effectiveness, but also to avoid potential side effects of bacterial breakdown products of the unabsorbed residue of medication.

24Parkinson's disease is associated with gastrointestinal tract dysfunction, where constipation is reported 25 as the most common gastrointestinal symptom. Aromatic bacterial metabolites are attracting 26 considerable attention due to their implication on gut homeostasis and host's physiology. Clostridium 27 sporogenes is a key contributor to the production of these bioactive metabolites in the human gut. Here, 28we show that C. sporogenes effectively deaminate non-proteinogenic aromatic amino acids, such as 29 levodopa, the main treatment in Parkinson's disease, and identify the aromatic aminotransferase 30 responsible for the deamination. The deaminated metabolite from levodopa, 3-(3,4-dihydroxyphenyl) 31propionic acid, is detected in fecal samples of Parkinson's disease patients on levodopa medication, 32 elicits an inhibitory effect on gut motility. Overall, this study underpins the importance of the metabolic 33 pathways of the gut microbiome involved in drug metabolism not only to preserve drug effectiveness, 34 but also to avoid potential side effects of bacterial breakdown products of the unabsorbed residue of 35 medication. 36 37Recently, small intestinal microbiota have been implicated in the interference with levodopa drug 54 availability (van Kessel et al., 2019;Maini Rekdal et al., 2019). Early in vivo studies showed that ~90% 55 of levodopa is transported to the circulatory system (Bianchine et al., 1972;Morgan, 1971; Sasahara et 56 al., 1981), leaving a ~10% non-absorbed fraction of residual levodopa that can act as substrate for other 57 bacterial species associated with the lower, more anaerobic regions of the GI-tract (Goldin et al., 1973). 58Such bacterial-residual drug interaction might act as bioactive metabolites with an impact on gut 59 homeostasis. 60 PD is often associated with non-motor symptoms especially of the gastrointestinal (GI) tract. GI-tract 61 dysfunction such as constipation, drooling and swallowing disorders occur frequently in PD patients, 62 especially constipation, which is reported in 80-90% of the PD patients (Fasano et al., 2015). in the esophagus, stomach, jejunum and ileum (Panagamuwa et al., 1994; Van Der Sijp et al., 1993) and 65 patients with constipation have a longer SI transit time compared to controls (Van Der Sijp et al., 1993). 66Only recently, SI dysfunction in PD was studied observing that the transit time in the SI was significantly 67 longer in PD patients compared to healthy controls (Dutkiewicz et al., 2015; Knudsen et al., 2017). 68Using wireless electromagnetic capsules, the median small intestinal transit time observed was 69 significantly increased between PD patients (400 min; n=22) and healthy controls (295 min, n=15) 70 (Knudsen et al., 2017). 71This study unravels the aminotransferase responsible for initiating the deamination pathway involved in 72 the transamination of (among others) levodopa and shows that C. sporogenes can effectively deaminate 73 levodopa to 3-(3,4-dihydroxyphenyl) propionic acid through the same pathway as the other ar...