Reduction of trimethylamine N-oxide to trimethylamine by the human gut microbiota: supporting evidence for 'metabolic retroversion'

Abstract

Dietary methylamines [choline, trimethylamine N-oxide (TMAO), phosphatidylcholine, carnitine] are present in meat, fish, nuts and eggs. Gut bacteria are able to use choline and carnitine in a fermentation-like process, with trimethylamine (TMA) among the main end-products. TMA is transported from the intestine via the hepatic vein to hepatocytes, then converted to TMAO by hepatic flavin-containing monooxygenases. TMAO present in urine and plasma is currently considered a biomarker for non-alcoholic fatty liver disease, insulin resistance and cardiovascular disease. However, circulating TMAO may play roles in protection from hyperammonemia, and glutamate neurotoxicity. Little is known about the reduction of TMAO (predominantly from fish) to TMA and other compounds by the gut microbiota. We screened 66 strains of human-associated gut bacteria on solid and liquid media for their ability to use TMAO, with metabolites in spent media analysed by 1H-NMR. Enterobacteriaceae produced most TMA from TMAO, with caecal/small intestinal isolates of Escherichia coli producing more TMA than their faecal counterparts. Lactic acid bacteria produced increased amounts of lactate and biomass when grown in the presence of TMAO, but did not appear to use TMAO as an alternative electron acceptor. Stimulation of the growth of gut Enterobacteriaceae in the presence of TMAO was confirmed in faeces-inoculated, anaerobic, stirred, pH-controlled fermentation systems. Feeding deuterated TMAO to C57BL6/J mice demonstrated microbial conversion of TMAO to TMA, with uptake of TMA into the bloodstream and its conversion to TMAO. Antibiotic-treated mice lacked microbial activity necessary to convert TMAO to TMA, instead taking up TMAO into the bloodstream by an unknown mechanism. This study demonstrates microbial reduction of TMAO to TMA followed by host-mediated oxidation of TMA to regenerate TMAO, i.e. metabolic retroversion.