Trimethylamine (TMA) is present in the diet (in fish) but primarily formed in vivo from the breakdown of choline. It is N-oxidised by FMO3 in the liver, the major isoform active towards TMA. Trimethylaminuria (TMAU; MIM:602079, fish-odour syndrome) is a human genetic disorder characterised by an impaired ability to convert the malodourous TMA to its odourless N-oxide. Patients emit a foul odour, which resembles that of rotting fish and can be a psychologically disabling condition (Messenger et al. 2013).

Flavin-containing monooxygenases (FMOs) are the second family of microsomal oxidative enzymes with broad and overlapping specificity. The major reactions FMOs catalyze are nucleophilic hetero-atom compounds such as nitrogen, sulfur or phosphorus as the hetero-atom to form N-oxides, S-oxides or P-oxides respectively. Despite the functional overlap with cytochrome P450s, the mechanism of action differs. FMOs bind and activate molecular oxygen before the substrate binds to the enzyme (picture). They also require flavin adenosine dinucleotide (FAD) as a cofactor. Unlike cytochrome P450 enzymes, FMOs are heat-labile, a useful way to distinguish which enzyme system is at work for researchers studying metabolism. Also, FMOs are not inducible by substrates, unlike the P450 enzymes.\n(1) NADPH binds to the enzyme and reduces the prosthetic group FAD to FADH₂. NADP⁺ remains bound to the enzyme.\n(2) Incorporation of molecular oxygen to form a hydroperoxide.\n(3) A peroxide oxygen is transferred to the substrate.\n(4) Water is released.\n(5) NADP⁺ dissociates returning the enzyme to its initial state.\n\nTo date, there are 6 isozymes of FMO (FMO1-6) in humans, the most prominent and active one being FMO3. The FMO6 gene does not encode for a functional enzyme although it has the greatest sequence similarity with FMO3 (71%), whilst the others range from 50-58% sequence similarity with FMO3. FMO1-3 are the ones that exhibit activity towards nucleophiles, the others are insignificant in this respect (Cashman 2003, Krueger & Williams 2005).

Trimethylamine (TMA) is present in the diet (in fish) but primarily formed in vivo from the breakdown of choline. It is N-oxidised by FMO3 in the liver, the major isoform active towards TMA. Defects in FMO3 can cause trimethylaminuria (TMAU; MIM:602079, fish-odour syndrome), a human genetic disorder characterised by an impaired ability to convert the malodourous TMA to its odourless N-oxide (Treacy et al. 1998). Mutations that cause TMAU include M66I, P153L, R492W, N61S and E32K (Zhang et al. 2003, Yeung et al. 2007).

Trimethylamine (TMA) is present in the diet (in fish) but primarily formed in vivo from the breakdown of choline. It is N-oxidised by FMO3 in the liver, the major isoform active towards TMA, to form trimethylamine-N-oxide (TMAO). Trimethylaminuria (fish-odour syndrome) is a human genetic disorder characterised by an impaired ability to convert the malodourous TMA to the odourless N-oxide form TMAO (Higgins et al. 1972, Humbert et al. 1970, Treacy et al. 1998). L-carnitine is an abundant component of red meat and contains a trimethylamine structure similar to that of choline. Gut microbiota is able to produce TMAO from L-carnitine. If high levels of L-carnitine via high red meat intake or dietary supplements is achieved, Koeth et al. have shown the resultant TMAO produced in the gut can accelerate atherosclerosis in mice and increase the risk of cardiovascular disease (CVD) (Koeth et al. 2013).

Tamoxifen (TAM) is an antiestrogen and currently used extensively for breast cancer therapy. FMOs, especially FMO1 can N-oxidze TAM to tamoxifen N-oxide (TNO). TNO can be reduced back to TAM by the P450 system. TNO appears to be just as potent as TAM but with fewer side-effects so this metabolic cycling could play a part in the use of TNO in the treatment of breast cancer.