Abstract
The mechanism of biosynthesis of trimethylamine oxide (TMAO) from dietary precursors in a seawater-adapted teleost, Nile tilapia Oreochromis niloticus, was investigated. Diets supplemented with quaternary ammoniums of choline, glycine betaine, carnitine or phosphatidylcholine were administered and significant increases in TMAO levels in the muscle were observed with choline alone. [Methyl-14C] and [1, 2-14C]-cholines were given through the diet and intraperitoneal injections but [14C]-TMAO was detected only in fish with dietary administration of [methyl-14C]-choline. Dietary treatment with [15N]-choline resulted in the formation of [15N]-TMAO in the muscle. The incorporation of radioactivity into TMAO was also observed after both dietary administration and intraperitoneal injection of [14C]-trimethylamine (TMA). There were marked increases in TMA levels when choline was introduced into the isolated intestine. These increases, however, were significantly suppressed in the presence of penicillin. [14C]-Trimethylamine derived from [methyl-14C]-choline was detected in the cavity of the isolated intestine. Introduction of [15N]-choline into the intestinal cavity resulted in the formation of [15N]-TMA. Reduction activity of TMAO to TMA was observed in intestinal microorganisms under microaerobic conditions. Trimethylamine monooxygenase activity was detected in the liver and kidney. It was concluded that marine teleosts possess the ability to produce TMAO from choline, which is related to intestinal microorganisms and tissue monooxygenase. Furthermore, TMA was suggested to be formed from dietary TMAO by the microbes in the intestine and thus reoxygenated to TMAO by the fish tissue monooxygenase before transfer to the muscle.
