Metabolism of Trimethylammonium Compounds by Acinetobacter

Abstract

L(-)-Carnitine (R(-)-3-hydroxy-4-trimethylaminobutyrate) is a ubiquitously-occurring substance which is essential for the β-oxidation of long-chain fatty acids in mitochondria. Bacteria are able to metabolise this trimethylammonium compound in different ways. Species of the genus Pseudomonas can utilise L-carnitine as sole source of carbon and nitrogen under aerobic conditions. After growth on L(-)-carnitine, the highly specific L(-)-carnitine dehydrogenase (EC 1.1.1.108) is induced in P. aeruginosa (Aurich et al., 1967) and P. putida (Kleber et al., 1978). A second group of carnitine-metabolising microorganisms comprising different Enterobacteriaceae, e.g., Escherichia coli, Salmonella typhimurium and Proteus vulgaris, can almost quantitatively convert L(-)-carnitine into ɣ-butyrobetaine under anaerobic conditions (Seim et al., 1980, 1982a,c,d). They do not assimilate the carbon and nitrogen skeleton. The metabolism of L(-)-carnitine by E. coli includes at least a two-step reduction of L(-)-carnitine to ɣ-butyrobetaine with crotonobetaine as intermediate (Seim et al., 1982d). The function of this reaction sequence is still unknown. Seim et al. (1982a,c,d) postulated that crotonobetaine serves as an external electron acceptor similar to (e.g.) nitrate (Haddock and Jones, 1977). One of the two enzymes involved, the carnitine dehydratase, was recently purified and characterised (Jung et al., 1989).