Bacterial catabolism of lipoic acid. Isolation and identification of a methyl ketone.

A soil bacterium, Pseudomonas putida LP, can be grown on lipoate as sole source of carbon, sulfur, and energy. In addition to the previously identified catabolites (bisnorlipoate, tetranorlipoate, beta-hydroxybisnorlipoate, lipoate thiolsulfinate, and two bisnorlipoate thiolsulfinates) isolated from cultures of the organism grown on [1,6-14C[lipoate, a methyl ketone (1,2-dithiolane-3-butyl-3'-one) has now been isolated and identified. This catabolite was isolated by solvent extraction and hydrophobic gel filtration and characterized by chromatographic mobilities and spray reactions and by UV, IR, PMR, and mass spectrometries. The methyl ketone presumably arises by decarboxylation of the beta-keto acid formed during the beta-oxidation of lipoate to bisnorlipoate by the microorganism.