[C-13]PROPIONATE OXIDATION IN WILD-TYPE AND CITRATE SYNTHASE MUTANT ESCHERICHIA-COLI - EVIDENCE FOR MULTIPLE PATHWAYS OF PROPIONATE UTILIZATION

The metabolism of propionate was examined in wild-type Escherichia col i and cells lacking citrate synthase by high-resolution C-13 n.m.r. Sp ectra of cell extracts from wild-type E. coli show that glutamate beco mes highly enriched in C-13 when C-13-enriched propionate is the sole carbon source. No glutamate labelling was detected when the tricarboxy lic acid cycle was blocked either by deletion of citrate synthase or b y inhibition of succinate dehydrogenase by malonate. The C-13 fraction al enrichment in glutamate C-2, C-3 and C-4 in wild-type cells was qua ntitatively and qualitatively different when [2-C-13]propionate as opp osed to [3-C-13]propionate was supplied. Approximately equal labelling occurred in the C-2, C-3 and C-4 positions of glutamate when [3-C-13] propionate was available, and multiplets due to carbon-carbon spin-spi n coupling were observed. However, in cells supplied with [2-C-13]prop ionate, very little C-13 appeared in the glutamate C-4 position, and t he remaining glutamate resonances all appeared as singlets. The unequa l and non-identical labelling of glutamate in cells supplied with [2-C -13]-as opposed to [3-C-13]propionate is consistent with the utilizati on of propionate by E. coli via two pathways, oxidation of propionate to pyruvate and carboxylation of propionate to succinate. These interm ediates are further metabolized to glutamate by the action of the tric arboxylic acid cycle. The existence of an organized tricarboxylic acid cycle is discussed as a consequence of the ability to block utilizati on of propionate in tricarboxylic acid-cycle-defective E. coli.