Evidence for the Essential Function of 2,4-Dienoyl-coenzyme A Reductase in the β-Oxidation of Unsaturated Fatty Acids in Vivo

For the purpose of assessing in vivo the importance of 2,4-dienoyl-CoA reductase (EC 1.3.1.34) in the beta-oxidation of unsaturated fatty acids, reductase mutants of Escherichia coli were isolated by selecting cells that were able to grow on oleate but not on petroselinic acid (6-cis-octadecenoic acid). One mutant (fadH) exhibited 12% of the 2,4-dienoyl-CoA reductase activity present in the parental strain with other beta-oxidation enzymes being essentially unaffected. Antireductase antibodies were used to show that the mutant contains a fadH gene product at a level similar to that observed in the parental strain. Thus, the mutation seems to have resulted in the synthesis of a fadH gene product with lower specific activity. The mutation was mapped in the 71-75-min region of the E. coli chromosome where no other gene for beta-oxidation enzymes has so far been located. Complementation of the mutation by F'141, which carries the 67-75.5-min region of the E. coli genome, resulted in an increase in the 2,4-dienoyl-CoA reductase activity to 80% of the level found in the parental strain. Measurements of respiration with petroselinic acid as the substrate showed rates to be linearly dependent on the 2,4-dienoyl-CoA reductase activity up to levels found in wild-type E. coli. 2,4-Dienoyl-CoA reductase, like other enzymes of beta-oxidation, was induced when E. coli was grown on a long chain fatty acid as the sole carbon source. It is concluded that 2,4-dienoyl-CoA reductase is required in vivo for the beta-oxidation of unsaturated fatty acids with double bonds extending from even-numbered carbon atoms.