1. Soluble extracts from rat heart and liver mitochondria were used to evaluate the early steps in the conversion of pent-4-enoyl-CoA into tricarboxylic acid-cycle intermediates. Hitherto the unresolved problem was the reduction of the double bond of pent-4-enoate. 2. Soluble extracts from heart mitochondria reduced pent-4-enoyl-CoA and penta-2,4-dienoyl-CoA in the presence of NADPH at rates (nmol/min per mg of protein) of 0.9 +/- 0.1 and 132 +/- 8 and from the liver mitochondria at the rates of 1.9 +/- 0.2 and 52 +/- 6 respectively. No reduction of acryloyl-CoA was found. 3. We show that primarily the double bond in position 4, not in position 2, of penta-2,4-dienoyl-CoA is reduced. 4. It is concluded that the principal metabolic pathway of penta-4-enoate is reduction of the double bond in position 4 after an initial oxidation of penta-2,4-dienoyl-CoA. The pent-2-enoyl-CoA thus formed can be further metabolized by the usual enzymes of beta-oxidation, and by the further metabolism of propionyl-CoA to tricarboxylic acid-cycle intermediates.
