Fatty acid oxidation in brain is limited by the low activity of 3-ketoacyl-CoA thiolase

In an attempt to establish why the brain is virtually incapable of oxidizing fatty acids, the activities of the β-oxidation enzymes in rat brain and rat heart mitochondria were measured and compared with each other. Although the apparent K/sub m/ values and chain-length specificities of the brain and heart enzymes are similar, the specific activities of all but one brain enzyme are between 4% and 50% of those observed in heart mitochondria. The exception is 3-ketoacyl-CoA thiolase (EC 2.3.1.16) whose specific activity in brain mitochondria is 125-times lower than in heart mitochondria. The partially purified brain 3-ketoacyl-CoA thiolase was shown to be catalytically and immunologically identical with the heart enzyme. The low rate of fatty acid oxidation in brain mitochondria estimated on the basis of palmitoylcarnitine-supported respiration and [1-14C]palmitoylcarnitine degradation may be the consequence of the low activity of 3-ketoacyl-CoA thiolase. Inhibition of [1-14C]palmitoylcarnitine oxidation by 4-bromocrotonic acid proves that the observed oxidation of fatty acids in brain is dependent on 3-ketoacyl-CoA thiolase and thus occurs via β-oxidation. Since the reactions catalyzed by carnitine palmitoyltransferase (EC 2.3.1.21) and acyl-CoA synthetase (EC 6.2.1.3) do not seem to restrict fatty acid oxidation in brain, it is concluded that the oxidation of fatty acids in rat brain is limited by the activity of the mitochondrial 3-ketoacyl-CoA thiolase