Regulation of fatty acid β-oxidation in rat heart mitochondria

Abstract

In an attempt to elucidate the mechanism by which the rate of fatty acid oxidation is tuned to the energy demand of the heart, the effects of changing intramitochondrial ratios of [acetyl-CoA] [CoASH] and [NADH] [NAD +] on the rate of β-oxidation were studied. When 10 mm l-carnitine was added to coupled rat heart mitochondria to lower the ratio of [acetyl-CoA] [CoASH] , the rate of palmitoylcarnitine β-oxidation, as measured by the formation of acid-soluble products, was stimulated more than fourfold at state 4 respiration while β-oxidation at state 3 respiration was hardly affected. Neither oxaloacetate nor acetoacetate, added to mitochondria to lower the [NADH] [NAD +] ratio, stimulated β-oxidation. Rates of respiration at states 3 and 4 were unchanged by additions of l-carnitine, oxaloacetate, or acetoacetate. Determinations of intramitochondrial ratios of [acetyl-CoA] [CoASH] by high performance liquid chromatography yielded values close to 10 for palmitoylcarnitine-supported respiration at state 4 and 2.5 at state 3 respiration. Addition of 10 mm l-carnitine caused a dramatic decrease of these ratios to less than 0.2 at both respiration states. Studies with purified or partially purified enzymes revealed strong inhibitions of 3-ketoacyl-CoA thiolase by acetyl-CoA and of l-3-hydroxyacyl-CoA dehydrogenase by NADH. Moreover, the activity of 3-ketoacyl-CoA thiolase at concentrations of acetyl-CoA and CoASH prevailing at state 3 respiration was 4 times higher than its activity in the presence of acetyl-CoA and CoASH observed at state 4. Altogether, this study leads to the conclusion that the rate of β-oxidation in heart can be regulated by the intramitochondrial ratio of [acetyl-CoA] [CoASH] which reflects the energy demand of the tissue. The thiolytic cleavage catalyzed by 3-ketoacyl-CoA thiolase may be the site at which β-oxidation is controlled by the [acetyl-CoA] [CoASH] ratio.