4-Bromocrotonic acid, an effective inhibitor of fatty acid oxidation and ketone body degradation in rat heart mitochondria. On the rate-determining step of beta-oxidation and ketone body degradation in heart.

4-Bromocrotonic acid was found to effectively inhibit respiration supported by either palmitoylcarnitine or acetoacetate in coupled rat heart mitochondria. Partial inhibition was observed when 3-hydroxybutyrate served as a substrate, whereas pyruvate-supported respiration was unaffected by the inhibitor. Thus, 4-bromocrotonic acid inhibits fatty acid oxidation and ketone body degradation. When the enzymes of beta-oxidation and ketone body degradation were assayed in mitochondria preincubated with 4-bromocrotonic acid, only 3-ketoacyl-CoA thiolase and acetoacetyl-CoA thiolase were found to be inactive. Evidence is presented for the enzymatic conversion of 4-bromocrotonic acid to 3-keto-4-bromobutyryl-CoA which effectively inhibits both thiolases. A kinetic evaluation of the inhibitions caused by 4-bromocrotonic acid in coupled rat heart mitochondria demonstrated that 3-ketoacyl-CoA thiolase and respiration supported by palmitoyl carnitine are inactivated at equal rates. However, acetoacetyl-CoA thiolase was inactivated more rapidly than was respiration supported by acetoacetate. It is suggested that the thiolase-catalyzed step is rate-limiting in beta-oxidation or is as slow as other reactions are. In contrast the thiolytic cleavage of acetoacetyl-CoA does not seem to be rate-limiting in ketone body degradation.