4-Bromocrotonic Acid, an Effective Inhibitor of Fatty Acid Oxidation and Ketone Body Degradation in Rat Heart Mitochondria ON THE RATE-DETERMINING STEP OF P-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 r spiration was unaffected by the inhibitor. Thus, 4-bromocrotonic acid inhibits fatty acid oxidation a d ketone body degradation. When the enzymes of 8-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 3keto-4-bromobutyryl-CoA which effectively inhibits both thiolases. A kinetic evalution 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, acetoacetylCoA thiolase was inactivated more rapidly than was respiration supported by acetoacetate. It is suggested that the thiolase-catalyzed step is rate-limiting in 8oxidation 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.