Identification of a novel malonyl-CoA IC50 for CPT-I: implications for predicting in vivo fatty acid oxidation rates

Published values regarding the sensitivity (IC50) of CPT-I (carnitine palmitoyltransferase 1) to M-CoA (malonyl-CoA) inhibition in isolated mitochondria are inconsistent with predicted in vivo rates of fatty acid oxidation. Therefore we have re-examined M-CoA inhibition kinetics under various P-CoA (palmitoyl-CoA) concentrations in both isolated mitochondria and PMFs (permeabilized muscle fibres). PMFs have an 18-fold higher IC50 (0.61 compared with 0.034 mu M) in the presence of 25 mu M P-CoA and a 13-fold higher IC50 (6.3 compared with 0.49 mu M) in the presence of 150 mu M P-CoA compared with isolated mitochondria. M-CoA inhibition kinetics determined in PMFs predicts that CPT-I activity is inhibited by 33% in resting muscle compared with >95% in isolated mitochondria. Additionally, the ability of M-CoA to inhibit CPT-1 appears to be dependent on P-CoA concentration, as the relative inhibitory capacity of M-CoA is decreased with increasing P-CoA concentrations. Altogether, the use of PMFs appears to provide an M-CoA IC50 that better reflects the predicted in vivo rates of fatty acid oxidation. These findings also demonstrate that the ratio of [P-CoAKM-CoA] is critical for regulating CPT-1 activity and may partially rectify the in vivo disconnect between M-CoA content and CPT-I flux within the context of exercise and Type 2 diabetes.