Carnitine and carnitine palmitoyltransferase in fatty acid oxidation and ketosis.
Carnitine is an essential factor in long-chain fatty acid oxidation. Carnitine acts as a carrier of fatty acyl groups from the cytoplasm to the mitochondrion. Long-chain acyl-CoA derivatives do not penetrate the mitochondrial inner membrane. Carnitine palmitoyltransferase A (CPT-A), located on the external surface of the inner membrane, catalyzes the conversion of cytoplasmic long-chain acyl-CoA and carnitine into acylcarnitine. The acylcarnitine is reconverted to intramitochondrial acyl-CoA by the action of carnitine palmitoyltransferase B located in the inner membrane. Now, the acyl-CoA is available for beta-oxidation in the matrix. An inner membrane carnitine-acylcarnitine translocase exchanges carnitine and acylcarnitine across the inner membrane but its role is long-chain acyl transfer has not been established. The tissue concentration of carnitine is important; liver carnitine is correlated with the rate of hepatic ketoacid production. In liver, malonyl-CoA, an intermediate in fatty acid synthesis, is proposed to regulate the activity of CPT-A. Studies using various purified transferases have not provided an answer to the question of whether the two activities expressed in mitochondria are separate enzymes. The absence of only CPT-A activity in isolated skeletal muscle mitochondria obtained from a patient with a lipid-storage myopathy suggests two separate activities.
D000138:Acidosis / Q000378:metabolism*; D000217:Acyltransferases / Q000378:metabolism*; D000818:Animals; D002331:Carnitine / Q000378:metabolism*; D002333:Carnitine Acyltransferases / Q000378:metabolism; D002334:Carnitine O-Palmitoyltransferase / Q000302:isolation & purification / Q000378:metabolism*; D002417:Cattle; D055598:Chemical Phenomena; D002621:Chemistry; D005227:Fatty Acids / Q000378:metabolism*; D007662:Ketosis / Q000378:metabolism*; D008930:Mitochondria, Liver / Q000201:enzymology / Q000378:metabolism; D008931:Mitochondria, Muscle / Q000378:metabolism; D010084:Oxidation-Reduction
D005227:Fatty Acids; D000217:Acyltransferases; D002333:Carnitine Acyltransferases; D002334:Carnitine O-Palmitoyltransferase; D002331:Carnitine