Fig. 1.

The carnitine shuttle.

Carnitine enters the cell through active transport by the high affinity carnitine transporter, organic cation transporter novel 2 (OCTN2). Long-chain fatty acid-CoA in the cytosol exchanges CoA for carnitine by the action of carnitine palmitoyltransferase I (CPT1) in the outer mitochondrial membrane. Acylcarnitine moves into the mitochondrial matrix by facilitated diffusion through a transporter, carnitine-acylcarnitine translocase (CACT), on the inner mitochondrial membrane, in exchange for carnitine. In the mitochondrial matrix, the acyl group is transferred to mitochondrial coenzyme A by carnitine palmitoyltransferase II (CPT2). Carnitine is then free to cycle back to the cytosol through the transporter. Carnitine acyltransferase (CAT) removes CoA from acetyl-CoA that is formed from β-oxidation to form acetylcarnitine. Acetylcarnitine can exit the mitochondria via CACT and enter the blood via OCTN2.