carnitine

Carnitine (β-hydroxy-γ-trimethylammonium butyrate) is synthesized endogenously from lysine and methionine in the liver and kidney, and is also obtained from dietary sources (primarily red meat and dairy). Its essential physiologic role is transporting long-chain fatty acyl-CoA molecules from the cytosol into the mitochondrial matrix, where beta-oxidation occurs to generate acetyl-CoA and ultimately ATP, particularly important during fasting states when fatty acid oxidation is the primary energy source. The carnitine shuttle involves three key enzymes: carnitine palmitoyltransferase I (CPT-I, located on the outer mitochondrial membrane, converts fatty acyl-CoA to fatty acyl-carnitine), carnitine-acylcarnitine translocase (transports fatty acyl-carnitine across the inner membrane), and CPT-II (on the inner mitochondrial membrane, reconverts fatty acyl-carnitine back to fatty acyl-CoA for beta-oxidation). CPT-I is inhibited by malonyl-CoA, linking fatty acid synthesis and oxidation regulation. Primary carnitine deficiency is an autosomal recessive disorder caused by mutations in the OCTN2 transporter, leading to impaired cellular carnitine uptake. Secondary carnitine deficiency can occur in various fatty acid oxidation disorders (e.g., MCAD deficiency), renal disease, or with certain medications (valproic acid impairs carnitine metabolism). Clinical presentation of carnitine deficiency includes hypoketotic hypoglycemia (due to inability to use fat for energy, forcing continued glucose/glycogen dependence without appropriate ketone production), skeletal myopathy with weakness and exercise intolerance, and dilated or hypertrophic cardiomyopathy due to the heart's high reliance on fatty acid oxidation for energy. Diagnosis involves measuring plasma free and total carnitine levels, acylcarnitine profile, and genetic testing. Treatment is oral L-carnitine supplementation. This topic frequently appears on boards in the context of fatty acid oxidation disorders, differentiating hypoketotic vs. ketotic hypoglycemia, and understanding the biochemistry of energy metabism during fasting.