Transport and functions of carnitine in muscles.

The transport, function and metabolism of carnitine are discussed with regard to their importance in clinical chemistry. In humans carnitine is synthesized from protein-derived trimethyllysine in liver, brain and kidney. Muscles take up carnitine from the blood in an exchange-diffusion process with endogenous deoxycarnitine, the immediate precursor of carnitine. Besides catalysing the transport of long-chain acyl groups in mitochondria, carnitine is necessary for the export of intramitochondrially produced short-chain acyl residues and for the trapping and the elimination of unphysiological compounds (benzoic, pivalic, valproic acids etc.). The detection and quantitation in urine of these physiological and unphysiological carnitine esters is necessary for the diagnosis of carnitine deficiencies. The carnitine esters may be eliminated in the urine and/or distributed in tissues, where some of them (acetyl-, propionyl- and isovaleryl-carnitine) may be utilized for specific purposes. The most important carnitine-dependent metabolic disorders are listed according to their causes.